# Phase 2 — Boot Protocol Emulation: Progress

**Status: substantial progress; boot handshake + i860 download solved; render
transport (FIFO fast path) identified as the remaining sub-protocol.**

The VPX device (`vpx-device/vpxlog.cpp`, env `VPX_RESPOND=1`) now drives the
production **BattleTech v4.10** (ALPHA_1 image) through the entire transputer/i860
boot sequence — far past the Phase 1 wall — and reaches the live renderer
protocol. Each layer below was solved by reading the normative sources on the
drive (`sda4/DPL3/VR_COMMS.C`, `DPL_HOST.C`, `VRENDER/VR_REMOT.C`, `LINKIO.C`,
`OUTSW.ASM`) and iterating against the running binary.

## Solved

### 1. iserver boot handshake
After the monitor (`VRENDMON.BTL`) download, `boot_xputer()` calls
`startup_handshake()`, which does **3 iserver transactions** (Phil's comment in
`VR_COMMS.C`: "the C run-time boots up and does 3 iserver transactions"). Each
is a `receive_protocol()` read of a 4-byte little-endian length/route word
(bit31=iserver, low16=payload length) + payload, answered by `iserver_action()`;
only success is checked. The device feeds **3 well-formed iserver "version"
requests** (tag 42, no request-content dependency) and the handshake completes.

### 2. i860 renderer download
`boot_860()` then streams the i860 image (`VRNOSTEX.MNG`) as framed renderer
messages on the slow byte path: `vr_860args` (21), `vr_860code` (18, ×118),
`vr_860data` (19, ×37), `vr_860bss` (20, ×9). The device parses the outbound
frame stream (`[length_word:4][action:4][data]`, `nb = length_word & 0xffff`)
and absorbs them, staying byte-aligned throughout.

### 3. The echo-action reply model (validated against board source)
Renderer request/reply calls (`velocirender_create`/`delete`/`flush`/`sync` in
`DPL_HOST.C`) send an action and require the **same action echoed back**. The
board side confirms this exactly: `VRENDER/VR_REMOT.C remote_velocirender()`
replies with its `data` buffer whose first word is still the received action
(`reply()` → `dN_send`). The device tracks the last outbound action and echoes
it; this is the general model for every render call, not a one-off.

## The remaining sub-protocol: the FIFO fast path

After the i860 boots, the host stops using the slow byte path and switches to a
**FIFO fast path** for the render protocol. This is why the current build still
stops at `velocirender_sync` — the device only implements the slow path
(0x150–0x153).

Discovered mechanics (from the capture + `OUTSW.ASM` + `LINKIO.C`):

- The VPX board is **dual-register**: beyond the C012 slow link
  (0x150 inputData / 0x151 outputData / 0x152 inputStatus / 0x153 outputStatus),
  there is a **FIFO data port** reached at **0x154/0x155** (observed) and an
  `ok_to_fifo` status at **0x160**.
- `send_FIFO_protocol()` / `velocirender_transmit()` (when `use_fifo`) call the
  assembly `outsw()` (`OUTSW.ASM`): it writes a `0x40` "use fifo" tag byte, then
  `add dx,3` and `REP OUTSW` — pumping the message as **16-bit words** into the
  FIFO port. In the capture these land as alternating byte writes to 0x154/0x155
  and decode to the init/args string (e.g. `/device~0x150~/video~svga~…`).
- The host switches to `use_fifo` once the board asserts `ok_to_fifo`
  (`fifo_ok_status()` reads 0x160 bit0); see the commented wait loop in
  `boot_xputer()`.

### Next step (concrete)
Extend the device to implement the FIFO transport:
1. Answer `ok_to_fifo` (0x160 bit0) at the right time so the host enables `use_fifo`.
2. Accept FIFO writes: the `0x40` tag, then word-stream data at the FIFO port
   (0x154/0x155). Reassemble into `[length_word][action][data]` messages
   (same framing as the slow path).
3. Serve FIFO reads for replies: `velocirender_receive()` in FIFO mode reads the
   reply (length_word + `[action][node]`) from the FIFO/status ports. Echo the
   sent action (model already validated), returning meaningful node handles for
   `vr_create` so later `vr_delete`/geometry ops have valid remotes.
4. Then the render loop (`vr_draw_scene` + frame-ack, `velocirender_frameack()`)
   — the Phase 3 boundary, where geometry/material/texture commands can be
   decoded into the OpenGL backend (formats already implemented in
   `restoration/divformats.py`).

## Reproduce

```
set VPXLOG=C:\VWE\TeslaRel410\emulator\vpxresp.txt
set VPX_RESPOND=1
emulator\src\src\dosbox-x.exe -conf emulator\respond.conf
```

Console progression now: `BattleTech v4.10` → `BTL4Application` → RIO check →
(handshake, i860 download succeed) → `unexpected action N in velocirender_sync`
(the FIFO-path render handshake, the next target). The log's `#` note lines
trace the device's decisions (handshake requests fed, frames parsed, actions
echoed, mystery ports resolved to 0x154/0x155).