BIOS COM ABI¶

Games reach the link only through these BIOS services (never the SIO registers) — so re-implementing them is enough to bridge the link. See implement it.

Call mechanism¶

Wrapper at 0x2304cd: service id in W, ×4, index the BIOS table at 0xFFFE00, call.

ld RW3, <id> ; call 0x2304cd     ; (0xFFFE00 + id*4) → call

Params in RB3, buffer ptr in XHL3, counts in RWA3, status in RA3.

Services¶

ID	Name	Inputs → result
`10`	`COMINIT`	initialize; `RA3=0` ok
`11`	`COMSENDSTART`	flush queued TX
`12`	`COMRECIVESTART`	enable RX
`13`	`COMCREATEDATA`	queue TX byte from `RB3`
`14`	`COMGETDATA`	RX byte → `RB3`; `RA3=0` ok / `1` empty
`15` / `16`	`COMONRTS` / `COMOFFRTS`	assert / deassert `/RTS`
`17`	`COMSENDSTATUS`	pending TX count → `RWA3`
`18`	`COMRECIVESTATUS`	RX byte count → `RWA3`
`19`	`COMCREATEBUFDATA`	send `RB3` bytes from `XHL3`
`1a`	`COMGETBUFDATA`	receive ≤ `RB3` bytes into `XHL3`

Legacy NeoPop/Mednafen HLE: TX byte queue, HDMA/int ch 11 after writes / ch 12 after reads, /RTS via I/O 0xB2.

Example — identity frame¶

COMCREATEDATA FC · 01 · 00 · 30 ; COMSENDSTART   → FC 01 00 30

Whole-ROM audit: all 32 COMCREATEDATA literals are 00 01 F8 F9 FC FD FE, in the link routines only.