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Minor Debugger improvements, notes, etc, while debugging the PDP-11 BASIC

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Jeff Parsons 2016-10-20 22:56:29 -07:00 • committed by Jeff Parsons
commit aa2a25c5ec
7 changed files with 163 additions and 94 deletions
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2
apps/pdp11/tapes/absloader/README.md
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@ -16,4 +16,4 @@ PCjs has archived the following DEC resources:
- [Absolute Loader](DEC-11-L2PC-PO.json)
- [LISTING OF PDP-11 ABSOLUTE LOADER (June 1975)](http://archive.pcjs.org/pubs/dec/pdp11/other/DEC-11-UABLA-A-LA_PDP-11AbsoluteLoaderListing_Jun75.pdf)
- "APPENDIX D: THE BOOTSTRAP AND ABSOLUTE LOADERS" from the [PDP-11 BASIC PROGRAMMING MANUAL (December 1970)](http://archive.pcjs.org/pubs/dec/pdp11/other/BASIC_Programming_Manual_Dec70.pdf)
- "APPENDIX D: THE BOOTSTRAP AND ABSOLUTE LOADERS" from the [PDP-11 BASIC PROGRAMMING MANUAL (December 1970)](http://archive.pcjs.org/pubs/dec/pdp11/basic/DEC-11-AJPB-D_PDP-11_BASIC_Programming_Manual_Dec70.pdf)

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apps/pdp11/tapes/basic/README.md
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@ -12,9 +12,55 @@ Try our [DEC BASIC Demo](/devices/pdp11/machine/1120/basic/).
PCjs has archived the following DEC resources:
- [BASIC (Single User)](DEC-11-AJPB-PB.json)
- [PDP-11 BASIC PROGRAMMING MANUAL (December 1970)](http://archive.pcjs.org/pubs/dec/pdp11/other/BASIC_Programming_Manual_Dec70.pdf)
- [PDP-11 BASIC PROGRAMMING MANUAL (December 1970)](http://archive.pcjs.org/pubs/dec/pdp11/basic/DEC-11-AJPB-D_PDP-11_BASIC_Programming_Manual_Dec70.pdf)
Third-party resources include:
- "[PDP-11 Paper Tape BASIC](http://www.avitech.com.au/ptb/ptb.html)", written March 24, 2013
---
Debugging Notes
---------------
One of the first things I noticed when debugging PDP-11 BASIC was its reliance on TRAP instructions.
For example, `TRAP 000` used to output the character in R2 to the terminal. Let's take a closer look at how its
TRAP handler works.
First, if you check the table of PDP-11 trap vectors, you'll see that the vector for TRAP instructions is 000034.
So let's dump the contents of the two-word vector at 000034:
>> dw 034 l2
000034 000100 000000
The BASIC TRAP handler is at 000100, and here's that code:
>> u 000100
000100: 011666 000002 MOV @SP,2(SP)
000104: 162716 000002 SUB #2,@SP
000110: 013646 MOV @(SP)+,-(SP)
000112: 006216 ASR @SP
000114: 103404 BCS 000126
000116: 006316 ASL @SP
000120: 062716 073654 ADD #73654,@SP
000124: 013607 MOV @(SP)+,PC
When the code starts, the TRAP instruction has already pushed two words onto the stack:
0(SP): previous PC
2(SP): previous PSW
The first instruction, `MOV @SP,2(SP)`, copies the *previous PC* onto the *previous PSW*, which is where we'll eventually want
*previous PC*, so that the handler can eventually return with a simple `RTS PC`.
The next instruction, `SUB #2,@SP`, subtracts 2 from the original *previous PC*, so that it now points to the TRAP instruction.
Then `@(SP)+,-(SP)` fetches the TRAP instruction while also "popping" the original *previous PC* and then "pushing" TRAP
instruction onto the stack, overwriting the original *previous PC*.
The next few instructions shift the TRAP right to see if bit 0 is set, and if it is not, then the TRAP instruction is restored
by shifting it left again, and then a large offset is added to it, transforming the TRAP instruction (which is now known to be
an *even* value) into a jump table index.
The final instruction, `MOV @(SP)+,PC`, moves the address at the jump table index into PC, while also removing the TRAP
instruction from the stack, leaving only the *previous PC* on the stack.