DEBUG16 - 65816 Disassembler and Trace Utility

Now that my Replica 1 is running with a 65816 processor, I wanted to try a program that was more substantial than the small demo programs I had tried.

The manual Programming the 65816 Including the 6502, 65C02 and 65802 available from The Western Design Center has a chapter describing and giving the source code for a program called DEBUG16. The program can perform disassembly and instruction tracing of 65816 machine code. It is about 1700 lines of assembler code. I thought I would try porting it to my Replica 1 using the CC65 assembler.

The first step was copying and pasting the assembler listings from the PDF file for the manual. That gave me a text file with the assembler listing of the original code.

Then I stripped out the the listing portion of the file to generate an assembler source file. It needed some changes to port it to the CC65 assembler, due to differences in it's assembler directives. After making a number of changes I had a file which would assemble and I could compare to the original listing.

It turns out there are a number of typographical as well as logic errors in the listing in the manual. A Google search shows at least one other person tried getting this code to work (about 10 years ago) and noticed the errors. After some detective work I think I was able to determine what the errors were, and obtained a file which could successfully build and generated the same code as in the original listing.

The original code was intended for running on an Apple //e with a 65816 card. The Apple specific code (e.g. for input and output) was clearly indicated in the source. I made the necessary changes to the input/output code to work on the Replica 1.

Next, I wrote a small main routine to call the LIST routine which disassembles 65816 code (I had it disassemble itself). Somewhat to my surprise, it actually produced reasonable output on the first try. I found a few errors in the code, such as an instruction or two that were disassembled incorrectly. I fixed that, although there may be some issues with a few instructions. It is at least correct enough for 6502 code that I can disassemble my entire JMON program and get the same output as from the 6502 disassembler that I wrote.

A sample of the output is shown here:

00:6013   08        PHP                                               
00:6014   18        CLC                                               
00:6015   FB        XCE                                               
00:6016   08        PHP                                               
00:6017   0B        PHD                                               
00:6018   F40000    PEA     $0000                                     
00:601B   2B        PLD                                               
00:601C   C220      REP     #$20                                      
00:601E   E210      SEP     #$10                                      
00:6020   649D      STZ     $9D                                       
00:6022   A580      LDA     $80                                       
00:6024   8584      STA     $84                                       
00:6026   A682      LDX     $82                                       
00:6028   8686      STX     $86                                       
00:602A   A780      LDA     [$80]                                     
00:602C   AA        TAX                                               
00:602D   8687      STX     $87                                       
00:602F   207762    JSR     $6277                                     
00:6032   204760    JSR     $6047                                     
00:6035   208F60    JSR     $608F                                     
00:6038   20CD61    JSR     $61CD                                     
00:603B   9005      BCC     $6042                                     
00:603D   20D462    JSR     $62D4                                     
00:6040   80DA      BRA     $601C                                     
00:6042   2B        PLD                                               
00:6043   28        PLP                                               
00:6044   FB        XCE                                               
00:6045   28        PLP                                               
00:6046   60        RTS                                               
00:6047   201D62    JSR     $621D                                     
00:604A   E230      SEP     #$30                                      
00:604C   A000      LDY     #$00                                      
00:604E   A586      LDA     $86                                       
00:6050   20F761    JSR     $61F7

The other function of DEBUG16 is a trace facility that allows stepping through 65816 code and seeing the current value of registers and disassembled instructions. This code has significantly more dependencies on Apple II functions and is harder to port. I spent some time on it, but it is tricky to debug the code as the 65816 changes in and out of native mode and 8/16 bit data and index register modes, and existing debug tools I have like Krusader's mini-monitor will not work in the 65816's native mode. I set this aside to look at later.

So the current status is that the disassembly routine is working well although it has not been tested exhaustively for all 65816 instructions. The trace function has not yet been tested or debugged. The code is available here.

While I was playing with the 65816, I took the time to make a little 65816 quick reference document. It can be printed on one double-sides page, and lists the register model, new addressing modes and instructions, and some other specifics of the 65816. It is most useful if you are familiar with the 6502 and are looking for a quick summary of what is added by the 65816. You can get it here in both OpenOffice.org and PDF formats.