Intentionally Unimplemented¶
Features and hardware behaviors we have chosen not to emulate (or to leave as permanent no-ops), as distinct from open work in Roadmap.md / the todo list.
Add new entries here when we decide something is out of scope, obsolete, or unused enough that a stub would only add complexity.
Apple IIgs — ROMBANK ($C028 / STATEREG bit 1)¶
| Hardware | On ROM 00/01 boards, access to $C028 toggles ROMBANK. The same bit appears as bit 1 of $C068 (STATEREG). |
| Effect | When language-card ROM is mapped at $D000–$FFFF, the toggle flips A14 into the system ROM for that window: normal $FF/D000–$FFFF vs alternate $FF/9000–$BFFF (shown in the $D000 window). It does not swap banks $FE/$FF. |
| ROM 01 firmware | Unused. Apple docs say STATEREG ROMBANK “must always be 0.” IIgs firmware and GS/OS never touch it. |
| ROM 03 | Removed from the hardware (CYA era). $C028 / the STATEREG bit are inert. |
| GSSquared | g_rombank exists in the STATEREG bitfield for completeness / debugger display, but $C028 is not handled and nothing remaps LC ROM from the bit. Behavior is “always 0,” which matches real software. |
Related (ROM image layout, not ROMBANK): Physical ROM01 dumps sometimes present banks $FE/$FF swapped relative to linear address space. Our ROM01 image is stored with $FE then $FF (bank $FF last) so the MMU can map the top N banks of the file linearly. ROM03’s four banks are already linear FC–FF that way. That normalization is independent of the ROMBANK softswitch.
Cassette interface ($C020 / $C060)¶
Listed in Documentation.md as no support planned.
The cassette port existed on II / II+ / IIe only (not IIc / IIgs). Almost no modern use case; disk images cover the same software. Softswitches may exist as stubs or float depending on platform wiring — we are not building encode/decode or host WAV I/O for tape.
II / II+ REPT key¶
The physical REPT key is not modeled. Host keyboards already supply autorepeat; mapping a separate REPT key would not improve compatibility for any known software we care about.
DMABank¶
The IIgs DMABank register is currently unimplemented.
Diagnostic register¶
The IIgs diagnostic register (used to read back the contents of the character rom) is unimplemented.
When to put something here vs. a todo¶
| Put it here | Put it on the todo / roadmap |
|---|---|
| Firmware never uses it, or later hardware removed it | Needed for compatibility or a planned machine |
| Obsolete host/peripheral with no demand | Incomplete but wanted (e.g. printer, MIDI) |
| Emulation choice that permanently simplifies the model | Temporary stub until the real behavior lands |
If a “by choice” item later turns out to matter for a real title or diagnostic, promote it to a tracked issue and remove or revise the entry here.