85 lines
4.0 KiB
Markdown
85 lines
4.0 KiB
Markdown
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# test_8086 — Intel 8086 as a velxio custom chip
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See [../autosearch/02_intel_chips_overview.md](../autosearch/02_intel_chips_overview.md#intel-8086-1978)
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for the spec.
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## Status
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📋 **Spec only.** Recommended **last chip to implement** — most
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complex bus, biggest ISA. Tackle after 8080 and Z80 prove the
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toolchain.
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## Pin contract (40-pin DIP, minimum mode)
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Minimum mode (MN/MX̅ tied high) keeps things sane. Maximum mode is a
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follow-up.
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| Group | Pins | Dir |
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| --------------- | ---------------------------------------------------- | --------- |
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| Multiplexed bus | `AD0..AD15` (low addr / data, multiplexed) | I/O |
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| Multiplexed bus | `A16/S3 .. A19/S6` (high addr / status, multiplexed) | out |
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| Bus control | `ALE`, `RD̅`, `WR̅`, `M/IO`, `DT/R`, `DEN̅` | out |
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| Bus arb | `HOLD`, `HLDA` | I/O |
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| Interrupts | `INTR` (in), `NMI` (in), `INTA̅` (out) | mixed |
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| System | `RESET`, `READY`, `TEST̅`, `CLK` | in |
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| Mode select | `MN/MX̅` (tie high for min mode) | in (fixed)|
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| Power | `VCC`, `GND` (×2 on real silicon) | power |
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| Status (min) | `BHE̅/S7` | out |
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Real silicon has ~40 pins; we register all of them.
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## Bus cycle reference (minimum mode read)
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```
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T1: drive A0..A19 onto AD0..AD15 + A16..A19 pins (low addr on AD).
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Drive ALE high then low to latch the address into an external 8282.
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T2: switch AD0..AD15 to input (read) or hold as data out (write).
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Assert RD̅ (read) or WR̅ (write).
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Assert M/IO appropriately (1 = memory, 0 = I/O).
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T3: sample AD0..AD15 (read) or hold the data (write).
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TW: while READY is low, stay in T3.
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T4: deassert RD̅/WR̅. Bus is free.
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```
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The chip itself does **not** demultiplex the address. An external
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"address latch" chip on the canvas (8282 equivalent) does that. See
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[../autosearch/05_open_questions.md](../autosearch/05_open_questions.md#q3-is-there-a-built-in-address-latch-primitive).
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## What's hard about the 8086
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| Concern | Strategy |
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| ---------------------------- | -------- |
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| AD bus multiplexing | Per-cycle direction switching (`vx_pin_set_dir`) — proven feasible by analogy with `mcp3008.c`'s state machine. |
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| 20-bit physical addr | Internal: `(seg << 4) + off`. Trivial. |
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| Variable-length instructions | ModR/M decode + displacement / immediate fetch. Big switch on opcode + helper tables. |
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| Prefetch queue (4–6 byte) | **Skip for the first cut.** Decode at IP. Full prefetch can come later. |
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| Segment register hazards | Honour the standard 8086 ordering: segment override prefixes, default segments per addressing mode. Reference any 8086 emulator. |
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| Min vs Max mode | Min only. Document that user must tie `MN/MX̅` high. |
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## Target demo sketch
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A 16-bit "hello world" assembly program that prints a string to a
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memory-mapped UART. Recognisable, modest scope, doesn't need DOS or
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BIOS emulation.
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Stretch: run a tiny subset of `8086tiny`'s BIOS to boot a ROM-based
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program. Real DOS booting is firmly out of scope until much later.
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## Implementation plan
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1. Spike: `MOV reg, imm` + `OUT` + `HLT` only. Wire to a ROM chip and
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a UART chip. Confirms the AD bus multiplexing.
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2. Add register file (8 × 16-bit gp + 4 × 16-bit segment + flags + IP).
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3. Add ModR/M decode and effective-address calculation.
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4. Add the rest of the ISA in waves: data movement, arithmetic,
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logical, control flow, string ops, interrupts.
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5. Run a known-good 8086 test suite (`8088_v1` test ROMs, etc.) —
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tracked in [../autosearch/05_open_questions.md](../autosearch/05_open_questions.md).
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## Files to create later
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- `8086.chip.json`
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- `8086.c`, `8086_decode.c`, `8086_modrm.c` (will likely split)
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- `address_latch.chip.json` + `address_latch.c` (the 8282 helper)
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- `roms/hello.bin`
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