Measure the blit on the 68000: the 38% estimate was 53.6%
First 68000 instructions in this project to draw a pixel. Everything before this was GVRAM filled from Lua, which costs zero 68000 cycles, so the blit figure the whole CPU budget rests on had never been validated. Four variants of a full-frame 256x192 paint, timed in MAME and each also hand-derived from the MC68000 timing tables beforehand; the two agree to 0.006-0.43%, which is what makes the result trustworthy after this project's history of false-good measurements. V1 movem.l blit from a word-expanded RAM frame 446,286 cyc 53.6% V2 naive move.b/move.w per pixel 1,284,174 cyc 154.1% V3 write-only floor, no source read 225,789 cyc 27.1% V4 same writes in 4x4 block order 637,971 cyc 76.6% Scope: MAME's gvram_w/gvram_r carry no timing at all, so these are instruction cycles against zero-wait-state memory -- a floor, not a hardware prediction. V1's output snapshots pixel-exact through verify_frame256.py, closing FINDINGS 23.5. The V1/V3 gap shows reading the source frame is exactly half the cost, which makes the architecture question live: decode-direct-to-GVRAM needs no RAM reference frame and scales with the non-SKIP block fraction, crossing compose-then-blit at 70% of blocks changed. That fraction is now the top priority and is already a by-product of vq_hybrid.py's mode decision. Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
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prosolis
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; Full-frame GVRAM blit cost on a stock 68000 @ 10MHz.
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;
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; Answers: what fraction of a 12fps frame budget (833,333 cycles) does simply
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; PUTTING a decoded 256x192 frame on screen cost, before any decoding?
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;
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; Geometry (tools/bench/crtc_mode.lua): 256-colour page, one pixel per WORD of
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; CPU address space, 1024-byte line stride, picture in rows 32..223 of a
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; 256-row page. So a row is 512 contiguous bytes of writes, then a 512-byte
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; skip. 192 rows = 98,304 bytes of GVRAM write traffic per frame.
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;
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; Confirmed from MAME 0.277 x68k_crtc.cpp:501 (gvram_w, case 0x0100): a CPU
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; write in 256-colour mode is masked to 0x00ff, so the HIGH byte of every word
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; written is discarded by the hardware. V1 exploits this -- it never has to
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; clear the odd bytes of its source.
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;
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; Three variants, selected by VAR, each looped ITER times:
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; V1 movem.l blit from a word-expanded RAM frame (96KB). The realistic
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; "decode to RAM, then blit" design. Reads 96KB, writes 96KB.
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; V2 naive byte-source expansion (move.b / move.w per pixel). The obvious
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; implementation, kept as the baseline V1 has to beat.
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; V3 write-only floor: registers preloaded once, no source read at all.
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; Nothing that puts this many pixels on screen can beat V3. The gap
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; V1-V3 is the price of reading a source frame at all.
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; V4 the SAME 96KB of writes, but issued in 4x4 BLOCK order instead of
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; row-linear order. This is the access pattern a decoder that writes
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; codewords straight into GVRAM actually has, and it is the number that
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; picks the decoder architecture: compose-in-RAM-then-blit (V1) versus
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; decode-direct-to-GVRAM (V4 scaled by the fraction of non-SKIP blocks).
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; Each block is 4 rows of 8 bytes at a 1024-byte stride, so the
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; destination displacements 0/1024/2048/3072 all fit a 16-bit offset and
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; the block needs only one base pointer. V4 deliberately scrambles the
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; picture (it reads a row-linear source in block order); it is a timing
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; probe, which is why the correctness snapshot is taken after V1.
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;
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; 12 registers per movem burst (d0-d7/a2-a5 = 48 bytes) is the maximum
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; available: a0=src, a1=dst, a6=end sentinel. The row counter lives in the
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; a1-vs-a6 compare rather than a d-register for exactly this reason.
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; 512 = 10*48 + 32, hence ten 12-register bursts and one 8-register tail.
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; Destination uses (d16,a1) displacement rather than post-increment because
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; movem cannot post-increment a destination; the displacement costs 4 cycles
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; per burst but saves an 8-cycle lea, so it is the cheaper of the two.
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FLAG = $18000 ; 0 idle / 1 running / $FF done
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VAR = $18004 ; variant selector, written by Lua
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ITER = $18008 ; iteration count, written by Lua
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SRCW = $60000 ; word-expanded frame 192*512 = 96KB
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SRCB = $80000 ; byte-per-pixel frame 192*256 = 48KB
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DST0 = $C08000 ; GVRAM + 32*1024 (first picture row)
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DSTE = $C38000 ; GVRAM + 224*1024 (one past last)
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org $10000
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start:
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move.l VAR.l,d0
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move.l #1,FLAG.l ; timer starts here
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cmp.l #1,d0
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beq v1
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cmp.l #2,d0
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beq v2
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cmp.l #4,d0
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beq v4
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bra v3
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; ---------------------------------------------------------------- V1
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v1: lea SRCW,a0
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lea DST0,a1
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lea DSTE,a6
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v1row: movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,48(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,96(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,144(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,192(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,240(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,288(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,336(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,384(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,432(a1)
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movem.l (a0)+,d0-d7
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movem.l d0-d7,480(a1)
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lea 1024(a1),a1
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cmpa.l a6,a1
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bne v1row
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subq.l #1,ITER.l
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bne v1
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bra done
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; ---------------------------------------------------------------- V2
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v2: lea SRCB,a0
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lea DST0,a1
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lea DSTE,a6
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v2row: move.w #255,d1
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v2px: move.b (a0)+,d0
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move.w d0,(a1)+ ; high byte is discarded by gvram_w
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dbra d1,v2px
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lea 512(a1),a1 ; skip the unused half of the line
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cmpa.l a6,a1
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bne v2row
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subq.l #1,ITER.l
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bne v2
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bra done
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; ---------------------------------------------------------------- V3
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v3: lea SRCW,a0
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movem.l (a0),d0-d7/a2-a5 ; load the burst once, outside the loop
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lea DST0,a1
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lea DSTE,a6
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v3row: movem.l d0-d7/a2-a5,(a1)
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movem.l d0-d7/a2-a5,48(a1)
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movem.l d0-d7/a2-a5,96(a1)
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movem.l d0-d7/a2-a5,144(a1)
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movem.l d0-d7/a2-a5,192(a1)
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movem.l d0-d7/a2-a5,240(a1)
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movem.l d0-d7/a2-a5,288(a1)
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movem.l d0-d7/a2-a5,336(a1)
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movem.l d0-d7/a2-a5,384(a1)
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movem.l d0-d7/a2-a5,432(a1)
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movem.l d0-d7,480(a1)
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lea 1024(a1),a1
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cmpa.l a6,a1
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bne v3row
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subq.l #1,ITER.l
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bne v3
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bra done
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; ---------------------------------------------------------------- V4
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v4: lea SRCW,a0
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lea DST0,a3 ; base of the current block row
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lea DSTE,a4 ; one past the last block row
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v4brow: move.l a3,a1
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lea 512(a3),a5 ; 64 blocks * 8 bytes
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v4blk: movem.l (a0)+,d0-d7 ; 32 bytes = one 4x4 block, expanded
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movem.l d0-d1,(a1)
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movem.l d2-d3,1024(a1)
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movem.l d4-d5,2048(a1)
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movem.l d6-d7,3072(a1)
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addq.l #8,a1
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cmpa.l a5,a1
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bne.s v4blk
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lea 4096(a3),a3 ; next block row is 4 picture lines
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cmpa.l a4,a3
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bne v4brow
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subq.l #1,ITER.l
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bne v4
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bra done
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done: move.l #$FF,FLAG.l ; timer stops here
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halt: bra.s halt
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