Learn Multi platform PDP11 Assembly Programming... With Octal!
This is work in progress... This tutorial is planned to start in 2020



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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.

If you want to learn PDP-11 get the Cheatsheet! it has all the 6502 commands, it also covers the extra commands used by the 65c02 and PC-Engine HuC6280



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
Example
Binary 0b
0b101
Octal (default) - or 0o
777
Decimal .
987.
Hexadecimal 0x
0xFF
Ascii ''
'A'
- `1234` -- octal (decimal in case you use `decimalnumbers` directive)
- `1234.` -- decimal
- `0b101010` -- binary
- `0o1234` -- octal
- `0x1234` -- hexadimical
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

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
R6
SP Stack pointer
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

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) (Rn+)
2 R
Auto Increment Value is taken from address in register.... register increased by bytes read (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



Missing commands
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
bis #177770,r0    ;effecitive AND #7












 

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