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Pace the ring, then read the DMAC config out of the IPL ROM: audio is cheap and the disk is not

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Two sessions that were never separated in the working tree, so they land as one
commit. check.sh ALL GREEN before and after both.

SESSION 19 -- the ring rig gets a frame clock (FINDINGS 51).

src/player/stream.s had no frame clock: it asked for record i the instant it
finished i-1, outran any finite pipe, and never let the ring back up. The 49.1
sweep passing at 48 KB was therefore a wrap-correctness result and nothing else.
PACE/PACEON ($18034/$18038) hold the decoder to 12 fps, so FR_HEAD-FR_TAIL
finally means what it reads as: whole frames the decoder could still draw with
delivery stopped dead. PACEON=0 free-runs and is what the wrap gate still uses,
so every figure in 49 is unmoved.

Paced, on the gate container: 64 KB holds 2 frames, 256 KB holds 7-8, 512 KB
holds 14-15, all pixel-exact. Tolerance is ceiling-1, measured by cutting the
pipe: 256 KB buys 500 ms of dead pipe, not 583.

SLACK IS ACCUMULATED, NOT OWNED. It is built out of pipe-wire and a seek spends
all of it. At 488 KB/s a 256 KB ring needs 4.83 s of play to reach its ceiling
from empty; 512 KB needs 8.42 s to reach 14. A bigger ring raises the ceiling
AND lengthens the climb, so a branch point does not ask "is the buffer big
enough" but "has there been enough play since the last one" -- and Dragon's
Lair's decision points are seconds apart. The rig now also says WHICH resource
is binding: at 460 KB/s every ring from 192 KB to 512 KB is rate-bound at
ceiling 4 and never fills, so larger rings are dead RAM in that scene.
20_seek_slack.py is the same model rewritten in Python from record sizes,
sharing no code with the Lua producer: 35/35 ceilings inside its bracket.

SESSION 20 -- the DMAC configuration was in the IPL ROM the whole time
(FINDINGS 52).

ROADMAP's "do this first" was to put the ADPCM stream on the bus. That needs a
clocks-per-byte figure for the audio channel, and 11_cpu_budget.py was charging
audio the DISK's rate -- 5 clk/B, its own help text calling it "single-address,
bus held". Audio was being charged the favourable end of B3, a 242 KB/s open
question.

It never had to be a guess. The IPL ROM programs all four HD63450 channels
itself and MAME boots the rig with it, so 21_iplrom_dmac.py reads the
configuration out of the image and decodes the MC68450 fields. Eight
(address, expected bytes, meaning) sites; a mismatch or an unknown revision
exits non-zero. In check.sh, no emulator, milliseconds.

ch3 DCR=$80, OCR=$32: dual address, 8-bit port, cycle steal WITHOUT hold,
REQG=10 external request. The DMAC arbitrates once per byte with no burst to
amortise the 5..8 + 2 over, so an audio byte is 16..19 clocks, not 5 -- the old
debit was 3.2x..3.8x small. And on the bus it is still nothing: 651 B/frame is
1.25%..1.48% of a frame, about 4% of what the decoder leaves. P6's bus risk
does not materialise. The unit worry was worth checking and nearly right: 15.6
kHz is 8 MHz/512 = 15,625 samples/s, two 4-bit samples to a byte = 7,812.5 B/s
exactly, and AUDIO_KBPS=7.8 is that in decimal kB while the tool multiplied by
1024.

THE DISK CHANNEL IS PROGRAMMED IDENTICALLY. ch1 (SASI) is DCR=$80 too, and so
is ch0. That is 16..19 clocks per delivered byte, where 42.4 brackets W at 5..12
and 42.5 has W=8 already missing 47/120 frames. The only worked example of a
disk DMA configuration on this machine sits above the entire bracket, and at
that price nothing fits at any container size. It is not scsiexrom.bin so B3
stays open -- what changed is that a cheap configuration is now the thing that
has to be SHOWN. W <= 12 is a requirement on the player's DMAC programming, not
a range the hardware hands us, and it is now the largest open number in the
project, ahead of the rate.

An unforced cross-check fell out: 15_bus_occupancy.py's new W sweep puts W=8 at
105.7% of the frame, agreeing with 42.5's 47/120, from mode histograms and bus
clocks respectively, two models sharing no code.

Also: ADPCM outranks the disk at the arbiter (CPR 1 against 2), so an audio byte
never waits and a video byte does -- relevant to 51's smooth-rate delivery model.

README MEDIA.

stream.lua gains DLX_SNAP_EVERY=1 (needs DLX_PACE, off by default, on no path
check.sh takes) and tools/media/make_readme_media.py turns the PNGs into
docs/img/. The stills and both clips are MAME's own screen pixels.

Building it turned up something worth recording. 116 of 119 captured frames are
pixel-exact against dlx.py; three are TORN -- frame n on top, frame n-1 below
the tear line -- because MAME captured the screen while the block loop was
partway down it. decode.s writes straight to the displayed page (one display
path, 28.1), so a real player tears the same way, and this is the first time
that consequence has been visible rather than argued. The script ASSERTS the
tear and refuses to build otherwise, rather than trimming three frames and
reporting "every frame I kept is exact". Second correction the capture forced:
the snapshot fires before frame n is decoded, so the obvious reading is that it
holds frame n-1 -- it does not, because MAME renders the screen at the end of
the machine frame, by which time the 68000 has finished frame n.

11_cpu_budget.py's "validated to within 1 pt" line is also corrected: the model
reads 2..10 pt HIGH and by more as the frame gets harder, which was already true
before either session.

src/player/decode.s is unchanged; decode.bin is still 1,296 B at the same MD5.

Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
This commit is contained in:
prosolis
2026-08-24 18:14:01 -07:00
parent b49bbdc939
commit 2f9f5cc995
49 changed files with 6276 additions and 375 deletions
Download Patch File Download Diff File Expand all files Collapse all files
tools/encoder/ratectl.py
+73 -10
View File
@@ -110,6 +110,58 @@ MU_FLOOR = 1e-4 # bisection is geometric, so lo must be > 0
# a frame that misses its deadline is simply late. FINDINGS 28.
FRAME_CYCLES = 10_000_000 / 12.0
# What one delivered byte costs the 68000, in clocks of that same budget. The
# SCSI DMA does not overlap with the CPU -- it steals the bus (FINDINGS 38.3) --
# and the MB89352 is an 8-bit port, so the DMAC pays PER BYTE and not per word
# (FINDINGS 43). 5.0 is the floor: single-address device-to-memory, bus held,
# zero drive wait (MC68450 Fig 4-25 sheet 2). Dual-address costs 9.
# 0.0 restores the pre-43 encoder, which priced a byte at nothing.
DISK_CLK_BYTE = 5.0
# Does the MODE DECISION see that debit, or only the fit test above it?
# FINDINGS 43.6 charged the disk inside the rate controller's ceiling but left
# vq_hybrid.decide() ranking modes with a free byte, so `mu` still bought
# cycles by moving V4 -> RAW: 47.8 cycles saved for 12 extra payload bytes,
# which at any c >= 4 clocks/byte is a net LOSS on the very budget mu is
# enforcing. True makes `decide` price a byte at `lam + mu*DISK_CLK_BYTE`.
#
# IT IS OFF, AND THAT IS A MEASUREMENT, NOT AN OVERSIGHT (FINDINGS 44). The
# inconsistency is real and the fix is a wash: on the 120-frame Singe window at
# c=5 it delivers 482.5 KB/s / 29.17 dB against 496.7 / 29.19, the same 1/120
# frames over budget, the same 112.9% worst frame, and 5,389 MORE clocks in the
# mean frame -- it buys 6,058 clocks of disk with 17,207 clocks of block
# decode. Scored at c=4 and c=9 as well, it is marginally the worse container
# at every price. `--joint-decide` turns it on.
JOINT_DECIDE = False
# The leaky bucket lets a quiet frame bank bytes for a busy one, because the
# player's ring buffer can hold them. True when a byte was only a byte. A byte
# is now also DISK_CLK_BYTE clocks of the frame's decode budget, and FINDINGS 28
# established there is no double buffer to decode ahead into -- so a frame that
# borrows bytes from the bucket borrows clocks it cannot repay (FINDINGS 43.5).
# True caps the borrow at what the clock budget can still absorb; the bucket
# then smooths only what is left after the disk is paid. It never caps BELOW
# the frame's own un-banked byte budget: that is rate control's job, not the
# clock budget's.
#
# IT IS OFF, AND THAT IS A MEASUREMENT (FINDINGS 44). At `--spans all`, the
# mode this project now recommends, the byte-side controller is INERT: a
# 32-frame bucket and an 8-frame bucket emit the same container byte for byte,
# and lam never leaves its floor on any of 120 frames. The cap is worth one
# frame of 120 at `--spans need` (2/120 -> 1/120, for +26 KB/s) and is a 2%
# regression at `all` (506.4 KB/s and 89.6% median frame against 496.7 and
# 87.9%), because capping the block payload only moves those bytes into the
# span section, which draws on its own flat pipe. Extending the cap to the
# span section too was measured and is much worse: 273.7 KB/s, 28.88 dB, and
# still 1/120 -- it starves the pass that was buying the deadline.
# `--joint-bucket` turns it on.
JOINT_BUCKET = False
def _byte_clk():
"""The debit the mode decision is allowed to see (0 = the old decision)."""
return DISK_CLK_BYTE if JOINT_DECIDE else 0.0
AUDIO_KBPS = 7.8 # MSM6258 ADPCM 15.6kHz mono -- comes out of the same budget
@@ -129,17 +181,18 @@ def _search_lam(ctx, allow, lam_lo, lam_hi, iters=12, mu=0.0):
not fit: that frame is emitted over budget on purpose. Past the FINDINGS 15
cliff a frame is not rate-controlled, it is destroyed, so a visible overrun
is the better failure (FINDINGS 26.2)."""
mode, sz = H.decide(ctx, lam_lo, mu)
bc = _byte_clk()
mode, sz = H.decide(ctx, lam_lo, mu, bc)
if sz <= allow:
return lam_lo, mode, sz, False
mode_hi, sz_hi = H.decide(ctx, lam_hi, mu)
mode_hi, sz_hi = H.decide(ctx, lam_hi, mu, bc)
if sz_hi > allow:
return lam_hi, mode_hi, sz_hi, True
lo, hi = lam_lo, lam_hi # lo does not fit, hi does
best = (lam_hi, mode_hi, sz_hi)
for _ in range(iters):
mid = float(np.sqrt(lo * hi))
mode_m, sz_m = H.decide(ctx, mid, mu)
mode_m, sz_m = H.decide(ctx, mid, mu, bc)
if sz_m <= allow:
hi = mid; best = (mid, mode_m, sz_m)
else:
@@ -172,12 +225,12 @@ def _search_mu(ctx, allow, lam_lo, lam_hi, cyc_budget, iters=10):
Returns (mu, lam, mode, size, cyc, over_bytes, over_cycles)."""
lam, mode, sz, ovr = _search_lam(ctx, allow, lam_lo, lam_hi, mu=0.0)
cyc = H.cycles(mode)
if cyc <= cyc_budget:
if cyc + DISK_CLK_BYTE * sz <= cyc_budget:
return 0.0, lam, mode, sz, cyc, ovr, False
lam_h, mode_h, sz_h, ovr_h = _search_lam(ctx, allow, lam_lo, lam_hi, mu=MU_CLIFF)
cyc_h = H.cycles(mode_h)
if cyc_h > cyc_budget: # cannot fit even frozen: emit late
if cyc_h + DISK_CLK_BYTE * sz_h > cyc_budget: # cannot fit even frozen
return MU_CLIFF, lam_h, mode_h, sz_h, cyc_h, ovr_h, True
lo, hi = MU_FLOOR, MU_CLIFF # lo overruns, hi fits
@@ -186,7 +239,7 @@ def _search_mu(ctx, allow, lam_lo, lam_hi, cyc_budget, iters=10):
mid = float(np.sqrt(lo * hi))
lam_m, mode_m, sz_m, ovr_m = _search_lam(ctx, allow, lam_lo, lam_hi, mu=mid)
cyc_m = H.cycles(mode_m)
if cyc_m <= cyc_budget:
if cyc_m + DISK_CLK_BYTE * sz_m <= cyc_budget:
hi = mid; best = (mid, lam_m, mode_m, sz_m, cyc_m, ovr_m)
else:
lo = mid
@@ -226,7 +279,7 @@ def _fit_spans(m, ctx, mode, sz, room, cyc_budget, span_mode, ib):
return mode, sz, H.cycles(mode), None
sel = SP.select(mode, src, m["nbx"], m["nby"], room,
need_clocks=(None if span_mode == "all" else cyc_budget),
idx_bytes=ib)
idx_bytes=ib, disk_clk_byte=DISK_CLK_BYTE, base_bytes=sz)
if not sel["spans"]:
return mode, sz, H.cycles(mode), None
nmode = sel["mode"]
@@ -292,16 +345,26 @@ def encode_rate_controlled(m, target_kbps, fps=12, bucket_frames=8,
# and a pipe cannot be saved up. None means "the same allowance the lam
# search had", which is what leaves spans nothing to buy with at a rate
# point the block coder has already spent (FINDINGS 41.2).
hard = None
if (JOINT_BUCKET and cycle_budget is not None and DISK_CLK_BYTE > 0):
# What can this frame's clock budget still absorb, once its own
# block decode is paid? Priced at the mode map the UN-banked budget
# buys, so the cap is a property of the frame's content rather than
# of how full the bucket happens to be.
_, mode0, _, _ = _search_lam(ctx, budget, lam_lo, lam_hi)
hard = (cycle_budget - H.cycles(mode0)) / DISK_CLK_BYTE
allow = min(allow, max(budget, hard))
span_allow = allow if span_budget is None else span_budget
sel = None
lam, mode, sz, ovr = _search_lam(ctx, allow, lam_lo, lam_hi)
mu, cyc, late = 0.0, H.cycles(mode), False
if span_mode and (span_mode == "all"
or (cycle_budget is not None and cyc > cycle_budget)):
or (cycle_budget is not None
and cyc + DISK_CLK_BYTE * sz > cycle_budget)):
mode_pre = mode
mode, sz, cyc, sel = _fit_spans(m, ctx, mode, sz, span_allow,
cycle_budget, span_mode, ib)
if cycle_budget is not None and cyc > cycle_budget:
if cycle_budget is not None and cyc + DISK_CLK_BYTE * sz > cycle_budget:
# The byte allowance could not buy the frame's deadline, so fall
# back to the controller that pays in picture -- and then offer
# spans the bytes the smaller mode map just freed.
@@ -311,7 +374,7 @@ def encode_rate_controlled(m, target_kbps, fps=12, bucket_frames=8,
mode_pre = mode
mode, sz, cyc, sel = _fit_spans(m, ctx, mode, sz, span_allow,
cycle_budget, span_mode, ib)
late = cyc > cycle_budget
late = cyc + DISK_CLK_BYTE * sz > cycle_budget
# Paint from the mode map as it was BEFORE spanning. A spanned run's
# blocks read SKIP in the emitted header, but SKIP means "hold the
# previous reconstruction" and on the first frame there is none -- and