# Boot & OS-call internals (reverse-engineered from the ROM under MAME)

Source: dynamic trace of `psionwamx` (v7.20f) under MAME — 170,160 instructions
of boot captured with correct bank mapping. Everything here is **reverse-
engineered / observed**, not from a manual, and describes internal mechanisms
below the documented C API.

## Reset & early boot

```
FFFF0: jmp  A000:0        ; reset vector -> ROM
A0000: jmp  A14A4
A14A4: di                 ; disable interrupts
       in/out 30h,...     ; ASIC hardware init via I/O ports
       mov ds0,190h       ; segment setup
       out 2h / 2Ch / 22h / 26h / 24h / 3Ch   ; ASIC control-register init
       jmp AE69C          ; signature check ([6896h]==0F0A5h) then continue
```
Early boot disables interrupts, programs the ASIC control registers through the
I/O ports, sets up segments, and validates a ROM signature word.

## OS service calls are software interrupts (not a single gate)

SIBO dispatches operating-system services through a **range of `INT` vectors**
(NEC V-series `brk` mnemonic), each vector a service group with a function
selector in a register. Vectors observed executing during boot, by frequency:

| Vector | Count (boot) |
| --- | --- |
| `0xA6` | 414 |
| `0x97` | 81 |
| `0x85` | 49 |
| `0x96` | 41 |
| `0xBA` | 41 |
| `0x8B` | 17 |
| `0xB9` | 15 |
| `0xAC` | 12 |
| others `0x81`–`0xB5` | few each |

Additional service vectors seen in driver code (not exercised at boot):
- **`INT 0xCF`** — the I/O executive: `CL` = function code, `BX` = channel
  handle, `DX` = argument, result in `AX`. This is the layer beneath `p_iow`.
- **`INT 0xD9`**, **`INT 0xD3`** — further service vectors (exact roles TBD).

The specific service behind each vector is not yet mapped; doing so means
breakpointing each ISR and correlating with the documented C calls. This table
is the starting point.

## ASIC I/O ports touched during boot

Distinct `out` port targets and write counts during boot (ASIC register access —
cross-reference the ASIC register maps in the hardware reference):

| Port | Writes | Likely role |
| --- | --- | --- |
| `0x28` | 1580 | display/LCD controller (dominant) |
| `0x15` | 88 | |
| `0x21` | 54 | |
| `0x10` | 17 | |
| `0x02` | 14 | ASIC control |
| `0x0A` | 13 | |
| `0x08` | 12 | |
| `0x2C` `0x24` `0x22` `0x26` `0x30` `0x3C` | few | ASIC control-register setup (from early boot) |

## Method (reproduce)

```
xvfb-run -a mame psionwamx -rompath roms -debug \
  -debugscript trace.txt -sound none -seconds_to_run 1
# trace.txt:  trace boot_trace.asm   /   go
```
Then post-process `boot_trace.asm` for instruction/port/vector frequencies.

## Next RE steps

1. Map each `brk` vector to its service (breakpoint ISR, correlate to C API).
2. Trace `SCANAPP`/`DEMMAN` to resolve the `WL2` scanner data-retrieval path.
3. Correlate the ASIC ports here with the HDK register maps.
