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Learn Assembly for the Greatest Classic Processors:  Z80 - 6502 - 68000
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Learn Multi platform 68000 Assembly Programming... By Magic!

Don't like to read? you can learn while you watch and listen instead!

Every Lesson in this series has a matching YOUTUBE video... with commentary and practical examples

Visit the authors Youtube channel, or Click the icons to the right when you see them to watch the Lessons video!

In the late 80's as the 8 bit machines died out, and the world evolved into 16 bit... computing monsters were battling out for supremacy!

Some people favored the Atari ST, others the Amiga... while some preferred consoles like the Genesis or Neo Geo... while in Japan, home computer users gazed in awe at the X68K with it's arcade perfect gameplay...

In fact, there was no battle between the CPU's... as all these were based on the 68000!... with the exception of the IBM PC's 8086 and the Super nintendo (which was based on a custom 16-bit 6502)... pretty much all the 16 bit machines of the 80's and early 90's were based on the 68000!

With it's 24 bit Address bus, it could access up to 16MB of memory... it was it's 16 bit Databus that defined it as a 16 bit processor... in fact all of it's registers are 32 bit!

The successor to the 8 bit 6800... the 68000 kept the powerful indirect addressing... added loads of new registers... and a huge command set!...

Adding a Supervisor mode,

Later models (the 68020,68030 and more) would add floating point, true 32 bit support and more... but it's the first generation 68000 that these tutorials will be looking at!

If you want to learn 68000 get the Cheatsheet! it has all the 68000 commands, and will allow you to easily see all the different commands, what they do, and how they affect the flags.

Platforms covered in these tutorials
Amiga 500
Atari ST
Neo Geo
Sega Genesis (Sega Mega Drive)
Sinclair QL
X68000 (Sharp x68k)

PDF resources
Pocket 68000 guide
Motorola 68000 manual

What is the 68000 and what are 16 'bits' You can skip this if you know about binary and Hex (This is a copy of the same section in the Z80 tutorial)
The 68000 is an 16-Bit processor with a 24 bit Address bus!
What's 8 bit... well, one 'Bit' can be 1 or 0
four bits make a Nibble (0-15)
two nibbles (8 bits) make a byte (0-255)
two bytes (16 bits) make a word (0-65535)
three bytes (24 bits) represent one address on the 68000  (0-16777215)
four bytes (32 bits) is the limit of what one register can store on the 68000  (0-4294967295)

Numbers in Assembly can be represented in different ways.
A 'Nibble' (half a byte) can be represented as Binary (0000-1111) , Decimal (0-15) or  Hexadecimal (0-F)... unfortunately, you'll need to learn all three for programming!

Also a letter can be a number... Capital 'A'  is stored in the computer as number 65!

Think of Hexadecimal as being the number system invented by someone wit h 15 fingers, ABCDEF are just numbers above 9!
Decimal is just the same, it only has 1 and 0.
Assemblers will use a symbol to denote a hexadecimal number, in 68000 programming $ is typically used to denote hex, and # is used to tell the assembler to tell the assembler something is a number (rather than an address), so $# is used to tell the assembler a value is a Hex number
In this tutorial VASM will be used for all assembly, if you use something else, your syntax may be different! 
Decimal 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ... 255
Binary 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111   11111111
Hexadecimal 0 1 2 3 4 5 6 7 8 9 A B C D E F   FF

Another way to think of binary is think what each digit is 'Worth' ... each digit in a number has it's own value... lets take a look at %11001100 in detail and add up it's total

Bit position 7 6 5 4 3 2 1 0
Digit Value (D) 128 64 32 16 8 4 2 1
Our number (N) 1 1 0 0 1 1 0 0
D x N 128 64 0 0 8 4 0 0
128+64+8+4= 204            So %11001100 = 204 !

If a binary number is small, it may be shown as %11 ... this is the same as %00000011
Also notice in the chart above, each bit has a number, the bit on the far right is no 0, and the far left is 7... don't worry about it now, but you will need it one day!

If you ever get confused, look at Windows Calculator, Switch to 'Programmer Mode' and  it has binary and Hexadecimal view, so you can change numbers from one form to another!
If you're an Excel fan, Look up the functions DEC2BIN and DEC2HEX... Excel has all the commands to you need to convert one thing to the other!

But wait! I said a Byte could go from 0-255 before, well what happens if you add 1 to 255? Well it overflows, and goes back to 0!...  The same happens if we add 2 to 254... if we add 2 to 255, we will end up with 1
this is actually usefully, as if we want to subtract a number, we can use this to work out what number to add to get the effect we want

Negative number -1 -2 -3 -5 -10 -20 -50 -254 -255
Equivalent Byte value 255 254 253 251 246 236 206 2 1
Equivalent Hex Byte Value FF FE FD FB F6 EC CE 2 1

All these number types can be confusing, but don't worry! Your Assembler will do the work for you!
You can type %11111111 ,  &FF , 255  or  -1  ... but the assembler knows these are all the same thing! Type whatever you prefer in your ode and the assembler will work out what that means and put the right data in the compiled code!

The 68000 Registers

The 68000 has a far more advanced register set than the 8 bit machines we are used has 8 Data registers for general use (like BC) , 8 Address registers for memory functions and even stack pointers! (Can be used as HL,DE and even
 SP!)... Calling the 68000 16 bit is rather understating the system, as all it's registers are actually 32 bit!

Register Bits Function
D0 31-24 23-16 15-8 7-0 Data Register
D1 31-24 23-16 15-8 7-0 Data Register
D2 31-24 23-16 15-8 7-0 Data Register
D3 31-24 23-16 15-8 7-0 Data Register
D4 31-24 23-16 15-8 7-0 Data Register
D5 31-24 23-16 15-8 7-0 Data Register
D6 31-24 23-16 15-8 7-0 Data Register
D7 31-24 23-16 15-8 7-0 Data Register
A0 31-24 23-16 15-8 7-0 Address Register
A1 31-24 23-16 15-8 7-0 Address Register
A2 31-24 23-16 15-8 7-0 Address Register
A3 31-24 23-16 15-8 7-0 Address Register
A4 31-24 23-16 15-8 7-0 Address Register
A5 31-24 23-16 15-8 7-0 Address Register
A6 31-24 23-16 15-8 7-0 Address Register
A7/SP (USP) 31-24 23-16 15-8 7-0 Address Register / User Stack Pointer
ISP 31-24 23-16 15-8 7-0 Interrupt Stack pointer (A7/SP in Supervisor mode)
PC 31-24 23-16 15-8 7-0 Program Counter

15-8 7-0 Condition Code Register (Flags)

Each register can be used as an 8,16 or 32 bit register depending on your requirements...smaller is faster, but it is not possible to use a 24 bit register - so for memory pointers you'll have to work at 32 bit.

A7 is generally the stack pointer, and can also be referred to as SP, however actually any address register can function as a stack pointer!

There are two 'Stack pointers' available, generally A7/SP will point to the 'User Stack Pointer' (USP) ... however when the processor is in supervisor mode, the Interrupt stack pointer (ISP) shadow stack pointer will be used. Supervisor mode is only used by the system firmware and operating system, and we won't really use it in programming our software.

The Program Counter is 32 bit, but the x68000 uses 24 bits of addressable memory.

The Condition Code Register is only 16 bit, and only 8 bit can be accessed outside of Supervisor Mode.

The Condition Control register's flags (CCR) and Status Register (SR)
While the Status register  is 16 bits, we can't use all of it in normal user mode, - the top 8 bits are protected, but we can access the first 8 bits (Called the CCR) anytime...
 F  E  D  C  B  A  9  8    7  6  5  4  3  2  1  0
T - S - - I I I - - - X N Z V C


Flag  Name Meaning
T  Trace bit 1 if tracing
S  Supervisor 1 if in supervisor mode
I  Interrupt interrupt level (0-7)
X  eXtend 1 if value of the C-bit  is 1
N  Negative 1 if topmost bit of result is 1
Z  Zero 1 if result equals zero
V  Overflow 1 if arithmetic overflow occurred
C  Carry 1 if the last operation resulted in a Carry/borrow

The Carry flag and eXtend flag are almost the same... its should be noted however that the eXtend flag is used by Add and Rotate commands... and the Carry flag is used by Branching commands

We have no SCF/CCF type commands, if we want to set a flag, we do it with ANDI or ORI...
For example, let's set the X flag (bit 4):
    ORI #%00010000,CCR
Now, let's clear the X flag:
    ANDI #%11101111,CCR

Addresses, Numbers and Hex... 68000 notification
We'll be using VASM for our assembler, and MOT syntax, but most other 68000 assemblers use the same formats... however coming from Z80, they can be a little confusing, so lets make it clear which is which!
Prefix Example Z80 equivalent Meaning
# #16384 16384 Decimal Number
#% #%00001111 %00001111 Binary Number
#$ #$4000 &4000 Hexadecimal number
#' #'a' 'a' Ascii 'a'
12345 (16384) decimal memory address
$ $4000 (&4000) Hexadecimal memory address

A I Q and X!
Some Registers can support special versions, for example ADD... there are special versions for different cases!
Letter Command Meaning Why you need it
Q ADDQ #1,d0 Add a small value to a register (Quick) Faster
A ADDA addr,A0 Add a value to an address register Automatic
I ADDI #999,d0 Add an immediate value Automatic
X ADDX D0,D1 Add with carry (extend) Limited use

if we use Q we can speed up our code, and X will be needed to use the carry (though with 32 bit registers we probably won't need it!)
Technically we should use A and I when using Address registers, or immediate values - but actually our assembler will do it for us!

Commands and data sizes
Many (but not all) of the 68000 commands can work on different data sizes, and working at smaller sizes will save us processing time... With commands like Move we have up to 3 different options

Command Bits Meaning
Move.B 8 Move Byte
Move.W 16 Move Word
Move.L 32 Move Longword

We don't have a 24 bit option! which is strange considering our addresses are 24 bits, but that's the way it is... we use L for 32 bits with addresses...
in fact commands where the destination is an address register always affect all 32 bits of the destination - and the source register is extended if it's smaller.

When using B or W with data registers, the other 24 or 16 bits of the will be left untouched... THEY PROBABLY ARE NOT ZERO - so beware if you use more bits later!

There are also some special commands like INC and DEC called ADDQ and DECQ... these can add or subtract a value upto (and including) 8... and do so as a 'single command' with no parameter when compiled to bytes

Addressing modes, and their format in the source code
The 68000 has multiple addressing modes, to make things a bit strange, modern notification has changed compared to the older one, lets take a look!
Mode New Example Old Notes
Immediate Data #n #1 #n Fixed value - use $ for Hex
Absolute Address $nnnnnn $100000 $nnnnnn Fixed memory address
Direct Data Register Dn D0 Dn Use Value in register
Direct Address Register An A0 An Use Value in register
Address register Indirect   (An) (A0) (An) Use value in address pointed to by register
Address register with Postincrement (An)+ (A0)+ (An)+ Use value in address pointed to by register, Then increase An... this can be used like a POP command!
Address register with Predecrement -(An) -(A0) -(An) Decrease An, then read the value pointed to by the register... this can be used like a PUSH command!
Address register indirect, with 16-bit displacement  (dd,An) (5,A0) d(An) Read a value from the address in A0, shifted by fixed value d
Address register indirect with indexing and 8-bit displacement (d,An,Xi) (5,A0,D0.L)   d(A0,D0.L) Read a value from the address in A0, shifted by  d+Dn (d can be 0 if you only want to shift by Xi)... Xi can be Dn.W or Dn.L 
Program counter relative addressing with a 16-bit offset (dd,PC) (5,PC) d(PC) Read from the current address plus a fixed value
Program counter relative addressing with an 8-bit offset plus an index register.  (d,PC,Xi)   (5,PC,D0) d(PC,Xi) Read from the current address plus a fixed value plus a register (d can be zero if you only want to add Xi)
Stack Pointers
technically, the 68000 has no Stack Pointer... and no Push Pop commands... but don't panic! A7 works as the stack pointer... and in fact we can use ANY address register as a stack pointer... and we can use the Postincrement, and Predecrement commands to do our stack commands... even better we can back up and restore MANY or ALL registers in one go!... lets take a look!

Action 68000 command
PUSH D0 move.l a6,-(sp)
POP DO move.l (sp)+,d0
PUSH EVERYTHING   moveM.L d0-d7/a0-a7,-(sp)
PULL EVERYTHING moveM.L (sp)+,d0-d7/a0-a7

SP and A7 are the SAME REGISTER.... when you specify SP the assembler will treat it as A7.

GCC wants to make our life hard with special formatting!
If you're working with code online, it may be in GCC forrmat, which is really annoying, here's the MOT equivalent.
move.l %d1,-(%sp) MOVE.L D1,-(sp) All registers are preceeded by %
move.l %a1, %sp@- MOVE.L A1,-(SP) @ symbol denotes memory address like brackets in MOT
move.l %a0@(0x34), %d0 MOVE.L (#$34,A0),D0 displacement specified in brackets
clr.l    %a5@+ CLR.L (A5)+ Postincrement specified after address

Code alignment
Because the 68000 is a 16 bit processor, commands need to be 16 bit aligned... you won't need to worry about this if you're only using command code, but if you define byte data (like strings or bytes) you need to ensure the next commands are word aligned, do this by putting an even command in before your next code line.

Effective Addresses
Unlike the 8 bit systems, on our computer systems like the (AtariST and Amiga), we can't be sure where our code will actually end up running
lets take an example where  we have a label (eg mystring):
    mystring: dc.b "Hello World"

Rather than loading it's address with MOVE.L mystring,A0... we should use LEA mystring (load effective address) or PEA mystring (push effective address... to stack) to get the address...
On some systems, our MOVE.L mystring,A0 command may actually work - due to address relocation (where the OS will alter the addresses in the code!)... but on others it will not, and you'll be left wondering why your code is malfunctioning! therefore, you should always use PEA and LEA to be sure!

The need for SPEED!
Compared to 8 bit processor's, the 68000 is so fast you may not need to worry about speed, but there are some things worth knowing,

Firstly, you'll notice there are no INC or DEC commands, but we don't need to keep using the slow MOVE commands...
This job is done by the 'QUICK commands'... why are they Quick?... because the 'parameter' is included in the command... so the whole line compiles to a single word... just like INC and DEC on the 8 bits!
We also have a CLEAR command... not only can it wipe registers... it can wipe memory addresses!
Action 68000 command RANGE
INC / DEC ADDQ.L #-1 , d0 -8 to +8
LOAD MOVEQ.L # , d0 -128 to 127
CLEAR (set to 0) CLR.L ($1000) does not work on address registers

Compare results... CMP Val1,Val2
Basic command  Comparison  6502 command  Z80 equivalent  68000 equivalent
if Val2>=Val1 then goto label >= BCS label JP NC,label BCS label
if Val2<Val1 then goto label < BCC label JP C,label  BCC label
if Val2=Val1 then goto label = BEQ label JP Z,label BEQ label
if Val2<>Val1 then goto label <> BNE label JP NZ,label BNE  label

The 68000 has much more flexibility than the 6502 - though some of it's commands are the same!...
When we're doing comparisons, the commands we use depend on whether our data should be treated as signed or unsigned.

CMP D0,D1   Signed   Unsigned
D1 <= D0 BLE BLS
D1 <> D0 BNE BNE
D1 >= D0 BGE BCC

Lesson 1 - Getting started with 68000
68000 is far more powerful than the 8-bit systems... the advantage is that it can do some things far more easily than the 8-bits... but the disadvantage is it can be a bit more tricky, and there are a few mistakes we can make!

Lets take a look at the power of 68000!

Vasm, Build scripts and Emulators

In these tutorials, we'll be using VASM for our assembly, VASM is free, open source and supports 6502, Z80 and 68000!

We will be testing on various 68000 systems, and you may need to do extra steps (such as adding a header or checksum)... if you download my DevTools, batch files are provided to create the resulting files tested on the emulators used in these tutorials.

Note: the Genesis and NeoGeo also have a Z80 processor for sound... you may want to Learn Z80 as well for those systems...
You can do some sound from the 68000 on the Genesis... but you MUST use the Z80 for sound on the NeoGeo

My sources will use a symbolic definition to define the platform we're buiilding for, if you use my batch files this will occur automatically, but if you're using your own scripts, you need to define this with an EQU statement.

Here's the platform, symbol I use, and emulators we'll be looking at!

Platform Symbol Definition Required   Emulator used
Atari ST BuildAST equ 1 Steem
Commodore Amiga BuildAMI equ 1 FSUAE
Genesis (megadrive) BuildGEN equ 1 Fusion
NeoGeo BuildNEO equ 1 MAME
Sharp X68000 BuildX68 equ 1 WinX68kHighSpeed

For these tutorials, I have provided a basic set of include files that will allow us to look at the technicalities of each platform and just worry about the workings of 68000 for now...

We will look at ALL of this code later, in the Platform specific series... but we can't do that until we understand 68000 itself!

The example to the right is split into 3 parts:
The generic header - this will set up the system to a text screen
The program - this is where we do our work
The generic footer - The functions and resources needed for the example to work

The example here will load HEX 69 as a 32 bit value into register D0 - then it will show the monitor

It's important to notice all the commands are inset by one tab... otherwise the Assembler will interpret them as labels.

Also, Make sure you do not put any spaces in between the parameters... on VASM  'move.l #$69,d0' will compile fine....'move.l #$69 ,d0' will NOT (notice the space after 69)... you will get an 'instruction not supported on selected architecture' error
The sample scripts provided with these tutorials will allow us to just look at the commands for the time being... we'll look at the contents of the Header+Footer in another series...

Of course if you want to do everything yourself that's cool... We're lerning the fundamentals of the 68000 - and they will work on any system with that processor... but you'll need to have some other kind of debugger/monitor or other way to view the results of the commands if you're going it alone!... Good luck!

Registers and Numbers
Compared to the 8 bits... The 68000 is a monster processor! it has 8 data registers (for storing maths) and 8 Address registers (for storing addresses for read and write)...

having 16 registers wouldn't be bad at all... but ALL these are 100% 32 bit registers... if you're used to accumulators, and one stack pointer - forget that!... all the Data registers are equally functional... and all 8 Address registers can be a stack pointer (Don't worry if you don't know what that means - we'll learn about stack pointers later!)

The Data registers are named D0-D7 - All are 32 bit - you can use them in any way you want.

The Address registers are named A0-A7 - All are 32 bit - even though the Address bus on the 68000 is only 24 bit (meaning 16mb ram max)... Note: the system uses A7 as the stack pointer...if you use 'SP' the assembler will translate this to A7... so you probably only want to use A0-A6 for your own use

Lets learn a command!... MOVE.L... Move is very powerful - it can move a 'fixed' value into a register... move a value from memory into a register... or from a register to memory... or even copy one register to another!

Take a look at the example to the right... we're going to load D0, D1 and D2... but notice... we're going to load them in different ways... D0 will be loaded with #$69... D1 will be loaded with #69... and D2 will be loaded with 69... what will the difference be??


Well here's the result... the values are shown in Hex...
so D0=69...  because specifying #$69 tells the assembler to use a HEX VALUE
but D1=45...  this is because without the $ the assembler used a Decimal value (45 hex = 69 decimal)
D2=0... why? well when we don't use a # the assembler gets the memory address.... so we read from memory address decimal 000069!... of course we can do $69 or $0069 to read from address hex 000069 too!

So #$xx = hex value  .... #xx = decimal value.... and xx means read from address!

This is the same as the 6502 - but different to the Z80!... Note - if you prefer, you can put addresses in VASM. move.l (69),d2 works the same as move.l 69,d2

If you forget the # you're code is going to malfunction - as the assembler will use an address rather than a fixed value!

It's an easy mistake to make, and it'll mean your code won't work... so make sure you ALWAYS put a # at the start of fixed values!... or you WILL regret it!

68000 also lets you put brackets around an address like Move.L ($FFFFFF),d0
The 68000 can address 24bits of memory... so memory addresses can be from $00000000 to $00FFFFFF... the top two digits of the 32 bit register won't work!

Beware though! 68000 isn't like 8 bit - there's an operating system handling the memory, and it may not like us 'Messing' with memory our program shouldn't have - we'll learn about this later!

Here are all the 68000 Assembler ways of representing values, and how they will be treated.
Prefix Example Z80 equivalent   Meaning
# #16384 16384 Decimal Number
#% #%00001111 %00001111 Binary Number
#$ #$4000 &4000 Hexadecimal number
#' #'a 'a' ascii value
or (12345)
(16384) decimal memory address
$ $4000
or ($4000)
(&4000) Hexadecimal memory address

What's this JSR thing?... Jump to SubRoutine!

We've been using this JSR command... but what does it do?

Well JSR jumps to a subroutine... in this case JSR monitor will run the 'monitor' debugging subroutine... when the subroutine is done, the processor runs the next command

In this case that command is 'JMP *' which tricks the 68000 into an infinite loop!

JSR in 68000 is the equivalent of GOSUB in basic or CALL in z80, it's identical to the 6502 JSR command!.... we'll look at how to make our own subroutine in a later lesson!
JMP is a jump command ... and * is a special command that means 'the current line' to the assembler... so 'JMP *' means jump to this line...

This causes the 68000 to jump back to the start of the line... so it ends up running the jump command forever!... it's an easy way to stop the program for testing!

Bytes, Words and Long!

Despite being sold as 16 bit (because it has a 16 bit data bus - meaning 16 'wires' coming out the cpu for data) Internally the 68000 is a 32 bit processor - all it's registers are 32 bit - that's not to say it isn't faster to work at 8 bit... the 68000 can work in Bytes, Words or Longwords!... lets take a look at what they mean!

Size Letter Bits Range (decimal integer) Range (hex)
Byte B 8 0-255 $00-$FF
Word W 16 0-65535 $0000-$FFFF
Long L 32 0-4294967295 $00000000-$FFFFFFFF

Did you notice that 'L' in the Move.L command we used before? did you wonder what it meant!... well it meant LONG!... you see, whenver we use a command, we can specify if we want to work in Bytes - Words or Longs!... some commands can't do all the kinds (see the cheatsheet)... but it will be fastest....
Note... it is not possible to load a Word or Long from an ODD address... we can do Move.W (68),d0  but we CANNOT do  Move.W (69),d0...
of course we can do
Move.B (69),d0... Byte reading can be on an odd or even address...

It's worth noting that FUSION seems to ignore the invalid command and make it work anyway.

The Move command has 3 different options Move.B to move 8 bits... Move.W to move 16 bits... or Move.L to move 32 bits

In this example we'll clear all 32 bits of registers D0-D2

Then we'll set all 32 bits of D0 using Move.L.... next we'll copy 16 bits to D1 with Move.W.... and we'll copy 8 bits to D2 using Move.B

Finally we'll copy to D3 without specifying B W or L... what will happen?
Here's the result - of course we set all 32 bits of D0 to $69696969

When we copied 16 bits to D1, the top 16 bits were still 0 (because of that original 32 bit MOVE of #0  to all the registers)

When we copied 8 bits to D2, the top 24 bits were still 0

When we didn't specify any length with D3 - the assembler assumed we were working at 16 bits... however I'd reccomend for clarity that you always specify B W or L for your own clarity... NOTE: Some commands ALWAYS work as L or B commands - like LEA or SBCD... so do not assume anything!

When you copy a B or W from one register to another - the remaining bits will be unchanged! this can cause problems... for example if you do
    Move.B #0,D1
    Add.L D0,D1
You will not be able to rely on the value of D1.... why? because the top 24 bits of D0 could have been ANYTHING.... and you just added them to D1! - either you should have done Add.B - or Cleared the top bits of D0

NOTE: There is no 24bit command - even though all our addresses are 24 bit - so we need to use Long commands (32 bit) for address registers!

The Many faces of Move... and why we probably don't need them! (Except CLR!)

Technically, the move command is different depending on what we do with it, for example if we're setting an address we really should use MoveA... this is because the Move command cannot set an Address register...

if we do Move.L $00111111,A0 it WILL work... even though it shouldn't!.... why? because VASM is kind - it knows we should have used MoveA - so it converts it for us... there is no negative to this - we're not 'Technically' correct - but who cares!

another is MoveQ.L - which only works with L (32 bit) this will set a register to a value between -127 and 128... why use it? well it's FAST!..... MoveQ will compile to 2 bytes  where as Move.L would take 6.... BUT VASM is kind again! it will detect if you could have used the MoveQ.L command - and convert your code to it automatically...

There is one similar command we may want to use... CLR.L D0.... this clears (sets to 0) a register, and works with B, W or L... VASM will NOT automatically convert MOVE.L #0,D0 to CLR.L #0,D0 because it can cause slight issues on the 68000 (but not on the 68010+... note the 'issue' is nothing we'll need to worry about)

We've used CLR.L to clear D1... this compiles to a small 16 bit command.

You can see we used MoveA.L to load $69696969 into A0... remember - while it's not 'technically correct'... we could have just used Move.L

Next we used MoveQ.L to move $01 to D1... note we could have used Move.L in THIS CASE... and VASM would have automatically converted it to MoveQ... but remember, ther is no MoveQ.W or MoveQ.B

If you're confused by all this MoveA MoveQ  and all this .B .W and .L - Just forget it!

Just use Move.L for everything!!! - it may not be quite as fast, but it will work, and the assembler will fix most of the times you should have used something else!
If you do not want this kind of optimization to occur, you can use the '-no-opt' switch in VASM

This will turn of the Optimisations Vasm would otherwise perform, which you may need to do if you're doing self modifying code!... though that's not something these tutorials will cover.

Adding and subtracting

The 68000 has an ADD and SUB command for adding and subtracting at 8, 16 or 32 bit with Add.B, Add.W or Add.L

Like with Move, there are some 'Special versions' we should use ... Like AddI and SubI with a # number (immediate number) or AddA / SubA for addreses - but once again VASM will worry about that for us!

One more interesting one is ADDQ and SUBQ... these allow the addition or subtraction of a value up to 7...  If you're familiar with Z80 - these are effectively our inc and dec commands - they're super fast, and compile to 2 bytes (the minimum for the 68000) - and we can inc or dec by up to 7 in one go!... these work with Data Registers, and Address Registers.... that said - the Assembler wil work out when we could have used them if we don't!
The commands will all work as we expect, but note, We could have used Add and Sub in all of these cases, and it would have worked just fine...

Just forget about AddI and SubI... the Assembler will do it for us... and don't worry too much about AddQ and SubQ if you don't understand them!

Storing back to memory!
The 68000 is a bit different to the 8 bits... usually there will be an 'Operating system' doing memory management for us,  and this may mean we don't know where our program ends up running, or what RAM we've been given for out variables...

Fortunately there is a simple solution... the LEA command will Load the Effective Address of a 'label' - the result... we don't need to worry about where the data really is!...

Labels always appear at the far left of the code - and mark a 'defined line' of the code... but don't worry too much about 'Labels' yet - we'll cover them in a later lesson

In this example 'UserRam' is a 'label' pointing to some RAM we want to use - and store some data into that ram...

We'll use a function Monitor_MemDumpDirect to show some bytes of RAM to the screen!

Note in this example we're working at the Byte level.
Here's the result of the programm running... you can see the bytes $11, $22 and $66 were written... these are the two values stored at the start... and then the result of these two added to the $33 loaded into D2

But Note: Depending on the system the program is compiled for - and even the operating system and running drivers, the address the program uses will change!

The 68000 is BIG ENDIAN... what's that mean... well... if you store &1234 in addresses &0000 and &0001 then &12 will be stored in &0000, and &34 is stored in &0001... Bytes are stored in descending order... BIG numbers at the front END

What? that sounds perfectly logical? Well maybe! but it's the opposite of the Z80 and 6502... not to mention the ARM and PC processors... all of which do it the other way round!
Don't go writing to areas you don't know about and expect it to work... on the 68000 many areas will have nothing there (unused areas)... or ROM - or hardware 'registers'...

If you write to one of those hardware registers something strange could happen!

Lesson 2 - Addressing Modes of the 68000
The 68000 has a wide variety of addressing modes - some of which are quite complex... and of course, all the registers are 32 bit - so we have a lot of flexability when it comes to memory addressing

Lets look at all the addressing modes and try them out!


Before we do anything in todays lesson, we need to prepare some 'test data' in memory and a couple of the registers.

We're going to load a bank of 32 bytes into ram, and point the A1 register at the middle of them... we're also going to set D1 to 1, and clear D0 to start.

These examples are designed for the GENESIS / Megadrive - because we're using a fixed memory address - they will not work on other systems - however the concepts of the different Addressing modes are universal, and will work on all 68000 systems.

Prepearation... the result...
We've loaded a variety of test values into memory - and we'll be using these with our indirect addressing modes

Usage with ASM commands
Lets's take a look at the cheatsheet

You'll see on various commands an <ea> marker.... this means Effective Address

In most cases, whenver you see this, you can use any of the Addressing modes we'll look at today.

We're going to try our all the addressing modes with READ commands... but most of these commands also work with WRITING - and in fact, a wide variety of commands!

1. Immmediate Addressing
Immediate addressing is where a 'fixed' value is used as a source... this means it will start with a # Symbol
In this example we've moved #$12345678 directly into D0 - this is known as immediate addressing!

2. Absolute Addressing
Absolute addressing is where we read from a 'fixed' memory address... of course the address is specified WITHOUT a # symbol

On a 68000 system, we need to be sure what we're doing, and what will actually be at the address we're reading/writing to... 
In this example we've loaded a WORD (+16 bits - 2 bytes) from memory address $FF0100 - the middle of our test data...

Note: the 68000 is BIG ENDIAN... this means D0 ends up with the value $F0F1
(on a little endian system it would be $F1F0)

Note... it's not possible to Read or Write a Word or Long to an ODD address... so we could not do Move.W $00FF0101,d0

This is a limitation of the CPU - it can only work with Words or Longs on even byte boundaries.

3. Data Register Direct Addressing
Data register direct is where the source is the value held in a Data Register D0-D7
The Specified Data register will be copied to the destination

4. Address Register Direct Addressing
Address register direct is where the source is the value held in a Address Register A0-A7
The Specified Address register will be copied to the destination

5. Address Register Indirect Addressing
Address Register indirect looks at the data held at the address in the address register...
In this example A1=$FF0100... so the result loaded into D0 will be read from address $FF0100

Note: The 68000 is BIG ENDIAN... so when a Word or Long is read in the larger value byte in the word or long  (in this case F0) is loaded from the first address... and the smaller from the second (in this case F1)

6. Address register Indirect Addressing with PostIncrement
Address Register with PostIncrement starts the same as Address Register Indirect,

The data is read from the address in the Address Register... however afterwards the address is incremented by the number of bytes read...
For example if we read in a Word... then 2 will be added to the address...

When used with SP/A7 This is effectively the 68000 POP command
In our example we're reading in a Word... we read in $F0F1 - and 2 is added to A1....  if we had read in a Long, 4 bytes would have been added to A1

Because A7 is the stack pointer it works differently to other Address registers - if we write 1 byte to A7 with Postincrement, it will actually move 2... this is because it needs to stay byte aligned to work with Words and Longs

7. Address register Indirect Addressing with PreDecrement
Address Register With Predecrement starts by decreasing the specified address register with the amount to be read... then it ends the same as Address Register Indirect - reading in the amount to be read from the address in the address register

When used with a WRITING command and SP/A7 This is effectively the 68000 PUSH command
In this case we're reading in a Word... so 2 is subtracted from the address register - and then the data is read from the address in the address register... if we were reading a Byte 1 would be subtracted...

This command is typically used with WRITING commands - and is effectively the PUSH command for the 68000

8. Address register indirect, with 16-bit displacement
This is another inderect addressing mode, however with a 'fixed' displacement - the data is read from the Address register plus the offset - however the address register is unchanged.

This is similar to (IY+n) on the Z80... it can also be used for reading values pushed onto the stack
In this example a byte was read from A1+2.... Note A1 was unchanged...

The displacement does not have to be positive - it can also be negative.

9.Address register indirect with indexing and 8-bit displacement
This is the most advanced addressing mode available... the address in the Address register is used PLUS the Data register PLUS a fixed offset ... the value is read from the resulting address...

If you want to do a command like  (A0,D0) then use this mode with a ZERO displacement - eg (0,A0,D0)
In this example D1=1 and our displacement is 2... the result is we're reading in from A1+3

As before A1, and D1 are unchanged by this command

10.Program counter relative addressing with a 16-bit offset
The 'Program Counter' is the current position in the program... this addressing mode allows for data to be read from and address relative to the program counter... this is useful for 'relocatable code'... where you don't know where your program will actually end up running...

That said, it's possible you will never need this addressing mode, so don't worry about it if you don't understand it!

Note: This command can only be used for READING
In this example we're going to load bytes $68 and $69 into registers D0 and D1...

For D0 we're going to specify the offset as a number... for D1 we'll use the more reliable option of using a label.

11.Program counter relative addressing with an 8-bit offset plus an index register.
This 'Program Counter' Relative addressing mode is similar, however it also allows for a Data register to be added to the offset and the result read from the final address... the Address and Data register are unchanged

Note: This command can only be used for READING
In this example we're going to load a byte from the 'Test Data' adding the D1 register as an offset (D1=1)

The first time we'll do this using a label... the second using a numerical value... either way the result is the same.

In the example we'll read a byte from the 'Test Data' - offset by D1 (1)... the result is we'll load in byte $69 into D0 and D2

The PC relative modes are something you may not need - they were never used in Grime 68000

But you need to know about them as they're pretty powerful!... it's also important to understand there are MORE addressing modes on the later processors... but we're only covering the 68000 in these tutorials

Lesson 3 - Loops and Conditions
We've looked at quite a few topics, but we've still not learned enough to really get started... this time, we'll learn about Loops and Conditions - and a bit more about Labels and Subroutines

Some overlooked fundamentals - Labels and Symbols and subs
We've been rushing ahead to learn some commands, but we've overlooked some real basics! Now Lrets fix that!

Labels are a 'Named' line in our compiled code...

Once we've used a label, we can use it again later to Jump to it with a JMP, or use it as a subprogram with JSR

A subprogram called with JSR is like a GOSUB in basic - it jumps to the named label runs some commands, then returns back to the line after the JSR... the return command in 68000 is RTS

We're going to call a simple subroutine called AddTwo, it adds 2 to D0, then returns - not much use, but you can see the subroutine worked between the two calls to Monitor D0

we can also use labels to mark data, and use that data with a LEA command!.... effectively the Label name is converted to the line number by the assembler

Jump and Branch... and why we don't need to worry!

The 68000 has two pairs of Commands, JMP and BRA for Jumping, and JSR and BSR for calling a subroutine...

While the result is the same, In theory BRA/BSR will save some memory - it saves the offset to the label - rather than the whole lable - saving a byte or two!... but VASM will optimize our code and do this for us, so we can just ignore BRA and BSR if we want!

Defining Fixed values with Symbols
We can give named 'symbols' a value with the EQU command

We can then use that named symbol later to get the same value... we can use symbols to store fixed values or memory addresses...

Symbols are not memory, however... once we've defined them they can never change, though we can use a symbol to define a memory address!

Comparing and Branching on conditions (Bcc)

Of course there will be many times we need to do things depending on the value of a register...

We can do this with the CMP command... this compares a register to another register, a register to an Immediate value, or a register to a memory address - effectively all the options we saw in Lesson 2

there are a wide variety of branch commands - they're often known collectively as 'Bcc' - where cc will be a two letter description of the command

These will 'Branch' (jump) to a label if the condition is true...

in this example we're using BranchCarrySet - this effectively checks if the value is Less than the register (for an 'usigned' register)...

The reason 'Carry is Set' is technical... it relates to the 'Carry flag' and the fact the 'CMP' command simulates a subtraction... don't worry about it yet, we'll cover all this later!

Instead of BCS - there are a wide variety of other commands we can use... when we do CMP D0,D1 we can use the following Bcc command depending on the branch we want to do.

CMP D0,D1   Signed   Unsigned
D1 <= D0 BLE BLS
D1 <> D0 BNE BNE
D1 >= D0 BGE BCC

We can Branch on a condition... but there's no BSR/JSR equivalent on conditions... If we want to Call a Subroutine on a condition being true, we need to SKIP a BSR command if the condition is not true... or Jump, and jump back of course!

Be careful of B W L with Branch!
This time we're going to use the BEQ command - this will branch if D0 is equal to the compared #0...

But Wait! It doesn't work!... can you spot why?

We set D0 to Zero at the Byte level with Move.B... but when we did CMP we didn't specify a level... so it worked at the word level!
if we change CMP to CMP.B - it will now work... alternatively we could change MOVE.B #0,d0 to MOVE.W #0,D0...

The important thing is we compare at the same B/W/L level as we set the register to #0 if we want the expected result.

This is very important, and you're likely to make the mistake a lot!

DBRA - Decrement and BRAnch - for easy looping!
Often we may want to loop a certain number of times, running a command 6 times for example...

We can do this easily with the DBRA command - it will decrease a register, and jump to a label if it's not gone below zero yet.

Note... DBRA works at the WORD level.. so we need to set d0 to the count with a Move.W

This command is similar to DJNZ on the Z80, but it's important to notice DJNZ on the Z80 stops AT zero... but DBRA stops when it goes BELOW zero

Multiple conditions for a Case statement
It's important to understand that ALL other languages convert to assembly... so anything Basic or C++ can do can be done in ASM!

We can chain multiple branches together to create 'If Then ElseIf' commands or even create 'Case' Statements in assembly, just by chaining multiple branch commands together.
The result is shown on screen

On the 6502 branch commands can only jump 128 bytes away, but the 68000 has no such limitation

We can enable the 'ds 1024' command in the axample above, to add a bunch of space - and the branches will still work fine!

Lesson 4 - Stack, Traps, and Maths!
Now we know how to do conditions, jumping and the other basics, it's time to look at some more advanced commands and principles of Assembly..

Lets take a look!

Stacks for temporary storage
'Stacks' in assembly are like an 'In tray' for temporary storage...

Imagine we have an In-Tray... we can put items in it, but only ever take the top item off... we can store lots of paper - but have to take it off in the same order we put it on!... this is what a stack does!

If we want to temporarily store a register - we can put it's value on the top of the stack... but we have to take them off in the same order...

Push and Pop.... or -(sp) and (sp)+ as they're known on the 68000
On the 68000 we use A7 - also known as SP as our stack pointer - we use predecrement and postincrement to alter the stack pointer

When we want to add a new item to the stack (PUSH) we use a command like:
Move.W D0,-(SP)

When we want to take an item of the stack (POP) we use:
Move.W (SP)+,D0

We can Push Registers, or Immediate values, really anything from the Addressing modes lesson last week!
We're going to use a fake stack at a fixed location - so we can easily see what's going on... this example will work on the GENESIS... but as always, the principles will work on all systems.

Remember A7 and SP are THE SAME THING...

We can see the result... the stack pointer is set to $FF0100 - and the preceding 16 bytes are shown to screen.... the current Stack pointer is shown as a Magenta Bar

First lets push a word to the stack ($1234)...  because we pushed two bytes, the Stack Pointer goes down by two to $FF00FE

You can see the two bytes have been inserted into the stack memory

This time we'll push a Long onto the stack... the Stack pointer will go down 4 bytes - which is what we'd expect!
This time we'll push two individual Bytes on the stack... what will happen?

well... we'd probably expect the stack to go down by 2... but it actually goes down by 4!

Why? well the stack needs to work with Bytes, Words and Longs... and W/L can only be used on even memory addresses - so when we push a byte to a7- the value is padded by 1 byte...

This does not happen with Move.B #$96,-(A6)... A7/SP is a special case!

When it comes to removing from the stack, the reverse happens,if we 'pop' off a byte,  the Stack pointer goes up by two

the register will be set to the popped off value

We don't 'have' to pop things off the stack in the same way we pushed them on...

In this case, we've popped off a Long... and got the Word we pushed before, and half another long!

The stack pointer doesn't care what we do - but if you don't know what you're doing your code may break

We'll pop off another Long....

Our Stack pointer is back to it's starting value

The Stack and JSR
We can use the stack pointer to backup and restore register values ... the processor uses it too, to handle calling Subroutines!... lets take a look!
We're going to do a test here... we'll show the stack to the screen...

Then we're going to use JSR to jump to subroutine StackTest.... we'll show the stack again... and for reference, we'll also see the address of 'ReturnPos'

Then we'll return to the main program and show the stack again... what will happen?
When we call the Subroutine... the Return address is pushed onto the Stack, and the stack pointer decreases by 1 Long (4 bytes)

When we do a RTS - the address is Popped off the stack - we can confirm this because the stack pointer has returned to it's original value...

For this reason, it's important to ensure that any changes we make to the stack during a subroutine are undone before the subroutine ends!

MoveM - Moving Multiple items onto the stack!

There will be times we may want to move multiple registers on to, or from the stack, we can do this with MoveM!

we can specify a range of registers with a minus  -

Lets try it out!
In this example, D0, D1 and D2 are pushed onto the stack
We can specify ranges of registers with a minus -,

We can specify 'unrelated' registers with a slash /

We can even combine the two, and push ALL the registers onto the stack, or pop them off

PEA - LEA for the stack!
We learned before We can use LEA to load the effective address of a label - which will work reliably if our code is relocated by the operating system, however there is another command.

There may be times when we want to push the address of a label onto the stack, we can do this with a single command PEA
We can see that the Address pushed on the stack with PEA is the same as the one we got with LEA

We often push the parameters a function will use onto the stack before calling the function - so PEA may be useful for pushing addresses straight onto the stack in this case!

Maths commands, NEG, DIVS, DIVU... MULS, MULU
Lets take a look at some more advanced maths commands!

Neg converts a positive number to a negative one...

Negative numbers in Hex can be calculated by flipping all the bits and adding 1... so $01 negated is $FF... and $02 negated is $FE... this  means a signed byte can go from -128 to +127

Mulu - Muls

Unlike the 8 bits, the 68000 has multiply! we can give two values - and Multiply them together!

MULU is for MULtiplying Unsigned numbers
MULS is for MULtiplying Signed numbers

Divu - Divs
But why stop there? We even have a Divide command for integer division!

The values returned are interesting, the returned long (4 bytes) are in the following format:

Byte 1 Byte 2 Byte 3 Byte 4

Q=Quotient (Successful divisions)

If we Divide 301 by 10, we would have a Quotient of 30, and a Remainder of 1

If we're using signed numbers, we need to use a different commands...
MULU is for MULtiplying Unsigned numbers
MULS is for MULtiplying Signed numbers

Be careful you don't divide by zero! anything divided by zero is infinity - which is a mathematical impossibility and will make the WHOLE WORLD EXPLODE!!!1

Well ok, that won't happen, but your computer will crash, so don't do it, K?

It's a TRAP!
'Traps' in 68000 assembly are responsible for Error handling, such as overflow within the processor... they are numbered 0-15 and resemble RST's on the Z80...

While we are unlikely to want to 'Cause' errors on our processor, some computers use them for operating system calls... for example on the AtariST TRAP #13 is effectively a bios call, and depending on your hardware, you may need to use them - but what the will do will depend on that system

For now you just need to understand that TRAP #n is a system call - and you'll need to research that system to work out what it does.

Lesson 5 - Bits and swaps!
We've looked at maths and comparison commands, but quite often we'll need to do things at the BIT level... fortunately the 68000 has an incredible range of commands to do everything we could want,

Lets take a look!
AND, OR and EOR... and NOT!

There will be many times when we need to change some of the bits in a register, we have a range of commands to do this!

AND will return a  bit as 1 where the bits of both parameters are 1
OR will set a bit to 1 where the bit of either parameter is 1
EOR is nothing to do with donkeys... it means Exclusive OR... it will invert the bits of parameter 2 with parameter 1 - it's called XOR on the z80!
NOT  is similar to EOR - it flips ALL the bits and doesn't take an extra parameter

Effectively, when a bit is 1 - AND will keep it... OR will set it, and EOR will invert it

A summary of each command can be seen below:

Command Parameter 1 Parameter 2 Result
OR 1

Command move.b #%10101010,d0
eor.b #%11110000,d0
move.b #%10101010,d0
and.b #%11110000,d0
move.b #%10101010,d0
or.b  #%11110000,d0
move.b #%10101010,d0
not.b d0
Result #%01011010 #%10100000 #%11111010 #%01010101
Meaning Invert the bits where the
mask bits are 1
return 1 where both bits are1 Return 1 when either bit is 1 Invert all the bits

In the Z80 tutorials, we saw a visual representation of how these commands changed the bits - it may help you understand each command.

Sample EOR %11110000 
Invert Bits that are 1
AND %11110000 
Keep Bits that are 1
OR %11110000
Set Bits that are 1

Lets try these commands on the 68000!

We'll use the commands at the byte level for simplicity, and try each of the commands on a different register.... however these commands also work at the Word or Long level.
The bits of each register will be set according to the logical command.

As with many commands on the 68000 - Technically we should use a special version of the command when using an immediate value, however actually the assembler does the work for us!

Normal command Immediate command
And AndI
Or OrI
Eor EorI

Testing and changing single bits with BTST,BSET,BCLR and BCHG
There will be many times we want to test a bit of a register - we could do this with AND commands, but there are more powerful commands.
we can specify a bit number, and a register and test that bit... the Zero flag will be set if the bit was ZERO - or cleared if the bit was ONE

This would effectively be the same as the 'bit' command on the Z80, but on the 68000 we can change the bit at the same time!

Command Meaning Bit Before  Bit After
BTST TeST a Bit - bit unchanged 1
BSET test a Bit - and SET 1
BCLR test a Bit - and CLeaR 1
BCHG test a Bit - and CHanGe 1

Lets try out all these commands... we'll set the bottom two bits of D0 to 1 and 0 ... and we'll try each command out on these bits.

We'll show the contents of the ZeroFlag with a function called ShowZero, and we'll show the results of each command on D0
The BTST command does not change the bits... all the other commands had the same effect on the Zero flag

BSET set both bits to 1
BLCR cleared both bits to 0
BCHG changed both bits to the opposite of what they were

We're using a fixed Immediate value in the examples... but commands like btst d1,d0 are valid too!

Remember, the 68000 has LOTS of different possibilities, so check the Cheatsheet, or the 68000 documentation for all the details

Rotating and shifting bits with ROL,ROR, LSL and LSR
There will be many times when we want to shift bits around... If we shift all the bits in a byte left, we'll effectively double the number - if we shift them right, we'll halve it
We may want to use 3 bits from the middle of a byte or word as a 'lookup' - and we'll need to get them in the right position...

You may not immediately see the need for bit shifting - but as you program, you'll come across many times you need to do it...

The 68000 has a variety of commands - this time we'll look at ROtating and Logical Shift commands...

Rotating is where we move the bits left or right, and any bit that goes off the top or bottom of the byte (or word/long) will come back on the other side of the byte... so if we rotate a byte 8 times, we'll end up with what we started!

Logically Shifting is where we move the bits left or right, and any bits that go off the top or bottom are lost - bits that are 'added' are always 0... we can use this command for halving or doubling an unsigned number.... if we Logically shift a byte 8 times, we'll end up with 0!

We can Rotate or Logically shift Left or Right... there are two other commands... Arithmetic Shift, and ROtate with EXtend - we'll cover these next lesson!

Command Left Right
ROtate with eXtend   ROXL   ROXR  
Logical Shift LSL LSR
Arithmetic Shift ASL ASR

Lets try out the ROtate and Logical Shift commands!

We'll set some bits, and rotate things around, and see the difference between the results!
We can see the difference between the rotate and shift commands....

as bits are Rotated they come back, when they are Shifted they don't!

Pretty obvious by the name of the commands really!

Don't forget -There are two other pairs of shift commands - ROXL/ROXR and ASL/ASR... but we've already covered a lot of ground here - we'll have a look at those next lesson!

Scc - Set on condition
There will be times we'll want to do things based on a condition, and unlike on the 8 bit's, with the 68000 we can set a byte of a register based on a condition!

If the condition is true - the register will be set to $FF (255 is considered to be true on the 68k)
If the condition is false - the register will be set to $00 (0 is considered to be true on the 68k)

the 'cc' part of Scc is an abbreviation - we can replace that cc with any condition code, so like BEQ and BNE we can use SEQ and SNE to set if EQual or Not Equal

The command only sets the register at the BYTE level, and can only set $FF or $00 - but you can use AND and OR commands to alter the values.

SWAP and EXG - For exchanging registers and words!
There will be times we want to move data around registers... it may be useful to swap two registers without using any temporary registers or the stack, and we can do this with EXG...

also we may want to swap the top and bottom two parts of a Long... and the SWAP command will do this for us - this may be useful for DIVide commands - remember - they return the Remainer in one part of the long, and the Quotient in the other

Let's test the EXG and SWAP commands with some sample values.
We can see in the first example the registers were swapped...

in the second, we swapped the top and bottom word of the register.

Rather than EXG, We could use the stack to exchange registers... but that would be slower - EXG is fast!

Also, we could swap parts of a register with Rotate or shift commands - but again SWAP is more powerful!

Lesson 6 - More Bits... Extends and Macros
We covered bits last time, but the 68000 has so many commands, there's still more to do!

The main thing we need to cover is the eXtend flag (X) !

The Extend flag (X) the CCR and the SR
The 68000 cpu is a bit odd in that it has an eXtend flag and a Carry flag... in most cases they will have the same value - but the commands use the eXtend bit and are ADDX not we'll refer to the commands as using the Extend flag, not the carry flag.

That said, the purpose of the Extend flag X is basically the same as the carry on the Z80 or 6502, it contains bits 'pushed' out of a register by an overflowing command with ADDX, or a rotation command with ROXL

In the 68000 cpu, the flags are held in and 8 bit register called the Condition Code Register (CCR)... the CCR is part of the 16 bit Status Register (SR).... because of the CPU protection, we can't normally change the SR, but we can read it at any time... we can however set the bits of the CCR - and we need to do this to set or the eXtend and Zero flag.

The SR / CCR bits
Here are the content and meaning of the CCR /SR Bits
 F  E  D  C  B  A  9  8    7  6  5  4  3  2  1  0
T - S - - I I I - - - X N Z V C


Flag  Name Meaning
T  Trace bit 1 if tracing
S  Supervisor 1 if in supervisor mode
I  Interrupt interrupt level (0-7)
X  eXtend 1 if value of the C-bit  is 1
N  Negative 1 if topmost bit of result is 1
Z  Zero 1 if result equals zero
V  Overflow 1 if arithmetic overflow occurred
C  Carry 1 if the last operation resulted in a Carry/borrow

Setting and Getting the CCR
When it comes to setting the CCR, we have three options, AND to clear values , OR to set values, and MOVE to move a combination of bits to the  ccr...

We can use these commands to make any changes to the CCR we require...

If we want to get the current state of the CCR, we cannot move the CCR to a register - however we can move the SR to a register - and the bottom byte of the SR *is* the CCR
The OR command can be used to set bits -we've set them all to 1... note the top 3 bits didn't do anything!

The AND command can be used to clear bits

The MOVE command can be used to set all the flags to any set of bits

If we want to get the contents of the CCR, we can use the Move command to move the SR to a register - the bottom byte will be the CCR

We can only write to the Status Register in Supervisor mode - and that may not be possible on all systems....

It doesn't matter, as it's probably not something you'll need anyway - so don't worry about it!

Rotating bits with extend
We've learned how to use ROL and ROR to move bits around a  register, but sometimes we may want to move them out of, or into a register - and we do that via the extend bit with ROXL and ROXR
When we rotate to the right with ROXR, the extend bit is pushed into Bit 7, and Bit Zero is pushed out into the extend bit ...

This can be done with Words and Longs too... and we can shift by more than 1 bit if we want with ROXR #2 or ROXR #n

And of course we can rotate Left with ROXL as well!
If we set the X flag to 1 before the rotate we can fill the new bits with 1's
As all the new bits are now 1, the byte will eventually be filled with 1s

You can use ROXL or ROXR with numbers of bits more than one, the eXchange flag will contain the final bit - this can be useful if you've (for example) read in a byte from a hardware device, and need a single status flag from the middle of the byte.

Arithmetic shifting with ASL and ASR
Arithmetic shift Left and Right store bits that are pushed out of the byte into the eXtend flag, but they do not fill extra bits with eXtend

ASL fills extra bits with 0,

ASR fills extra bits with the last top bit - this is so that ASRing a negative number will STAY negative.

You can see the results of these commands here!
By combining ASL and ROXL we can use 2x 32 bit registers to effectively double 1x  64 bit value!
We can see the result here!

We can work on 64 bit or greater numbers by using SUBX, ADDX and NEGX - these use the eXtend bit to allow carry from each command to flow into the next register

We'll use D0 and D1 and a 64 bit pair - and repeatedly Add and Subtract 1 from the pair... we'll show the pair to screen each time to see how they roll over as the pair go below Zero

The NEGX command will invert multiple registers... 

One odd thing... if we do NEGX both times, the Z flag is not set - so if we want to do a compare after the commands, we need to set the flag manually before we start.
The results can be seen here... any 'carry' or 'borrow' from D0 will pass through to D1

The formula for negating a number is 'flip the bits and add one'... for this reason, we can't just use NEG on both DO and D1 - as one would be subtracted from both, not just the lowest one

EXT - Sign Extending Byte to Word... or Word to Long
The EXT command 'Sign Extends' a register - effectively the top bit is expanded to fill the register - this means a negative byte is converted to a negative word, or a negative word is converted to a long
The result with a negative number is the sign is extended...

With a positive number there is no appararent effect.


NOP - Doing nothing.... and Macros for saving typing!
the NOP command does nothing - It's main uses are for slowing down code, or for 'self modifying code' where code is changed while it's running...

Because the 68000 is so fast... NOP doesn't slow things down much...

Using the assembler we can create something called a 'macro' - we can create a set of commands and give them a name - when we use that name later, the assembler will replace the name with the lines of code - note this is not the same as a call...

We'll use a macro here to create an EightNoOps command to slow things down more
This example will swap the two registers - if we remove some of the NOPS it will speed up

Conditional compilation
There may be times you want to compile two versions of a program with parts of the program different...

One option would be to use branches and conditions - but this adds size to the binary and slows down the program.

Another option is conditional compilation - we can define a symbol with EQU - and use IFD, and ENDIF  to define a range of code that will ONLY compile if the symbol is defined... and ELSE or IFND

This allows us to compile two versions of the code depending on if the symbol is defined or not.

The results of the program will be different depending on if the symbol is defined...
with Testsym defined

with Testsym not defined:

Remember - code excluded by the condition DOES NOT EXIST in the resulting file...

These IFD are how the example code is able to compile for so many systems - different modules are compiled in and out depending on the platform we develop to...
Symbols can be defined on the VASM command line - so different batch files can compile different versions of your program

Learn Assembly for the Greatest Classic Processors:  Z80 - 6502 - 68000
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