The 6800 was too expensive for the mainstream, and it had many of its features cut, and was released as the 6502...  it's second accumulator gone, it's command set cut back - and everyone forgot about the 6800.... But the 6800 came back - as the 6809... with new previously unheard-of powers!... armed with twin stack pointers , 16 bit Stack,X and Y registers- 16 bit capabilities and advanced addressing modes and even a MULTiply command (unheard of in most 8 bits)... the 6809 is the 'missing link' between  the 6502 and the 68000! Powering the Dragon 32, the FM-7 machines - and the unique Vectrex... Lets see what the 6809 can do!

Platforms Covered in these tutorials

Other ChibiAkumas 6809 Tutorials

6309 Tutorials

Lesson 1 - Getting started with 6309 Assembly [DGN]

Lesson 2 - Shifts, Compares and logical ops [DGN]

Lesson 3 - More new commands! [DGN]

6809 Hello World Series

Lesson H4 - Hello World on the Dragon / Tandy CoCo [DGN]

Lesson H5 - Hello World on the Tandy CoCo Disk BIN file [DGN]

6809 MultiplatformLessons

Lesson M1 - Random Numbers and Ranges

Lesson M2 - Binary Coded Decimal

6809 Simple Samples

Sprite Movement on the Dragon 32- Simple 6502 ASM Lesson S1

Sprite Movement on the CoCo3 - Simple 6502 ASM Lesson S2

Object Movement on the Vectrex - Simple 6502 ASM Lesson S3

6809 Assembly: Lesson S4 - Sprite Movement on the FM7

6809 Platform Specific Lessons

Lesson P1 - Keyboard and Joystick reading on the Dragon / CoCo [DGN]

Lesson P2 - Joystick and Key reading on the Fujitsu FM7 [FM7]

Lesson P3 - Joystick reading on the Vectrex [VTX]

Lesson P4 - Bitmap Drawing on the Dragon / Tandy CoCo [DGN]

Lesson P5 - Graphics on the Fujitsu FM7 [FM7]

Lesson P6 - Vector drawing on the Vectrex [VTX]

Lesson P7 - Making Sound with the AY-3-8910 on the Vectrex and FM7 [VTX] [FM7]

Lesson P8 - Sound on the Dragon / Tandy CoCo [DGN]

Lesson P9 - 16 color Bitmap Drawing on the TRS-80 CoCo 3 [CCO]

Lesson P10 - Speed Tile (8x8 sprite) drawing on the FM7

6809 SuckShoot Series

Lesson SuckShoot1 - Suck Shoot on the Vectrex [VTX]

Lesson SuckShoot2 - Suck Shoot on the Dragon [DGN]

Lesson SuckShoot3 - Suck Shoot on the FM7 [FM7]

If you want to learn 6809 get the Cheatsheet collection! it has all the 6809 commands, It will give you a quick reference when you're stuck or confused, which will probably happen a lot in the early days! it also covers the 6309.

We'll be using Macroassembler AS for our assembly in these tutorials... VASM is an assembler which supports rarer CPU's like 6809 and 65816 and many more, and also supports multiple syntax schemes... You can get the source and documentation for AS from the official website HERE

What is the 6809 and what are 8 '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 6809 is an 8-Bit processor with a 16 bit Address bus!... it has two 8 bit accumulatrors, A and B, that can be combined to make up one 16 bit accumulator D (AB)
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)

And what is 65535? well that's 64 kilobytes ... in computers Kilo is 1024, because four bytes is 1024 bytes
64 kilobytes is the amount of memory a basic 8-bit system can access

the 6809 is 8 bit so it's best at numbers less than 256... it can do numbers up to 65535 too more slowly... and really big numbers will be much harder to do! - we can design our game round small numbers so these limits aren't a problem.

You probably think 64 kilobytes doesn't sound much when a small game now takes 8 gigabytes, but that's 'cos modern games are sloppy, inefficient,  fat and lazy - like the basement dwelling losers who wrote them!!! 6809 code is small, fast, and super efficient - with ASM you can do things in 1k that will amaze you!

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.

In this guide, Binary will shown with a % symbol... eg %11001100 ... hexadecimal will be shown with & eg.. &FF.

Assemblers will use a symbol to denote a hexadecimal number, some use $FF or #FF or even 0x, but this guide uses & - as this is how hexadecimal is represented in CPC basic All the code in this tutorial is designed for compiling with WinApe's assembler - if you're using something else you may need to change a few things! But remember, whatever compiler you use, while the text based source code may need to be slightly different, the compiled "BYTES' will be the same!

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 usefull, 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 6809 Registers
Compared to the 6502, , the 6809 is seriously powerful - and even gives the Z80 something to think about!

16 Bit 8 Bit Use cases accumulator A A 8 Bit Accumulator accumulator B B 8 Bit Accumulator 16-Bit Accumulator  D A B A+B combined to make a 16 bit accumulator Condition Code Register CC Flags Indirect X X Indirect Register Indirect Y Y Indirect Register User Stack Pointer U User Stack Hardware Stack  Pointer  S Stack  Program Counter PC Running Command Direct Page (8 Bit) DP x Zero page is relocatable on 6809

Flags: EFHINZVC Name Meaning E Entire Flag Regular/Fast Interrupt flag (Entire registers pushed) F FIRQ Mask Fast Interrupt mask (1=Fast interrupts disabled) H Half Carry Bit 3/4 carry for BCD I IRQ Mask 1 = interrupts disabled. N Negative 1= negative Z Zero 1= true V oVerflow 1= overflow C Carry 1= Add one, or Subtract one with ADC/SBC

The Direct page is like the 6502 Zero Page, however it does not need to be at zero! We can load A with a value, then TFR A,DP to set the direct page... we need to tell the assembler where the direct page is, otherwise some commands may malfunction, we do this with ASSUME dpr:$xx - this is called SETDP on some assemblers

Like the 68000, the 6809 is BIG ENDIAN... this means a 16 bit pair stored to an address like $6000 will save the high byte to $6000, and the low byte to $6001

The Carry flag works like the 68000, not the 6502... on the 6502, we set the carry before a subtract command to 'disable it'... but on the 6809, a set carry is 'Enabled' and subtracts an extra 1.

Special Memory addresses on the 6809
Unlike the 6502, The 6809 has full 16 bit Stack pointers, U and S.... the 'Zero Page' (AKA Direct Page) can also be re positioned

Like the 6502, there are a variety of 'Interrupt Vectors' with fixed addresses...

Address	Vector (Address)	Registers Auto-pushed onto stack
$FFF2	SWi 3 Vector	D,X,Y,U,DP,CC
$FFF4	SWI 2 Vector	D,X,Y,U,DP,CC
$FFF6	FIRQ Vector	CC (E flag cleared)
$FFF8	IRQ Vector	D,X,Y,U,DP,CC
$FFFA	SWI 1 Vector	D,X,Y,U,DP,CC
$FFFC	NMI Vector	D,X,Y,U,DP,CC
$FFFE	RESET Vector	NA

The 6809 Addressing Modes
The 6502 has 11 different addrssing modes... many have no comparable equivalent on the Z80

Group	Addressing Mode	Details	Example
	Inherent Addressing	Commands that don't take a parameter	ABX
	Register Addressing	Commands that only use registers	TFR A,DP
Imm	Immediate Addressing	Direct Address of command	ADDA #$10 ADDD #$1000
DirectPg	Direct Page addressing	Read from DP (zero page)	ADDA $10
Ext	Extended Direct addressing	Read from an address	ADDA $1234
Index	Indexed Addressing	Uses a 2nd setting byte - allows for Autoinc	,R offset,R label,pcr ,R+ ,-R []
Index	Extended Indirect Addressing	Read from the address specified... then get the value from that address	ADDA [$1234]
Index	Indexed Addressing: Zero Offset	Just use the address in the register	LDA ,Y LDA 0,Y LDA Y
Index	Indexed Addressing: Constant Offset from base register	5 / 8 / 16 bit offset from X,Y,U,S ... Can be negative or positive	LDA 1000,Y
Index	Indexed Addressing: Constant Offset From PC	8 / 16 bit offset from PC	LDA $10,PC
Index	Program counter relative	PCR is like PC, but is calculated by the assembler	ADDA label,PCR
Index	Indirect with constant offset from base register	Load from the address in the register + offset	LDA [1,X]
Index	Accumulator offset from Base register	Add accumulator (A/B/D) to a X,Y,U,S (not PC)	LDA B,Y
Index	Indirect Accumulator offset from Base register	Load from the address made up of a X,Y,U,S Plus the accumulator	LDA [B,Y]
Index	AutoIncrement	Add 1 or 2 to the register (no offset)	ADDA X+ ADDA ,X++
Index	AutoDecrement	Subtract 1 or 2 from the register (no offset)	ADDA -X ADDA ,--X
Index	Indirect AutoIncrement	Load from the address in Register, then add 2 (can't add 1)	ADDA [,X++]
Index	Indirect AutoDecrement	Subtract 2 then Load from the address in Register (can't subtract 1)	ADDA [,--X]
	Program relative	Offset to PC	BRA label

Indexed Mode Summary

Category	Version	Direct Eg	Indirect Eg
Constant Offset from base register	None 5 bit 8 bit 16 bit	,X 1,X 32,X 16384,X	[,X] {n/a - 8 bit only} [32,X] [16384,X]
Accumulator offset from Base register	A B D	A,X B,X D,X	[A,X] [B,X] [D,X]
AutoIncrement / AutoDecrement	+ ++ - --	X+ X++ X- X--	{n/a} [X++] {n/a} [X--]
Constant Offset From PC	8 bit 16 bit	32,PC 16384,PC	[32,PC] [16384,PC]
Extended Indirect	16 bit	{n/a}	[$8000]

Hints
Saving a byte on return:

Rather than returning, if your last command is a pop, just pop the PC with your other registers:
PULS B , X , PC
   
Addresses, Numbers and Hex... 6809 notification
We'll be using ASW for our assembler, but most other 6502 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 value
	12345	(16384)	decimal memory address
$	$4000	(&4000)	Hexadecimal memory address

Missing Commands!
Commands we don't have, but might want!

DEX/DEY/INX/INY	Tfr X,D ;replace X with Y if required DecB ;or IncB as required Tfr D,X ;replace X with Y if required
CLC	AndCC #%11111110
SEC	OrCC #%00000001
DeX	LEAX -1,X
ABY (Add B to Y)	LEAY B,Y

Lesson 1 - Getting started with 6809 Lets learn the basics of using 6809...  In this lesson we'll set some registers, and do a few simple maths operations. We'll be testing on a Dragon 32 or XM7 (XM7 shown)

The 6809 has a 16 bit accumulator (D - made up of AB), Two index registers (X,Y) and two stack pointers (U,S) These are LoaDed with LD?...where ? is a register name... an immediate value can be loaded with # followed by a number. We can specify Hex Values using $ We can specify Decimal without the $ we can specify binary with % we can even load the Stack.. but beware, the stack is used by calls, so we need to be sure the new value will work OK
The 16 bit Accumulator D is made up of two 8 bit accumulators A and B (A is the High byte of D - B is the low byte) We can load these from an 8 bit immediate. We can specify an Ascii value with '
Here are the results

You'll see lots of JSR command - this is like a GOSUB in basic, it jumps to a subroutine... RTS is the equivalent of RETURN... JMP is like GOTO... don't worry too much though... we'll see these again later.

Just like the 6502, If we don't specify the # then the number specified is an address... we can use ST? to Store register ? To an address
Here are the results
Loading works the same way... We don't need to specify a two byte address... if we specify just one byte (like $60) then the data will be loaded from the DIRECT PAGE. This is like the 6502 Zero page, though register DP is used for the top byte of the address, and it doesn't have to be $00xx
We've loaded bytes from memory, And stored two words to the Direct Page (Zero Page)

Specifying addresses in the direct page uses One byte, compared to the normal Two, so memory in this area saves program code and is faster. The Direct page should be used as a 'Temporary store' for values you don't have enough register for.

As well as immediates and memory, we have commands to transfer between registers. We can Copy the value from one register to another with TFR... the source register is on the LEFT of the comma, the Destination is on the RIGHT We can swap registers with EXG... the two registers values are swapped.
Here are the results

We can use INC? and DEC? to add 1 or subtract 1 from a register - these are great when doing loops We can also zero a register using CLR?... these only work on 8 Bit Registers A or B, not X,Y or D
Here are the result
We have versions for memory addresses too... INC, DEC and CLR - these are also all 8 bit.
Here are the results

Addition and Subtraction

We have ADD? and SUB? commands... these can work with 8 bit registers... We also have ADDD and SUBD which work with the 16 bit accumulator We also have ABX - which adds B to X (X=X+B)... A is not used... ABX is the only such command, there's no AAX, ABY or ADX
Here is the result

There will be many times in our program that we want to call subroutines, or jump to different places in our code. When we want to mark part of our code as a destination for a jump or a subroutine we use a Label - these are always at the far left of the source - whereas normal commands are tab indented. If we want to jump to a different position in our code we can use JMP - this sets the program counter to the address of the specified label (of course a fixed address can be used too) If we want to jump to a subroutine - and come back once the subroutine is done we use JSR.... the end of the subroutine is marked by a RTS return command... execution will continue on the line after the JSR command

You'll notice that many of the commands on the 6809 are the same as the 6502, but many are different on the 6809 Both have their 'roots' in the 6800, and the 6809 has some compatibility with the old cpu, but code would need recompiling for the 6809... Next time we'll take a look at addressing modes - and we'll learn about all the impressive options the 6809 offers.

Lesson 2 - Addressing modes on 6809 Lets look into more detail of the 6809... our commands need to load from or save to somewhere, and the 6809 offers us a wide range of 'addressing mode' choices for this purpose, Lets try them all out, and learn about them.

Inherent addressing is commands which do not take a parameter, the 'destination' of the command is built into the command. In this case... INCA will always use A as a source and destination
here is the result

Register addressing is where we specify a register by name as a source or destination
The source and destination are registers - we copied A to B!

We've seen Immediate addressing before, this is where a fixed value is specified with the command with a # symbol, and that value is used as the parameter
Here is the result

Direct Page Addressing is basically the same as Zero Page addressing... however thanks to the DP register the Direct Page can be at any address! We specify a single byte (without a #)... this is used as the bottom byte of an address - the top byte is taken from the DP register... that address contains the source parameter for the command
In this example we specified $80 , and DP=00... so we loaded from address $0080

Indirect Direct Page Addressing

Indirect Direct Page Addressing also uses the DP - however the two bytes at the specified DP address are used as an address... and the parameter is loaded from that address! The one byte DP address is in square brackets
The address specified is $81 in the example... so as DP=00 we load in from $0081... the two bytes here are $1213... this becomes the source address for our parameter  - at address $1213 is $42 - so this is the value that ends up being loaded into A

Don't forget! Unlike the 6502 the 6809 is BIG ENDIAN... the value $1234 will be stored in memory as $12 $34 It sounds logical if you're used to the 68000 - but it will be a shock to the 6502 or z80 users!

Extended Direct Addressing sounds complex, but it's not... If we specify a one byte address, we load from the direct page, but if extend our address to two bytes, we specify the Direct address to load from
In this case we specify $2000 - so the parameter is read from address $2000

Extended Indirect Addressing

Extended indirect Addressing is where we load our parameter from the address at an address (specified in 16 bits - a full address) The two byte DP address is in square brackets
In this example we specified [$2000]... at this address is $1A1B (Big Endian)... so the parameter loaded into A is read from $1A1B ($CC)

Indexed Addressing

Indexed Addressing uses a register plus an offset... the offset comes first, followed by a comma, then the register The offset can be Zero - in which case we can omit it. The offset can be positive or negative, and the register can be X,Y,S or U (not D)
Here are the results. Y was pointing to $2000 (the 1A)... U was pointing to $2002 (the 1C)
We can use a symbol to give a numbered offset a label. On the 6809 we can even have a 16 bit offset!
Both these work just fine!
There are multiple ways of specifying a zero offset
Here are the results

Actually, the offset doesn't have to be a fixed number - we can use the Accumulator as an offset! This has a different name though... "Accumulator offset from Base register"

We can specify a label as a parameter by offset using PCR as the parameter, an the label as the offset, the assembler will calculate the offset  for us.
The data has been loaded in

Rather than reading a parameter from a register+offset, we can read the parameter from the address at the address register+offset... We specify an offset and a register in Square brackets [] this is Indirect With Constant Offset Addressing
in the first example we loaded from [$80,X]... at address $0080+1 is $1213 - this becomes the source address of the parameter... at address $1213 is value $42 in the second example we loaded from [$2000,X]... at address $2000+1 is $1B1C - this becomes the source address of the parameter... at address $1B1C is value $AB

Rather than a fixed offset, we can use an accumulator (A,B or D) as an offset from an index register (X,Y,S,U) this is known as Accumulator Offset From Base Register in this example The parameter will be loaded from the address in A+X or B+X
Here are the results

Rather than use the address Accumulator+Base as the address of the parameter... we can indirectly use the address at that address as the source.
For Y X=$0080 and A=1... so we look at address $0080 - at this address is $1213... so this becomes the source address of the parameter - at address $1213 is value $4243 For D X=$0080 and A=2... so we look at address $0080 - at this address is $1314... so this becomes the source address of the parameter - at address $1314 is value $3334

AutoIncrement will read from the address in a register, and add 1 or 2 to the register AFTER the read. We just put a + or ++ at the end of the register - the register can be X,Y,S or U
Here are the results
AutoDecrement will decrease the register BEFORE the read by 1 or 2 We just put a - or -- at the end of the register - the register can be X,Y,S or U
Here are the results

Just like with Post Increment we can change the index register - but we can use that indirectly, and the address at the address that register points to becomes the address of the parameter We just put a +- or ++ at the end of the register for an Inc of 1 or 2... we use Square brackets [] to denote indirection - the register can be X,Y,S or U
Here are the results... The registers X and Y are Incremented... and the address in those registers is used to generate an address that is the source of the parameter
We can also use indirection with Pre-Decrement, and the address at the address that register points to becomes the address of the parameter We just put a - or -- at the start of the register for an Dec of 1 or 2... we use Square brackets [] to denote indirection - the register can be X,Y,S or U
Here are the results... The registers X and Y are Incremented.. and the address in those registers is used to generate an address that is the source of the parameter

Branch commands use a 'relative offset' to the destination, represented in the byte code as a positive or negative number. This is calculated automatically by the assembler... BRA uses a single byte offset , so can only jump short distances... LBRA (Long Branch) can jump long distances
Here are the results!

All the commands above will calculate an an address, and use the parameter at that address. However, there may be times when we want to calculate the address, but save it for later... Load Effective Address does this for us! This is a handy 'time saver' if we need to use a calculated address several times - it's faster to store the address in a register, then use that pre-calculated address for future commands.

In the First example, we used an offset of $100 from Y. As Y=$2000, the result loaded into U is $2100 In the Second Example, we PreDecremented Y by 2. As Y was $2000, Y Went down to $1FFE, and that value is loaded int X In the Third Example, we loaded the indirect address at $0080... at address $0080 is  $1112 - so this value is loaded into S

Lesson 3 - Carry, Branch, Test We need to learn about conditions and branching... which brings up the topic of Flags! Lets learn about it all!

The 6809 has a wide variety of Branch commands - these use a single byte relative offset to the current code position. While most of these are conditional, the 6809 actually has a 'Branch to Subroutine' command (BSR) - this is the same as JSR, but uses a single byte for the address (saving 1 byte over JSR) on the 6809 All branch commands have a 'Long' Equivalent (in this case LBSR), this uses a 16 bit offset, allowing a branch to anywhere in ram.

The subroutine will show an ! onscreen

When we do a mathematical operation like ADD or SUB, there is the possibility that the mathematical operation will cause the value in the register to go over 255 or under 0, causing a 'Carry' or 'Borrow' If we want, we can use an extra register as a second byte (in a 16 bit pair - or more) An ADD or SUB command will set the carry (also used as borrow)... we then use ADC for Add with Carry, or SBC for Subtract with carry - which will Add or subtract a value PLUS the Carry flag (if set)

Here are the results... when the Carry flag was set, the High byte (A) is affected by the ADC or SBC.

We've looked at ADD and SUB, but many other commands affect and use the carry flag, Rotate commands will shift bits around the register, and some will use the carry too.

We've learned ADD can set the carry... but what if we want to do a different command depending on the Carry flag? We can use BCC to Branch if Carry Clear (no carry) We can use BCS to Branch if Carry Set (carry)
Depending on the Carry flag, one of the two branches will occur!

To properly learn about how commands affect these flags, and the branches, you really need to download the source, and change the commands. If you REM (;) out the last ADDA, you'll leave the carry flag set, and the BCC branch will happen. What? you thought you didn't need to try things yourself? Good luck with that!

When we want to test a value in a register, we can do this with Compare (CMP).... this will set the flags, but not change any register - we have a whole range of CMP commands for different registers. The easiest Branch we can try out is BEQ and BNE... our Compare command will set the Zero flag if the difference between the register and the compared value is Zero... BEQ (Branch if EQual) will branch if the Zero flag is set BNE (Branch if Not Equal) will branch if the Zero flag is set

Which Branch occurs will depend on whether the Zero flag was set or not.

The CMP commands are actually a 'simulated subtraction' - in the sense that they set the flags the same as a SUB would, but they leave the register unchanged. That's why Z is set if the two compared values are the same.

Depending if the values in our registers are Signed or Unsigned numbers we need to use different branch commands for our comparison If our number is UNSIGNED (0 to 255 or 0 to 65535) then we use the following commands: BHI will branch if Higher (>) BHS will branch if Higher or Same (>=) BLO will branch if Lower (<) BLS will branch if Lower or Same (<=)
In this example D contained 28672... we compared to 24576, so BHI occurred - showing a >

Comparisons of Signed numbers

Finally we have to exceptions that don't look at the flags BRA will BRanch Always BRN will BRanch Never BRN is useless really, it's just a 'quirk' of the instruction set.
Unsurprisingly BRA occurred!

BRN is pretty useless, but it could be handy for self modifying code! Self modifying code is code that re-writes itself, for example, you may wish to modify a jump to turn it off.. changing BRA to BRN will do this!

Lesson 4 - The StackS! (Yes... there's two!) * In Assembly, We often need to temporarily store values for a short while and bring them back later, we use the Stack for this! Unlike most 8 bits, however the 6809 doesn't just have one stack... it has TWO! S and U - Twice the stacky goodness!

Stack attack!

'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... The stack will appear in memory, and the stack pointer goes DOWN with each push on the stack... so if it starts at $01FF and we push 1 byte, it will point to $01FE

Like most CPU's, the stack on the 6809 moves DOWN memory as items are 'pushed' onto it... We'll need to point our stack pointers to the top of a spare area of memory. When we want to set the stack up we use commands LDS and LDU (LoaD Stack and LoaD User stack)
When we want to put an item on the stack we use a PUSH command PSHS/PSHD... we get it back with PULL command PULS/PULU.... The stack is in reverse order, so the items are pulled off the stack in the opposite order they were pushed... Pushed items can be pulled into different registers later. Multiple registers can be pushed or pulled at the same time... the order we list the registers in the source does not affect the order they are processed by the command. We can push one register onto the stack
Here are the results. With each push the stack pointer S or U will go down... with each pull S or U will go up

We can push and pull any of the registers onto the stack... 8 Bit accumulators A or B, 16 bit registers X, Y or D, the Stack pointers themselves S, U the program counter PC, or the 8 bit flags (CC)

We can push the 8 bit accumulators onto the stack - when we do so a single byte is pushed onto the stack We can even then pull them off into a 16 bit register. We should always make sure our routines leave the stack as they found them - with the stack pointer in the same position at the end as the start, however we don't actually need to pull all the items off the stack - in this case we pushed the Stack register onto the User stack - and popped it off at the end... restoring the Stack pointer to it's starting state!

Here are the results.

Subroutines and returns also use the main stack (S) When we use JSR, the return address is pushed onto the stack... for this reason we need to ensure our stack is the same at the end of the subroutine as when it started.
In this example we can see the Return address, surrounded by the Other values pushed onto the stack

While JSR pushes the address of the next command onto the stack, RTS effectively pops an item off the main stack (S) into the Program counter (PC) JSR always uses the main stack (S) not the user stack (U).

we can also back up the flags (CC)! In this example, our subroutine backs up X,Y and CC Normally our subroutine would end with RTS... but as RTS effectively pops the program counter (PC) off the stack, we can do this with the PULS, and save the command!
Here are the results

No matter how many or few registers we push or pull the command always ends up being two bytes. Therefore, if we need to end our sub with a PULS we can save one byte by adding the PC to the list... PC is always the last register popped off the stack if it's in the PULS list.

Lesson 5 - More Maths - Logical Ops, Bit shifts and more We've covered a wide range of of commands, but we've overlooked a wide range of common maths commands. Lets look at them now and learn what they do

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 the accumulator and parameter are 1
OR will set a bit to 1 where the bit of either the accumulator or the parameter is 1
EOR is nothing to do with donkeys... it means Exclusive OR... it will invert the bits of the accumulator with the parameter - it's called XOR on the z80!

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	Accumulator	Parameter	Result
AND	1 0 1 0	1 1 0 0	1 0 0 0
ORA	1 0 1 0	1 1 0 0	1 1 1 0
EOR	1 0 1 0	1 1 0 0	0 1 1 0

Command	lda #%10101010 eor #%11110000	lda #%10101010 and #%11110000	lda #%10101010 ora  #%11110000
Result	#%01011010	#%10100000	#%11111010
Meaning	Invert the bits where the mask bits are 1	return 1 where both bits are1	Return 1 when either bit is 1

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	ORA %11110000 Set Bits that are 1

Our commands need to specify the register we want to work on. We can only operate on A or B (Not D)... we can also use ANDCC to set Condition codes, or ORCC to clear them. AND will clear bits where the parameter bit is 0 OR will set bits where the parameter bit is 1 EOR will flip bits where the perameter bit is 1 COM will flip all the bits (complement) - it doesn't take a parameter NEG will negate a number (turn a positive to negative or vice versa) - effectively flipping the bits and adding 1

The bottom bits of the AND Test were 0, so these were cleared in the test The bottom bits of the OR test were 1, so these were set in the test The bottom bits of the EOR test were 1, so these were flipped in the test When tested COM flipped all the bits When tested NEG negated the value - of course if we run it twice, we end up with the value we started

Rotations and Shifts

The 6809 has 3 kinds of rotate command, these can work to the Left, or Right, and can work on A, B or an address... they always work on a single byte Operation   Left   Right  Rotate bits around through Carry ROL ROR Arithmetic Shift (Signed) ASL ASR Logical Shift (Unsigned) LSL LSR
ROL and ROR rotate the bits by 1 to either the left or the right... Of course the register contains 8 bits, but the Carry acts as a 9th bit - so any bit pushed out of the register goes into the carry, and any carry is pushed into the "other side" of the registers
ASL and ASR are designed to work with signed numbers... When the bits are shifted to the right with ASR, any new bit will be the same as the previous top bit (in this case 1)... ASL will fill any new bits with 0 ... though it's effectively the same as LSL ASR effectively halves signed number, but keeps the sign intact... so 8 will turn to 4, or -8 will turn to -4 ASL effectively doubles numbers so 4 becomes 8
LSL and LSR are designed to work with unsigned numbers... When the bits are shifted to the right with LSR or ASL, any new bits with 0 ... LSR effectively halves unsigned number, but breaks signed numbers... so 8 will turn to 4 ASL effectively doubles numbers so 4 becomes 8

You need to download the source code and unrem the alternate shifts to see them in operation! What do you mean, you can't be bothered! Grr... I don't know, the youth of today are sooo lazy!

There may be times when you want to sign extend an 8 bit register to 16 bits - effectively filling the top byte with the top bit of the low byte. We can do this with the dubiously named 'SEX' command.... this sign extends B into A... so the D register now contains a 16 bit version of the signed number In this example we load some junk into A - then we load a test number into B - and sign extend it.

When the top bit was zero... A was  set to %00000000 ($00) When the top bit was one... A was  set to %11111111 ($FF)

Test bits and set flags

We have some commands to set flags based on a register without changing it the BIT commands allow us to provide a bitmask - this is effectively the equivalent of an AND command - but does not actually change the register TST sets the flags according to a register (or memory address) and sets the flags accordingly - we can use it to check if a register contains zero
BIT will compare the selected bits, and set the Z flag if the bits that were 1 in the parameter are 0 in the tested register. TST will set the flags according to the register or memory address... in this case we set the Zero flag when A=0... and the Negative flag when A was <0

Binary coded decimal is where we use a byte to store two decimal digits (one per nibble).... Actually they are stored as 'hexadecimal' however the digits never go over 9... for example $09 + 1 = $10 and $0099 +1 = $0100 DAA will decimal adjust the A accumulator (it cannot work on B)... this should be done after addition, and will correct the accumulator, converting numbers like $0A to $10
The Two nibbles act as a pair of decimal numbers (even though they are actually base 16)

The MUL command will multiply A * B and store the result in D
in this example we multiplied $69 by $10 ... The result? $0690

We've learned how to Multiply... Wondering where the Divide command is?... Well, um, there isn't one! You've got a MUL command - that's more than the Z80 or 6502 had... stop being so demanding!