PCjs Machines

DEC PDP-11 Bootstrap Loader

The Bootstrap Loader is a small program that loads the Absolute Loader, which is then used to load other PDP-11 tapes in the “Absolute Format,” such as BASIC (Single User).

Here’s what the Bootstrap Loader looks like:

Location  Instruction
 037744     016701
 037746     000026
 037750     012702
 037752     000352
 037754     005211
 037756     105711
 037760     100376
 037762     116162
 037764     000002
 037766     037400
 037770     005267
 037772     177756
 037774     000765
 037776     177550

Using any PDPjs machine with the built-in Debugger, such as this PDP-11/20 Boot Monitor, the Bootstrap Loader is easily entered with a single Debugger EDIT (“e”) command:

e 037744 016701 000026 012702 000352 005211 105711 100376 116162 000002 037400 005267 177756 000765 177550

You can immediately disassemble the code using the Debugger’s UNASSEMBLE (“u”) command u 037744 040000:

037744: 016701 000026          MOV   037776,R1
037750: 012702 000352          MOV   #352,R2
037754: 005211                 INC   @R1
037756: 105711                 TSTB  @R1
037760: 100376                 BPL   037756
037762: 116162 000002 037400   MOVB  2(R1),37400(R2)
037770: 005267 177756          INC   037752
037774: 000765                 BR    037750
037776: 177550                 .WORD 177550

To run the above code, set the PC to 037744 and start the machine:

r pc=037744
g

Pre-loading DEC’s Bootstrap Loader

I pasted the disassembled code (above) into a listing file, BOOTSTRAP-16KB.txt, and then ran FileDump to produce a BOOTSTRAP-16KB.json that can be automatically pre-loaded into any machine:

filedump --file=BOOTSTRAP-16KB.txt --format=octal --output=BOOTSTRAP-16KB.json

For example, this PDP-11/20 Bootstrap Loader Demo pre-loads the Bootstrap Loader using the <ram> component’s optional file attribute:

<ram id="ram" addr="0x0000" size="0x4000" file="/software/dec/pdp11/boot/bootstrap/BOOTSTRAP-16KB.json"/>

I also edited BOOTSTRAP-16KB.json to include the following properties:

"load": 0x3FE4,
"exec": 0x3FE4,

so that the <ram> component knows exactly where to load and execute the file. However, the <ram> component can also specify those value(s) itself, if needed:

<ram id="ram" addr="0x0000" size="0x4000" file="/software/dec/pdp11/boot/bootstrap/BOOTSTRAP-16KB.json" load="0x3FE4" exec="0x3FE4"/>

Finally, if you prefer octal and you’re sure everyone will be using an ES6-capable browser, you can do this:

<ram id="ram" addr="0o00000" size="0o40000" file="/software/dec/pdp11/boot/bootstrap/BOOTSTRAP-16KB.json" load="0o37744" exec="0o37744"/>

All PCjs configuration files use hex and/or decimal exclusively, since those number formats have always been supported, but as time goes by, the new octal and binary formats will be almost as universal.

[GitHub Source]