Dragon-s-Lair-X68k

51 Commits 1 Branch 0 Tags

3 Commits

Include renames

Author	SHA1	Message	Date
prosolis	c419251266	Put the frame clock on the 68000, and find that the 12 fps frame does not exist ROADMAP P3 said "needs MFP timer or VBL" and neither can do it. The MFP's timer clock is 16 MHz/4, its prescalers stop at 200 and its data register is 8 bits, so the slowest tick any single timer can make is 78.125 Hz -- 6.5x faster than a frame -- and 4e6/12 is not an integer, so no setting reaches 12 Hz at all. The raster has no whole divide near 12 either: 4 refreshes is 13.86 fps and 5 is 11.09. tools/analysis/23_frame_clock.py walks all 7x256 timer settings rather than asserting it. src/player/clock.i takes the V-DISP falling edge on MFP GPIP4 -- the start of vertical blanking, which is when a player would present -- and adds fpsVTOTAL per edge to a 16-bit accumulator, emitting a tick at 31,500 and keeping the remainder. The long-run rate is fpsVTOTAL/VTOTAL = 12.000000 fps exactly, and both constants are read out of the CRTC at init, so the clock is derived from the registers that generate the raster it counts. Measured over 3,000 refreshes: 3,000 interrupts, 649 ticks where 649.1429 were due. It costs 181.35 clocks per V-DISP, 838 per frame, 0.1006% of the budget -- timed by the 68000 itself, because the host's granularity is 17.64 ms and the interrupt is microseconds. The loop's own cost was calibrated rather than looked up and landed on 38.000002 clocks, which both licenses the subtraction and confirms buscost.py's model; the 181.35 then decomposes exactly, leaving 43.99 clocks for the interrupt exception -- the textbook 44, measured. THE ONE THAT MOVES SOMETHING: 12 fps on a 55.4577 Hz raster is 4.6215 refreshes, so a frame is shown for 4 refreshes (72.13 ms) or 5 (90.16 ms), 37.9% of them short. The 833,333-clock budget every figure in this project is priced against is the MEAN slot, and the short one is 13.4% under it. The cadence was already in the tree unnamed: stream.lua's tick is sampled at frame boundaries, so its gaps were always 4 or 5, and every host-paced result in FINDINGS 49/51 carried it. P3 moved who produces it onto the machine and made it visible. It is not a dropped frame -- the pace gate lets an overrun eat the next frame's idle -- and on the gate container it costs 4 frames of 120 their idle against 1 for the nominal model, most of that the frame-0 transient at 111% of budget. stream.s counts it now, and the rig matches an offline model of the divider exactly. Also struck: MAME's raster runs 2.22% fast. refresh_mode() builds the frame period from scr.max_x*scr.max_y with scr.max_x = m_htotal - 8, one character cell short and an inclusive bound used as a count, so it runs at 56.6901 Hz where the registers say 55.4577 -- agreeing to six digits with the arithmetic. Every "1/55.46 s granularity" note in this tree was wrong and is 1/56.69 s, corrected in six files with the derivation put once in crtc_mode.lua. No conclusion changes and no 68000 cycle figure moves; the CPU clock is unrelated to the screen. But anything paced by the raster runs fast under MAME, so the rig reports both rates and prices the interrupt against the hardware's. decode.s and frame.i are unchanged; decode.bin is still 1,296 B at the same MD5. The pace gate's wait loop is byte-for-byte the one FINDINGS 51 measured and the free-running path executes none of the new code. check.sh gains two stages: the clock's own measurement, and 120 frames decoded pixel-exact with nothing outside the machine deciding when a frame may start. Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6	2026-08-24 20:55:34 -07:00
prosolis	c520a89e14	Measure the finer chain tail: 84/120 becomes 18/120, and the derivation was right by cancellation blit.s gains v7 -- v6's 24-pixel movem chain plus a second chain whose unit is one `move.l (a0)+,(a2)+`. Measured over 13 span lengths by span.sh, every config pixel-exact: cycles = 66.0 per span + 9.143 per COARSE pixel + 9.978 per FINE pixel fitting all 13 to within 0.2%. v5 and v6 re-measure to FINDINGS 30 exactly, so the harness has not drifted underneath the new variant. Rescored against the same scsi window and the same additive model, v7 takes 84/120 frames over budget to 18/120 -- exactly what FINDINGS 39.4 derived, and that agreement is two cancelling errors: the derivation's 2-register movem tail is 29% too dear per pixel, and its "nothing per span" for the second chain entry is 22.3 clocks too cheap. The plain post-incrementing move.l is the right tail instruction, and it makes the padding quantum 2 pixels, which a run of 4x4 blocks pads to exactly zero. The DMAC stays dropped on a measurement now rather than an argument: v7 takes back 37 of the 43 frames the array chain would, with no reserved channel and no timing neither emulator here can verify. Break-even against all-V1 moves from L=4 blocks to L=2. The fine displacement is carried mid-stream rather than in the span record, so the decoder holds nothing across the copy and keeps all 12 payload registers -- which is the whole reason the coarse unit is 24 pixels. span.sh is now -seconds_to_run 200 (30 s wall, 36 configs) and takes its expected snapshot count from the generated metadata instead of a literal 23. Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6	2026-08-23 19:02:23 -07:00
prosolis	29eb78a599	Measure the span: the mode survives, and it is an encoder format FINDINGS 29 priced a literal-span mode at 4(50 + 4L9.08) cycles and labelled the whole section DERIVED. Session 8 step 0 was to measure it before optimising over the mode set it implies. Two variants in blit.s, one stream per span length from prep_spans.py, timed by span.lua, driven by span.sh in ~25 s: v5, handed (x, npix) and left to work the copy out: 97.9/span + 10.459/px v6, handed an address and a jump displacement: 43.7/span + 9.152/px 29 assumed 50.0/span + 9.080/px So 29's arithmetic was right about a format nobody had written. The difference is not tuning: v5 spends ~122 cycles a span computing a destination, dividing npix into bursts and handling a 0..15 remainder, all of which the encoder knows at build time. v6's record is {u32 absolute GVRAM address, u16 jump displacement} into an unrolled chain of 24-pixel copy units -- no loop, no remainder, no arithmetic -- and it fits 11 span lengths to 0.3%. Three things that measurement showed and derivation could not: - The per-pixel cost is a function of REGISTER PRESSURE. FINDINGS 24's 9.08 was a fixed blit with 12 registers free; v5 can spare 8 and pays 10.46; v6 gets 12 back only because the encoder holds the state. - Short spans die in the remainder path -- a 12-pixel span costs MORE than a 16-pixel one -- and the fix is padding, not avoidance. - Odd-x alignment is free (259.0 vs 261.8 cycles/span), as a 16-bit bus implies but nobody had checked. Re-priced against the unchanged mode maps, sasi: median 74.4% -> 52.0% (29 said 43.0), misses 37 -> 10/120 (29 said 8), 448.0 KB/s. Break-even moved from runs of 2 blocks to runs of 4. 29.4 survives: a scene cut needs x >= 0.196 of the frame as spans and the bus allows x <= 0.373, so it fits at 12fps. All 23 timing configs are also checked pixel-exact, so none of this was timed against a decoder that quietly skipped work. FINDINGS 30. Next: lever B, the cost-aware mode decision. Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6	2026-08-23 15:45:51 -07:00

Author

SHA1

Message

Date

prosolis

c419251266

Put the frame clock on the 68000, and find that the 12 fps frame does not exist

ROADMAP P3 said "needs MFP timer or VBL" and neither can do it.  The MFP's
timer clock is 16 MHz/4, its prescalers stop at 200 and its data register is 8
bits, so the slowest tick any single timer can make is 78.125 Hz -- 6.5x faster
than a frame -- and 4e6/12 is not an integer, so no setting reaches 12 Hz at
all.  The raster has no whole divide near 12 either: 4 refreshes is 13.86 fps
and 5 is 11.09.  tools/analysis/23_frame_clock.py walks all 7x256 timer settings
rather than asserting it.

src/player/clock.i takes the V-DISP falling edge on MFP GPIP4 -- the start of
vertical blanking, which is when a player would present -- and adds fps*VTOTAL
per edge to a 16-bit accumulator, emitting a tick at 31,500 and keeping the
remainder.  The long-run rate is fps*VTOTAL/VTOTAL = 12.000000 fps exactly, and
both constants are read out of the CRTC at init, so the clock is derived from
the registers that generate the raster it counts.  Measured over 3,000
refreshes: 3,000 interrupts, 649 ticks where 649.1429 were due.

It costs 181.35 clocks per V-DISP, 838 per frame, 0.1006% of the budget -- timed
by the 68000 itself, because the host's granularity is 17.64 ms and the
interrupt is microseconds.  The loop's own cost was calibrated rather than
looked up and landed on 38.000002 clocks, which both licenses the subtraction
and confirms buscost.py's model; the 181.35 then decomposes exactly, leaving
43.99 clocks for the interrupt exception -- the textbook 44, measured.

THE ONE THAT MOVES SOMETHING: 12 fps on a 55.4577 Hz raster is 4.6215 refreshes,
so a frame is shown for 4 refreshes (72.13 ms) or 5 (90.16 ms), 37.9% of them
short.  The 833,333-clock budget every figure in this project is priced against
is the MEAN slot, and the short one is 13.4% under it.  The cadence was already
in the tree unnamed: stream.lua's tick is sampled at frame boundaries, so its
gaps were always 4 or 5, and every host-paced result in FINDINGS 49/51 carried
it.  P3 moved who produces it onto the machine and made it visible.  It is not a
dropped frame -- the pace gate lets an overrun eat the next frame's idle -- and
on the gate container it costs 4 frames of 120 their idle against 1 for the
nominal model, most of that the frame-0 transient at 111% of budget.  stream.s
counts it now, and the rig matches an offline model of the divider exactly.

Also struck: MAME's raster runs 2.22% fast.  refresh_mode() builds the frame
period from scr.max_x*scr.max_y with scr.max_x = m_htotal - 8, one character
cell short and an inclusive bound used as a count, so it runs at 56.6901 Hz
where the registers say 55.4577 -- agreeing to six digits with the arithmetic.
Every "1/55.46 s granularity" note in this tree was wrong and is 1/56.69 s,
corrected in six files with the derivation put once in crtc_mode.lua.  No
conclusion changes and no 68000 cycle figure moves; the CPU clock is unrelated
to the screen.  But anything paced by the raster runs fast under MAME, so the
rig reports both rates and prices the interrupt against the hardware's.

decode.s and frame.i are unchanged; decode.bin is still 1,296 B at the same MD5.
The pace gate's wait loop is byte-for-byte the one FINDINGS 51 measured and the
free-running path executes none of the new code.  check.sh gains two stages: the
clock's own measurement, and 120 frames decoded pixel-exact with nothing outside
the machine deciding when a frame may start.

Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6

2026-08-24 20:55:34 -07:00

prosolis

c520a89e14

Measure the finer chain tail: 84/120 becomes 18/120, and the derivation was right by cancellation

blit.s gains v7 -- v6's 24-pixel movem chain plus a second chain whose unit is
one `move.l (a0)+,(a2)+`. Measured over 13 span lengths by span.sh, every config
pixel-exact:

    cycles = 66.0 per span + 9.143 per COARSE pixel + 9.978 per FINE pixel

fitting all 13 to within 0.2%. v5 and v6 re-measure to FINDINGS 30 exactly, so
the harness has not drifted underneath the new variant.

Rescored against the same scsi window and the same additive model, v7 takes
84/120 frames over budget to 18/120 -- exactly what FINDINGS 39.4 derived, and
that agreement is two cancelling errors: the derivation's 2-register movem tail
is 29% too dear per pixel, and its "nothing per span" for the second chain entry
is 22.3 clocks too cheap. The plain post-incrementing move.l is the right tail
instruction, and it makes the padding quantum 2 pixels, which a run of 4x4
blocks pads to exactly zero.

The DMAC stays dropped on a measurement now rather than an argument: v7 takes
back 37 of the 43 frames the array chain would, with no reserved channel and no
timing neither emulator here can verify. Break-even against all-V1 moves from
L=4 blocks to L=2.

The fine displacement is carried mid-stream rather than in the span record, so
the decoder holds nothing across the copy and keeps all 12 payload registers --
which is the whole reason the coarse unit is 24 pixels.

span.sh is now -seconds_to_run 200 (30 s wall, 36 configs) and takes its
expected snapshot count from the generated metadata instead of a literal 23.

Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6

2026-08-23 19:02:23 -07:00

prosolis

29eb78a599

Measure the span: the mode survives, and it is an encoder format

FINDINGS 29 priced a literal-span mode at 4*(50 + 4L*9.08) cycles and labelled
the whole section DERIVED. Session 8 step 0 was to measure it before optimising
over the mode set it implies. Two variants in blit.s, one stream per span length
from prep_spans.py, timed by span.lua, driven by span.sh in ~25 s:

  v5, handed (x, npix) and left to work the copy out:  97.9/span + 10.459/px
  v6, handed an address and a jump displacement:       43.7/span +  9.152/px
  29 assumed                                           50.0/span +  9.080/px

So 29's arithmetic was right about a format nobody had written. The difference
is not tuning: v5 spends ~122 cycles a span computing a destination, dividing
npix into bursts and handling a 0..15 remainder, all of which the encoder knows
at build time. v6's record is {u32 absolute GVRAM address, u16 jump
displacement} into an unrolled chain of 24-pixel copy units -- no loop, no
remainder, no arithmetic -- and it fits 11 span lengths to 0.3%.

Three things that measurement showed and derivation could not:

  - The per-pixel cost is a function of REGISTER PRESSURE. FINDINGS 24's 9.08
    was a fixed blit with 12 registers free; v5 can spare 8 and pays 10.46; v6
    gets 12 back only because the encoder holds the state.
  - Short spans die in the remainder path -- a 12-pixel span costs MORE than a
    16-pixel one -- and the fix is padding, not avoidance.
  - Odd-x alignment is free (259.0 vs 261.8 cycles/span), as a 16-bit bus
    implies but nobody had checked.

Re-priced against the unchanged mode maps, sasi: median 74.4% -> 52.0% (29 said
43.0), misses 37 -> 10/120 (29 said 8), 448.0 KB/s. Break-even moved from runs
of 2 blocks to runs of 4. 29.4 survives: a scene cut needs x >= 0.196 of the
frame as spans and the bus allows x <= 0.373, so it fits at 12fps.

All 23 timing configs are also checked pixel-exact, so none of this was timed
against a decoder that quietly skipped work.

FINDINGS 30. Next: lever B, the cost-aware mode decision.

Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6

2026-08-23 15:45:51 -07:00