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fix(blitter): exact ADDDSEL semantics in the hi-res sub-sample mirror

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Copilot review: the Stage 2 ADDDSEL sub-sample derivation diverged from
the stock DSTA2 data path in two ways -- an unsigned clamp where stock
does a SIGNED saturated byte add (source sign-extended) when TOPBEN is
clear, and a saturate where stock MASKS to 12 bits when TOPNEN is clear.
Mirrored exactly. Also documents why PIXEL_OFFSET_16(sub) is correct:
the macro token-pastes into sub_x/sub_y/sub_width/sub_pitch, reusing the
stock addressing formula over the explicit 16.16 position rather than
transcribing it.

make TEST_EXPORTS=1 test exit 0; Checkered Flag 2x box-replication gate
still exact (398/398, 0 bad blocks).

Co-Authored-By: Claude Fable 5 <[email protected]>
This commit is contained in:
Joseph MattielloandClaude Fable 5 committed 2026-08-08 23:54:48 -04:00
1 parent f66af4c3ed
commit 836baf665f
1 file changed
+28 -5
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src/tom/blitter.c
+28 -5
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@@ -447,6 +447,16 @@ static int32_t a1_clip_x, a1_clip_y;
static int shadow_hires_read_src16_a1(int32_t sub_x, int32_t sub_y,
uint32_t *out)
{
/* PIXEL_OFFSET_16 token-pastes its argument into <arg>_x/_y/_width/
* _pitch, so PIXEL_OFFSET_16(sub) evaluates the STOCK A1 addressing
* formula over the explicit 16.16 (sub_x, sub_y) position and the
* locals below -- reusing the formula by construction rather than
* transcribing it. */
/* PIXEL_OFFSET_16 token-pastes its argument into <arg>_x/_y/_width/
* _pitch, so PIXEL_OFFSET_16(sub) evaluates the STOCK A1 addressing
* formula over the explicit 16.16 (sub_x, sub_y) parameters and the
* two locals below -- reusing that formula by construction instead
* of transcribing it. */
int32_t sub_width = a1_width;
int32_t sub_pitch = a1_pitch;
uint32_t addr = (a1_addr + (PIXEL_OFFSET_16(sub) << 1)) & 0xFFFFFF;
@@ -485,13 +495,26 @@ static void shadow_hires_sub_fast(shadowfb_sub *out, uint32_t cmd,
}
if (ADDDSEL)
{
/* Mirror of the DSTA2 ADDDSEL data path below. */
/* Exact mirror of the DSTA2 ADDDSEL data path below: signed
* saturated byte add when TOPBEN is clear (source sign-extended,
* clamped to [0,0xFF]), then the 12-bit MASK (not saturate) when
* TOPNEN is clear. */
v = (s & 0xFF) + (dstdata & 0xFF);
if (!(TOPBEN) && v > 0xFF)
v = 0xFF;
if (!TOPBEN)
{
int16_t ss = (int16_t)((s & 0xFF) | ((s & 0x80) ? 0xFF00 : 0x0000));
int16_t dd = (int16_t)(dstdata & 0xFF);
int16_t sum = (int16_t)(ss + dd);
if (sum < 0)
v = 0x00;
else if (sum > 0xFF)
v = 0xFF;
else
v = (uint32_t)sum;
}
v |= (s & 0xF00) + (dstdata & 0xF00);
if (!(TOPNEN) && v > 0xFFF)
v = 0xFFF;
if (!TOPNEN && v > 0xFFF)
v &= 0xFFF;
v |= (s & 0xF000) + (dstdata & 0xF000);
}
else