a2-hrcg-teardown
HRCG teardown
This is my attempt to disassemble some of the routines on the DOS Toolkit disk, also known as the Applesoft Tool Kit. Don Lancaster’s Tearing Into Machine Language Code suggests disassembling the Apple Hi-Res Character Generator (HRGC), and I started with its building blocks, the relocatable module loaders.
So far I’ve disassembled:
- RBOOT – loads and relocates RLOAD.
- RLOAD – loads and relocates HRCG, APA or other relocatable module.
And mostly finished:
- HRCG – Hi-Res Character Generator. The disassembly is of the HRCG code itself. The header (code length) and footer (relocatable table entries) are normally generated by EDASM.
Afterward I found Dr John B Matthews had already done an analysis of RBOOT/RLOAD for ProDOS. Mine was just for fun anyway, but since the DOS 3.3 version has a lot of differences, it might be useful.
RBOOT/RLOAD for DOS are described in applesoft_toolkit__part_2_.pdf (chapter 4) on Asimov.
HRCG is mostly described in part 1 (chapter 2).
Bugs
HRCG doesn’t bounds check (or keep track of) the number of alternate charsets you loaded. If you select a charset number (^A n) out of range, you will get garbage.
Below I loaded 2 alternate charsets (ROMAN.SET and FLOW.SET) and then pressed ^A 1 <ENTER> to select alternate 1, ^A 2 <ENTER> … ^A 5 <ENTER>. Charsets 3 and 4 are garbage – actually HRCG program code at HRCG +$0000 and HRCG +$0300, since the charsets are loaded directly below it. Charset 5 is, surprisingly, legible. That’s because charset 5 lands at HRCG +$0600, and the builtin charset stretches from +$0500 .. +$07FF. The offset of $0100 into the font effectively shifts uppercase to lowercase, including symbols.
