Learn Multi platform PDP11 Assembly Programming... With Octal!

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Before the ZX Spectrum, the CPC and Nintendo, computing was literally a 'bigger' thing... the PDP-11 by DEC was an early computer...

Impressively advanced for it's time, it's programming language resembles the 68000, and some of it's other design ideas can be seen in other processors today.

The front of the PDP unit is lit up with LED's and covered in switches... these aren't just a sci-fi prop though, the PDP panel can be used as a hardware monitor and debugger, allowing any address in the CPU's memory to be read and altered.... in fact booting the PDP-11 may require setting many of these switches to program a 'boot sequence' into ram.

A PDP/40

The PDP-11 front panel.

Useful Resources:

Programming Card - Quick Programming Reference

PDP11 04-05-10-35-40-45 processor handbook 1975-76 - Original programming manual

Macro 11 Manual - official MACRO-11 documentation

Tutorial Document - Good summary of the commands

Peripherals handbook - PDP-11 hardware documentation

ChibiAkumas Tutorials

If you want to learn PDP-11 get the Cheatsheet! it has all the PDP-11 commands.

You may also want the programmers card

What is the PDP and what the heck is 'Octal'?
The PDP-11 is an 16-Bit processor - in the 1970s it was classed as a 'MiniComputer'... the CPU unit was around the size of a microwave, and when connected to disk drives and other hardware, the whole computer was the size of a single 'rack'

Octal is a numbering system known as 'Base-8' - it uses digits 0-7, effectively grouping digits from sets of 3 bits.

Octal allows binary to converted to digits, without using any extra 'letters', allowing for easier display and usage on conventional hardware like calculators.

If we're going to use the PDP-11 we need to get used to octal, as that's how all the documentation is written... fortunately, because it only uses 'digits' it won't be too strange, and we can mostly not worry about it!

While Octal is the 'norm' in PDP, Our Assembler can use other formats with the following syntax.

Base	Symbol	Alternate	Example	Radix command
Binary	^b		#^b101	.RADIX 2
Octal	(default)	^o	#777	.RADIX 8
Decimal	.	^d	#987.	.RADIX 10
Hexadecimal	^x0		#^x0FF	.RADIX 16
Ascii	'		#'A

^FF seems to work for HEX as well.

the PDP 11 is a Little Endian system... Low bytes are stored at even numbered addresses, and High bytes are stored at odd addresses

Oct	Dec	Hex	Binary
1	1	1	1
2	2	2	10
3	3	3	11
4	4	4	100
5	5	5	101
6	6	6	110
7	7	7	111
10	8	8	1000
11	9	9	1001
12	10	A	1010
13	11	B	1011
14	12	C	1100
15	13	D	1101
16	14	E	1110
17	15	F	1111
20	16	10	10000
100	64	40	1000000

The PDP-11 and UKNC content on this site wouldn't have been possible without the help of 'aberrant'... who's also working on a UKNC port of Chibiakumas!... not only is it an impressive feat, but the code is incredibly well commented, and worth looking at for UKNC insights

You can check out aberrant's work here!

Using The PDP-11 Front panel

The PDP 11 has an advanced front panel, which effectively can operate as a debugger, and is also able to program words of data into memory... The

There are 18-22 SWITCHes numbered 0-17/21 - these are used to set a value to be used as an address or as a value to store. the ADDRESS leds show the address being set or probed, the DATA leds show the value at an address/register

You can try out the PDP interface online... Here!

The other switches and leds have the following functions:

The PDP-11 Registers

The PDP-11's registers are ALL 16 bit.

Register	Details	Notes
R0
R1
R2
R3
R4
R5	LK	Often Used to remember the return address during calls
SP (R6)	SP	Stack pointer
PC (R7)	PC	Used to remember running address

If an Even register number (R0,R2,R4) is used for certain commands (for example MUL)- the register and it's following will be used as a 32 bit pair when the result is returned

Flags: -------- PPPTNZVC

	Name	Meaning
-	unused
P	Priority
T	Trap
N	Negative	1=Negative
Z	Zero	1=Result Zero
V	Overflow
C	Carry	1=Carry

Unlike many CPUs, the flags on the PDP-11 are not stored in a register, but are stored at memory address 777776-7 (in octal).

The PDP-11 Addressing Modes
The PDP-11 has 8 different addressing modes... many are similar to much later systems... in addition it has 4 'effective modes' which are defined by using Reg 7 as a parameter

'Deferred' addressing is known as indirect addressing on other systems

Octal Representation	Mode	Description		Sample Command	68000 Equivalent	effective result
0 R	Register	Value is taken from Registers itself	Rn			R
1 R	Register Deferred	Value is taken from address in register	(Rn) or @Rn		(Rn)
2 R	Auto Increment	Value is taken from address in register.... register increased by bytes read	(Rn)+			(Rn+)
3 R	Auto Increment Deferred	Value is taken from address at address in register.... register increased by 2	@(Rn)+			((Rn+))
4 R	Auto Decrement	Value is taken from address in register.... register decreased by bytes read	-(Rn)
5 R	Auto Decrement Deferred	Value is taken from address at address in register.... register decreased by 2	@-(Rn)
6 R	Indexed	Value is taken from address in register + a fixed number	n(Rn)		(2,R2)	(Rn+n)
7 R	Indexed Deferred	Value is taken from address at address in register + a fixed number	@n(Rn)			((Rn+n))
2 7	Immediate	Fixed numeric value	#n		#n
3 7	Absolute	Value from fixed address	@#A		A
6 7	Relative	Value from relative address	A
7 7	Relative deferred	Value from address in address	@A

MACRO-11

Macro-11 is the Assembler that is used by RT-11... Macro-11 is case insensitive, it also only uses the first 6 characters of labels and symbols... this limit extends to file names -which must be 6 characters or less... eg 123456.MAC or ABCDEF.asm

We can complement (flip the bits) of a constant with ^C eg #^C123

Missing commands

AND	We don't have an AND command, but we can fake one, we flip all the bits of our register, and use BIS (OR) Here is the equivalent of AND #7	bic #177770,r0 ;effecitive AND #7 or com R0 ;flip bits bic r0,r1 ;clear
CALL RETURN	We need to specify a register to use with calls	JSR PC,\addr RTS PC
PUSH reg POP reg	Use AutoInc and AutoDec	MOV \reg,-(SP) MOV (SP)+,\reg

Lesson 1 - Getting started with PDP-11!
For our PDP-11 development we'll be using R11 as an assembler , and "UKNC back to life" as an emulator...
The UKNC is PDP-11 compatible (we can also run the tests on R11 itself)
There's a video of this lesson, just click the icon to the right to watch it ->

Character Limits: The joy of 6!

MACRO-11 is limited in many places to 6 characters... this is especially true in filenames... for labels and symbols we can use more letters, but only the first 6 will be used - the rest will be ignored.

You will notice we do use longer labels in many cases in these tutorials - this is to give the functions easy to understand names.

Please bear in mind that the assembler we're using only processes the first 6

Structure of an ASM (.MAC) source file

Lets look at a simple file (Min.MAC)

we have a header - Our program is defined to start at #1000 in octal

In our body we're running a simple monitor program - this is where you would put your code

In our footer we're including some useful files (with a .INCLUDE statement), we're then defining a block of 128 bytes - this is for the stack, which will default to the top of our program area on R11

MACRO-11 files often have the extension .MAC rather than .asm

Notice the JMP . (full stop)? - A full stop in Macro-11 is the same as * in other systems, it returns the current line address..

So Jmp . is effectively an infinite loop.

Loading Values

WE have 6 general purpose (R0-5)registers we can use for whatever we want, Lets look at the MOV command - it move a value into a register When we set a register, our source is on the left, and the destination is on the right This is a 16 bit command, the value will be two bytes, and by default the number will be in OCTAL - #377 in octal is 255 in decimal
The register will be loaded with the value
By default we're specifying Octal, but we can specify other values... We can specify Decimal by putting a . at the end of the number (or ^d at the start) We can use Hex by putting ^x at the start we can use ascii by putting ^' at the start (we don't need the second ') By default we're set to Octal (Base 8 / Radix 8) but we can force octal with ^o All the previous examples loaded a 16 bit value, but we can load an 8 bit value by putting a B at the end of MOV... the byte will be sign extended to a word (bit 7 will fill bits 8-15
Here are the results! - note B5 was sign extended, filling all the unused bits with 1's

By default PDP-11 works in 16 bit, but many commands can have B tagged on to the end to work in bytes...

Check out the cheatsheet to see all the details of the commands.

If we don't want to default to octal when we specify a # number, we can do so with the .RADIX command

This changes the base as we require, we can always override the base with the ^ commands we already looked at.

We can use Binary, Octal, Decimal or Hexadecimal

Although it's tempting to default to HEX or DEC - we'll stick with OCTAL in these tutorials - as it's how the systems was designed.

here is the result

We'e looked at moving 'immediate' values (numbers with a # in the command) - but we can also transfer values from one register to another We can also load zero with the CLR (clear) command - this uses less memory than MOV #0,r4
Here are the results

INC and DEC

Just like the CLR command can quickly set a register to Zero - we'll frequently want to take a register and add 1 or subtract 1 We'll need this for loop counters, reading data, and plotting pixels to a screen... well we have special commands to do this INC and DEC by default INC and DEC are 16 bit, but as before we can use INCB and DECB
When we did the INC the word value up by two... when we did the DEC it reduced again... When we did INCB - the Byte went up, but when it went over #^o377 (Decimal 255) it rolled back to zero because 255 is the limit of the byte... the DECB commands had the expected effect

ADD and SUB

Of course,we can ADD and SUBtract other values! We just specify the ADD/SUB with two parameters , the immediate value or register on the left , and the destnation register on the right
Here are the result

JSR and CALL / RTS and RETURN

The JSR and RTS functions on PDP-11 perform CALLS (Jump To Subroutine) and RETURNs (Return from Subroutine)

Each function takes a register - JSR pushes the register onto the stack , then puts the program counter in that register... RTS pops the register off the stack, and moves it to the program counter..
.
Generally we'll just want to use JSR PC,{label} and RTS PC.... but rather than typing PC every time, we can use CALL {label} and RETURN {LABEL} - these are shortcuts supported by the assembler.

However... we can use a different register! If we use JSR R4,{label} and RTS R4 - then R4 will get the calling address... handy for getting values in .BYTEs following the CALL - or just getting the running address.

here are the results... not R4 took the PC of the calling address

Not just Registers!

We've looked at registers and immediates - but of course PDP can do far more! Lets take a look at Writing to ram! We can write to an address by using a label and putting a @ in front of it.
The value was saved to Ram... There's lots more of options for saving and loading... but we'll leave them until next time!

We can define BYTEs and WORDs in our code for our reading or writing... but PDP-11 is 16 bit and needs to be WORD aligned - we should put an .EVEN command after the data.

Lesson 2 - Addressing modes
We looked at the basics of PDP last time, but it's time to start getting serious!

Although it's relatively old, PDP-11 has a huge number of addressing modes... and now it's time to learn about all of them.

0R - Register

Register addressing is something we've already looked at... its where we're transferring data between registers! In this example, we transfer the value in R1 into R0 We just specify the register name R0-R5.... Note: R6 is called SP, and R7 is PC
We transferred R1 to R0

Wondering what 0R and 1R Mean?
We'll these are the octal representation in the bytecode..R would be a register number - and this code would be used in the source or destination field to specify the
value source...
You don't actually need to know these for programming - but we'll include them here for completeness

1R - Register Deferred

Register deferred sounds confusing, but it doesn't need to be!... this would be called 'Indirect register addressing' on other systems....

Basically, we're going to read from an address - and that address is held in a register... to do this we put the register in bracket.

Notice that whether we read a word from an odd boundary or even one, we get the same result!... this is because the PDP-11 requires the word data to be aligned on even boundaries.

Low bytes are stored at even numbered addresses, and High bytes are stored at odd addresses So we get the same value as last time,even though address is +1

The values are read in from the address in R1 into R0

2R - Auto Increment

We read from a register's address last time... we can also get that address to increment after each read...

We can use a Register as a source address - and sequentially read from consecutive memory addresses (for example if we're processing a bitmap file, or a text string)

We just put the register in brackets () and and add a + to the end... this is because the value in the register goes up AFTER the read

If we're reading in words, the address will increase by 2 - if We're reading in bytes, the address will increase by 1

MOV (SP)+,Rn is effectively our POP command

Here are the results

3R - Auto Increment Deferred

This is a combination of the last two - we'll read from the address in the address in a register - then increment the register by 2 Effectively the register should point to an entry in a vector table (a table of address pointers) We specify this addressing mode by using @(Rn)+
Here are the results

4R - Auto Decrement

This is the opposite of "2R - Auto Increment" We specify -(Rn) ... note this time the decrement occurs BEFORE the read MOV Rn,-(SP) effectively our PUSH command
Here are the results

5R - Auto Decrement Deferred

This is the reverse of 3R - we'll decrement the address by 2, and then we'll read from the address in the address in that register This is useful for reading from a list of pointers backwards... We specify @-(Rn) to use Auto Decrement Deferred mode
Here are the results

6R - Indexed

Indexed addressing takes the value from an address calculated from a register PLUS an offset This allows us to point a register to a bank of data (for a player, or device driver for example), and give an offset for read/write operations... The format is v(Rn) - where v is a positive or negative offset from the register.
We've read in the values from the addresses

7R - Indexed Deferred

Indexed Deferred is a combination of the last two... the address in a register plus an offset is used as a source address in a vector table - and that address is used as the source address for the final value. To specify this we use @v(Rn) where v s
Here are the results

If the register parameter is the program counter (R7 - PC) we have 4 extra 'effective' options...

We don't have to worry about this - The assembler handles this for us as needed.

27 - Immediate

We've seen Immediate values many times, these are just values specified in the line of the code... They are specified with #number
here are the results

37 - Absolute

Absolute addresses are where we're reading in from a fixed memory location... Normally we will probably use a label, but we can also use a numeric address.
Here are the results

67 - Relative

Relative addressing is where we are using a value read from memory relative to the Program Counter (the current running code) . The program counter can be addressed with . - this returns the current line of code. we can use this as part of a relative calculation
Both calculations load from the same address

During Execution in the processor the PC register always points to the next command (the next word after the current command).
the full stop (.) is the assembler directive, which represents the address of the current command, so is 2 bytes before the Program Counter.

77 - Relative deferred

Once again - Relative deferred uses an offset from the program counter, but this time it loads the address from the resulting address - and gets the value from that address for the parameter
The value at TestD2 (000777) was loaded into both registers

Lesson 3 - Conditions, branches and loops
We've looked at loading in fixed values, but we need to have some way of controlling what happens depending on the values in registers and memory...

This is where conditions come in!... lets learn all about them.

Looping with SOB!

on some systems looping can be a son of a bitch... but when we need to do a loop with PDP, there's no need to cry* - the PDP makes it easy with SOB! (Subtract One and Branch)

This will subtract 1 from the specified register, then loop back to the specified label if the register isn't zero - we can use this as our loop (like Z80 DJNZ)

In this example we add 2 to R0 each loop

*Excuse me for the terrible jokes.. i'm done now!... probably...

The loop ran 3 times based on the count in R1 - because it stops when it reaches zero, there was no 4th iteration

Fun with Flags.

As Branches are usually based on conditions, Before we look at them, lets learn how to set and clear the flags - there are special commands to do this!

Each flag has a 'set' command and a 'clear' command - we'll learn what each flag means and how to use it in a moment.

Action	N	Z	V	C	All
Set	SEN	SEZ	SEV	SEC	SCC
Clear	CLN	CLZ	CLV	CLC	CCC

We're using a function called 'MonFlg' to show the flags, but it's function is outside the scope of today's lesson

The flags will be set accordingly

We can back up and restore the flags in one of our registers with MFPS and MTPS

MFPS will back up the flags to a register

MTPS will restore the flags from the register

It should be noted that these commands do not exist on many earlier PDP-11 versions.

We backed up the flags into R0 ... zeroed them and restored them from R0

You may think that's a lot of 'set' and 'clear' commands - but that's just the half of it!

You can actually 'create' commands with the correct combination of bits to set or clear any combination of flags in one go! - you'll need to see the byte data the command compiles to to figure it out though - and it's probably not that useful!

Zero flag (Z)... Equal / Not Equal

Whenever we do a MOVe, CoMPare, ADD or SUB the flags will be set.

If the result is ZERO - Z will be true - otherwise it will be false... Like many systems CMP sets the flags like a subtract command, but doesn't change the registers.... so if the difference is zero

BEQ will branch if Z is set (Branch if EQual
BNE will branch if Z is not set (Branch if Not Equal)

There is a special branch... BR - this will ALWAYS branch!

Also notice the TST command - this will update the flags according to a register - but does not change the register itself.

The sample will show = if the result is Zero... or != if the result is not zero

BRanch and JMP do the same thing in the sense that they always jump... but BRanch takes fewer bytes - BR uses an offset in the 2 byte command, whereas JMP uses an absolute address (total of 4 bytes)... but BRanch cannot 'jump' far away from the current label (approx -256 to +255 bytes away)

You'll notice lots of remmed out code - this is other tests you can do to see how the flags are affected - you really need to download the code and try it yourself,or just watch the video!

Unsigned - Carry (C), Negative (N) and oVerflow (V)

Because of the way the CMP command works like a subtraction, with unsigned numbers the Carry flag will be set

Signed numbers are a little more tricky, we'll have to consider the oVerflow and Negative commands - fortunately the CPU handles this for us, and gives us 4 commands for each

Condition	Signed	Unsigned
<	BLT	BLO
<=	BLE	BLOS
>=	BGE	BHIS
>	BGT	BHI

The flags will be shown, and a < or > will be displayed onscreen

oVerflow (V) - those pesky signs!

Because when considering a signed number 32767 is 32767, but 32768 is -1... we have to be careful that adding a number, or subtracting one won't cause the sign flag to unexpectedly change... this is what the overflow flag (V) is for! BVS will branch if overflow is set BVC will branch if overflow is clear
O will be shown on Overflow... NO will be shown if there's No Overflow

Negative (N) - Sign bit

The N flag will allow us to check a sign - it effectively takes the value of the top bit of the result of the last operation. BMI will branch if the value is negative (minus - N set) BPL will branch if the value is positive (plus - N clear)
The flags and a + or - will be shown

Carry flag (C)

When an addition or subtraction goes over the limit of a register, or a bit shifting operation pushes a bit out the register the Carry flag is set (it acts as a borrow with subtraction) BCS will Branch if Carry is Set BCC will Branch if Carry is Clear
C will be shown if the Carry is set ... - will be shown if it isn't

BIT Test

If we want to test one or more bits in a register we can do so with the BIT command! This is effectively an AND command - but it does not change the registers - it will set the Zero flag depending on if the bits are zero or not.
Z or NZ will be shown to the screen.

Lesson 4 - Stack and More Maths
We looked at the AutoINC and AutoDEC addressing modes before that effectively work as PUSH and POP - but we didn't explain those!... lets look now at the stack and how JSR/CALL works

We'll also have a look at Multiplication and Division... and using the Carry to combine two registers for 32 bit maths!

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

(This screenshot is from the 6502 - but the principle is the same for the PDP-11)

Macro-11 has MACROS!

In todays example we're going to define a MACRO!
This will create a 'command' that contains a series of other commands - the assembler will replace any instance of 'ShowStack' with the commands in the macro...

This is just a simple example - Macro-11 also supports parameters: .MACRO testmac param1,param2 would define two parameters for the macro... these can then be used within the macro.

For full info see the Macro-11 documentation

Using the Stack for temporary storage

There will be many times we need to back up a register temporarily and restore it later - the stack is great for this We can use MOV Rn,-(SP) to move values onto the stack... we do it twice with R5 in this example. We can then restore it later with MOV (SP)+,Rn... we also do this twice with R5
First We pushed 017777 onto the stack via R5 Next We pushed 016666 onto the stack via R5 Then we loaded R5 with with 015555 We then restored R5 popping it off the stack - getting back 16666 We then restored R5 popping it off the stack - getting back 17777 Notice how SP changes as values are pushed and popped... SP always points to the Last item pushed onto the stack

JSR/CALL and the stack

JSR will push a register onto the stack, and move the program counter into that register...
The CALL command is the same as JSR PC - and is the simplest way to call a subroutine - whenever we do this the return address will be pushed onto the stack

in this example we'll push an item onto the stack... call a sub... push an item... call a sub and push a final item,

We'll then pop all the items off the stack and return - in the reverse order

The Three Pushes (1,2,3) can be seen on the stack... with the two Return addresses (1,2) in between

Because the stack is used by the returning address, we'll probably want to make sure the stack is in the same position at the end of the subroutine as it was at the start... so POP off what you PUSHED on!

If you're super clever you may want to do something tricky with the stack and calls - but for starters keeping the stack the same at the end as the start of a sub is best.

Add and Subtract in 32 bits with ADC and SBC

Just like many other CPU's addition and subtraction that exceeds the limit of a single register (16 bit on the PDP-11) will affect the carry flag

When Adding... the carry is effectively any bit that 'overflowed when the value went over 65536
When Subtracting... the carry is effectively a Borrow, and will be 1 if the calculation went below zero.

To perform 32bit+ addition, first we do the ADD command as normal for the low word... then we do ADC for the high word
To perform 32bit+ subtraction, first we do the SUB command as normal for the low word... then we do SBC for the high word

The first example is the ADD/ADC one - when the bottom word (R1) overflows - ADC adds one to R0

The first example is the SUB/SBC one - when the bottom word (R1) goes below zero - SBC subtracts0 one from R0

Multiplication and Division

When we perform a MULtiply command - we can perform an 16 or 32 bit multiplication, this is decided by the number of the destination register...

If an EVEN numbered register is used with MUL (like R2) the result will be stored in 32 bit pair (R2,R3)

if an ODD numbered register is used with MUL (Like R3) the result will be stored as a single 16 bit register (R3)

DIVide can only work with even registers... using destination R2 will mean the divided value is in 32 bit pair (R2,R3) - the integer result will be stored in R2 - the remainder will be in R3

Using DIVide with an odd pair will not work (R3 is no good)

in Example 1 We MULtiplied R0 by R2 and stored the result in 32bit pair R2,R3

in Example 2 we DIVided 32bit pair R2,R3 by R0... R2 stored the whole number result... R3 stored the remained

In Example 3 we MULtiplied R1 by R3, and stored the result in 16bit register R3

Example 4 did not work - we tried to use odd register R3 with DIV - this does not work.

Lesson 5 - Logical and Bit operations
We've looked at some mathematical operations, but we've really overlooked a lot so far... but we'll catch up now!

Lets take a look at the remaining major commands we'll need!

Swap Bytes with SWAB

All our registers are 16 bit - but we can swap the two bytes within a register with the SWAB command Here we'll swap the two bytes of R0 with SWAB then swap them back
The top and bottom nibbles were swapped - then swapped back again!

Sign Extending a 16 bit value to 32 bit with SXT

If we want to extend a 16 bit register to a 32 bit register pair we can do so with SXT... effectively this command fills a register with the sign bit - so in this example we update the flags with TST
In the first example SXT extended the 1 sign bit, filling R0 with ones In the second example SXT extended the 0 bit, filling R0 with zeros

Flipping bits with COM - Positive to negative with NEG

If we want to flip all the bits in a register, we can do this with COMpliment... We may want to convert a positive to negative (or visa versa) we can do this with NEGate... this effectively flips all the bits, and adds one
COM flipped all the bits... NEG flipped all the bits and added 1 (effectively sign flipping the value)

Logical Ops... XOR, BIS ,BIC

PDP-11 Logical OPS are slightly different

XOR (eXclusive OR) will flip the bits in the destination when the parameters bits are 1... this is known as EOR on some CPUS
BIS (BIt Set) will set (1) the bits in the destination when the parameters bits are 1... this is known as OR on other CPUs
BIC (BIt Clear) will clear (0) the bits in the destination when the parameters bits are 1... this is the opposite of AND on other CPUS

Operation	XOR	BIS	BIC
Destination Reg	10101010	10101010	10101010
Parameter Reg	11110000	11110000	11110000
Result	01011010	11111010	00001010

if we want to do an AND we'll have to be tricky... We need to flip the bits of the value we want to AND and do a BIC - this will have the same effect... we can flip the bits with a COM command... or by getting the assembler to calculate a compliment with the ^C prefix to a value

The results of the XOR, BIS, BIC and fake AND command can be seen here

^C tells the assembler to calculate the logical compliment (bit flipped) value... we can chain this with other statements, so ^C^D3 would complement decimal value 3... ^C^D0F would compliment Hex value 0F

Bit Shifts ROR, ROL, ASR, ASL

Like most systems, PDP-11 has a range of shifting and rotating operations ROR - ROtate Right using carry ROL - ROtate Left using carry ASR - Arithmatic Shift Right ASL - Arithmatic Shift Left We'll test each command with a loop.
The result of each command can be seen here. ROR - Bit 0 will be moved to the carry - the old carry moves to bit 15 ROL - Bit 15 will be moved to the carry - the old carry moves to bit 0 ASR - Arithmatic Shift Right - new Bit 15 will be the same as last bit 15 ASL - Arithmatic Shift Left - new Bit 0 will be zero

Bit Shifts ASH n bits... ASHC for 32 bit shifts

If we want to move by more than one bit we can use ASH A positive shift will shift Left A negative shift will shift Right ASH will shift a single register... ASHC will use a pair of registers (eg R0,R1) as a 32 bit pair
Here are the results... ASH only affects one register... ASHC affects the pair of registers