# X68000 hardware facts (verified, not assumed)

Target baseline: X68000 ACE/Expert class. 68000 @ 10MHz, 2MB RAM, SCSI HDD.

## Cycle budget
Content is cel animation shot on twos -> 12fps effective.
10,000,000 / 12 = **833,333 cycles per frame**. This is the wall.

## GVRAM layout  [verified: JC-000/x68000-dev-guide docs/graphics.md]
- 512KB physical, mapped at $C00000-$DFFFFF as aliased page windows.
- **One 16-bit word per pixel position, in EVERY color mode.**
  Bit depth does NOT change VRAM bandwidth. It only subdivides the word.
- Address formula (512-wide page): `addr = page_base + y*1024 + x*2`
- Horizontally adjacent pixels are 2 bytes apart in all modes.

| Mode  | Pages | Word subdivision            | Page aliases                      |
|-------|-------|-----------------------------|-----------------------------------|
| 16    | 4     | nibble per page (P3..P0)    | $C00000/$C80000/$D00000/$D80000   |
| 256   | 2     | byte per page (P0=lo,P1=hi) | $C00000 (P0) / $C80000 (P1)       |
| 65536 | 1     | whole word                  | $C00000                           |

Writes via a page alias are **auto-masked and shifted by hardware** into that
page's field. No software read-modify-write is needed for 16-color mode.

## Chosen mode: 256 colors, 256x192 active area
Rationale: since every mode is one word-access per pixel, low bit depth buys
no speed. 256-color halves on-disk data vs 65536-color for identical blit cost.

Coalescing trick: in 256-color mode a `move.l` spans two pixel positions across
BOTH pages. We sacrifice page 1 as a double-buffer and let it take duplicate
data, which lets us use `movem.l` bursts:
  movem.l d0-d7/a0-a3,(a6)   ; 12 regs = 48 bytes = 24 pixels

Full-frame refresh cost: 49,152 px / 24 = 2,048 bursts
  ~104 cyc/burst + VRAM waits (~1.5x) = ~320k cycles  << 833k budget
A hard scene cut fits. Delta frames cost far less.

## Audio
MSM6258 ADPCM via HD63450 DMAC. ~7.8KB/s at 15.6kHz mono, near-zero CPU.
22 min ~= 10MB.

## Storage estimate
~15,800 frames. Target ~3-5KB/frame compressed -> 50-80MB video + 10MB audio.
Stream rate ~40-60KB/s. Well within SCSI sustained throughput.

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## Corrections applied after session-1 measurement
- Storage estimate below was optimistic. Measured naive codec gives ~470MB @24fps.
  See FINDINGS.md §3. VQ codec targets ~40MB.
- "Content is cel animation shot on twos -> 12fps effective" — **the transfer has
  zero duplicate frames**; 12fps requires explicit decimation. See FINDINGS.md §3.
- 10MHz models are **SASI**, not SCSI. See FINDINGS.md §5.