Skip to content

Latest commit

 

History

History
75 lines (66 loc) · 4.12 KB

notes.md

File metadata and controls

75 lines (66 loc) · 4.12 KB
Raw

Notes

This file documents the process used to disassemble and document the SAM source code from the C64 version.

Disassembling

  • In VICE, load SAM and RECITER into low memory
  • Execute code at $97e2-97e8 to map LOMEM at $A000-$BFFF
  • Create dump file (need $7d00-$bfff)
  • Trace entry points with WFDis
    • $9500 installs the SAM wedge
    • $950D executes the wedge, which reaches the rest of the code
  • Mechanically convert to CA65 syntax
    • A lot of this was semi-automated with Vim macros and regexes:
      • Add : after labels: '<,'>-1s/^\([LS][0-9a-f]\{4\}\)/\1:^M /g
      • Convert hex into '.byte' statements:
        • Insert , $ between bytes: '<,'>-1s/\(\w\) \(\w\)\1, $\2/g
        • Add .byte prefix: '<,'>-1s/^\(\s*\)\1.byte $/g
    • Node scripts were used to decode some tables (see /reference/tools/*)
  • Authored custom 'c64.cfg' to place SAM at $7D00

Running SAM

Author stub 'startup.s' to install wedge (jsr $9500).

Important Addresses

Documented on page 35 of the C64 manual.

HEX Function Example Notes
$9500 (Re)install S.A.M. Wedge SYS 38144
$97E0 Throat POKE 38880, n (Default 128)
$97E1 Mouth POKE 38881, n (Default 128)
$9A00 S.A.M. from BASIC SYS 39424
$9A03 S.A.M. from ML JSR $9A03
$9A06 RECITER from BASIC SYS 39430
$9A09 RECITER from ML JSR $9A09
$9A0E SPEED POKE 39438, n (Default 72)
$9A0F PITCH POKE 39439, n (Default 64)
$9A10 LIGHTS POKE 39440, n (Default 0 = off)
$9A11 INTERRUPTS POKE 39441, n (Default 0 = disabled)
$9A15 TEXT (ASCII string terminated by $9b)

Tidying

Replace Hardcoded Addresses

Identified by relocating SAM and using a memory breakpoint to detect access to the previous memory region.

  • $9271, $928b: Split pointer table to alphabetic rules at $7d00-$8cda
  • $92a5: LTS rules for non-alphanumeric characters
  • $Lb3c0: Sample table
  • $a800-$af00: Rendering tables (Note: Must be page aligned)

Replace padding/uninitialized vars with '.res'

Identified through code inspection aided by memory breakpoints.

Bugs Fixed

During the process of documenting SAM, I found and fixed a few minor/benign bugs:

  • '?' did not cause a rising inflection when using RECITER due to '(?)=.'
  • A benign bug where 'e_rules' pointed to $8039 instead of $803f.
  • 'jsr' to $a439 (which is mid-instruction). Probably benign as I think this code is unreachable.

Documenting SAM

The reverse-engineered comments borrow liberally from pre-existing efforts:

Reference