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docs/future-work: reflect shipped sprite_flicker + HUD/scroll fix

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Two stale items cleaned up after the VRAM-buffer HUD work:

- §G mentions `examples/hud_demo.ne` as the realistic companion
  to the minimal `vram_buffer_demo.ne`, and documents the
  `$2006`/`$2005` reset the NMI drain now performs so the PPU's
  scroll latch stays clean across frames.
- Design question 3's sprite-flicker half is answered — both
  `cycle_sprites` and `game { sprite_flicker: true }` ship — so
  the question narrows to the still-open `draw ... priority:
  pinned` modifier.

https://claude.ai/code/session_01F7dHsgh7UX7SAK3wZ7JiKc
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Claude 2026-04-18 23:12:04 +00:00
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docs/future-work.md
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@ -340,13 +340,18 @@ explicit-stack pattern (small `u8[N]` stack + `u8` top).
`nt_set(x, y, tile)`, `nt_attr(x, y, value)`, and
`nt_fill_h(x, y, len, tile)` ship today — see
`examples/vram_buffer_demo.ne`. The runtime ring lives at
`$0400-$04FF` (gated on the `__vram_buf_used` marker; the analyzer
bumps the user-RAM bump pointer to `$0500` when the buffer is in
use). Each append lays down `[len][addr_hi][addr_lo][data…]` and
writes a fresh `0` sentinel; the NMI drains the buffer at vblank
via `LDA $0400,X / STA $2007` indexed-absolute (4 cycles per data
byte, no ZP cost).
`examples/vram_buffer_demo.ne` (minimal drain test) and
`examples/hud_demo.ne` (realistic score/lives HUD with
shadow-compare writes and one-shot attribute paint). The runtime
ring lives at `$0400-$04FF` (gated on the `__vram_buf_used`
marker; the analyzer bumps the user-RAM bump pointer to `$0500`
when the buffer is in use). Each append lays down
`[len][addr_hi][addr_lo][data…]` and writes a fresh `0` sentinel;
the NMI drains the buffer at vblank via `LDA $0400,X / STA
$2007` indexed-absolute (4 cycles per data byte, no ZP cost),
then resets `$2006`/`$2005` to zero so the PPU's scroll latch is
clean for the next frame (otherwise trailing `$2006` writes shift
the rendered image by however many cells we wrote past `$2000`).
Still TODO:
@ -584,13 +589,12 @@ Remaining gap items in order of user value:
would be cheap but would invalidate any copy-pasted ca65 fragments.
2. **Debug port address.** $4800 is conventional but not universal. Should
we support multiple debug output methods?
3. **OAM allocation strategy.** Sequential allocation remains the default;
the `cycle_sprites` opt-in keyword rotates the DMA offset each frame so
scenes past the 8-per-scanline budget flicker instead of dropping the
same sprite every frame. Open question: should automatic cycling become
a `game` attribute (`sprite_flicker: true`) that emits the increment
without requiring a per-frame call, and/or add a `draw ... priority:
pinned` modifier for HUD sprites that must stay at low OAM slots?
3. **Pinned OAM slots for HUD sprites.** Automatic sprite cycling is
decided — both the manual `cycle_sprites` keyword and the
`game { sprite_flicker: true }` attribute ship today (see §M).
Open question: should we add a `draw ... priority: pinned`
modifier that opts a specific sprite out of the rotation so HUD
elements stay at low OAM slots?
4. **Error recovery granularity.** How aggressively should the parser
recover? More recovery means more errors per compile but also risks
cascading false errors.