Put the transport on the 68000, and find PIO costs 87 clocks a byte

ROADMAP P4b. src/player/xfer.i answers src/player/ring.i's XF_* mailbox with a
real READ(10) to a real MB89352 in place of tools/bench/stream.lua's modelled
transport: 120 records, 4,488,588 B, pixel-exact out of a 256 KB ring, with a
real mid-stream seek in a second pass. The tiling is the SAME 18 wraps and
14.7 KB mean hole that 49.4's host producer and 55.4's modelled transport
produced -- a third transport, same placement, which is the assertion that
ring.i could not tell which side of the seam answered it.

What it costs is the finding. tools/bench/xfer_cost.sh subtracts the same 120
frames run twice and gets 87.28 clocks per delivered byte, against the 68000's
own cycle table for the loop, which says 87.15 -- 0.2% apart, so the cost is the
instruction stream and not MAME's device model, and it is the first number this
rig has produced that survives leaving the emulator. That is 391.8% of a 12 fps
frame; the machine's own V-DISP clock agrees from the other end at 2.57 fps.
Against the ladder, W=5 held is 22.4% of a frame and W=19 is 85.3%, so P4a is
worth 4.6x the worst DMA configuration in this tree and 17.5x the best -- where
before this session it was worth 9 against 19. W itself did not move by a clock.

"UNDERRUNS: 0/120" is vacuous with a synchronous transport, and stream.lua now
prints that argument next to the zero: a frame cannot start before its record
has landed because the decoder IS the transport. The counter that means
something is NO IDLE, 119/120 with a worst overrun of 441 whole ticks. Same
class of error as 49.7.2's free-running ring passing at 48 KB.

58.3: a record is not a sector -- 117 of 120 start part way into one, and
reading whole sectors into the ring corrupts the neighbours rather than wasting
bytes (49.2, no bounds check). scsi.i reads the covering sectors and stores only
the window, which is free in PIO and stops being free the moment P4a succeeds.
tools/analysis/26_sector_align.py prices the three ways out and sector-aligned
records win on both axes: +0.43% wire and zero clocks, against +1.34% and a
bounce copy at +5 clk/B. ROADMAP now carries a four-item re-encode bundle and
P4a should be attempted against a sector-aligned container.

check.sh gains two stages and was ALL GREEN before and after. decode.bin is
unchanged at 1,296 B and the same MD5.

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

tools/bench/pace_run.sh

@@ -28,6 +28,13 @@
 # needs DLX_PACE=2, because rebasing the frame clock across a pass is the
 # machine's to do and a host-written tick would carry on counting.
 #
+# DLX_XFER=scsi replaces the MODELLED transport with a real one (ROADMAP P4b,
+# src/player/xfer.i): the machine gets a CZ-6BS1 and the same volume the SCSI
+# gate reads, this script stops moving bytes altogether, and every record is
+# fetched by the 68000 with READ(10). It needs DLX_RINGOWN=1 -- the mailbox it
+# answers is ring.i's -- and it FORBIDS a modelled rate, because there is no
+# longer anything for one to model.
+#
 # DLX_PACE selects WHO KEEPS THE TIME: 1 (default) is the host writing the tick,
 # 2 is the 68000 writing it off the CRTC's V-DISP (ROADMAP P3, FINDINGS 54).
 # Everything else about the run is identical, which is the whole point -- the
@@ -41,6 +48,30 @@ DLX=${DLX:-tmp/rc_fr_singe_scsi_span.dlx}
 PACE=${DLX_PACE:-1}
 OWN=${DLX_RINGOWN:-0}
 ITERS=${DLX_ITER:-1}
+XFER=${DLX_XFER:-model}
+# EMULATED seconds the run is allowed. A pass that is cut short compares a
+# half-drawn screen and reads as a wrap bug, so this is raised deliberately
+# rather than left to a timeout: a DLX_XFER=scsi pass costs ~46 s of emulated
+# time against the modelled transport's ~7, because the CPU moves every byte
+# itself (FINDINGS 58.2), and two of them do not fit in 90.
+SECS=${DLX_SECONDS:-90}
+if [ "$XFER" = scsi ]; then
+  [ "$OWN" = 1 ] || { echo "DLX_XFER=scsi needs DLX_RINGOWN=1: the transport in"
+    echo "src/player/xfer.i answers src/player/ring.i's mailbox, and with the"
+    echo "host owning the ring there is no mailbox to answer."; exit 2; }
+  # A rate is not merely ignored here, it is REFUSED. The bytes now arrive on
+  # the emulated machine's own time, and a run labelled "488 KB/s" that did not
+  # deliver at 488 KB/s is exactly the kind of number this project has twice
+  # paid for. There is no rate in a DLX_XFER=scsi run, and the log says so.
+  [ "$KBPS" = 0 ] || { echo "DLX_XFER=scsi takes kbps 0. The transport is real,"
+    echo "so nothing here delivers at a modelled rate -- and MAME's device"
+    echo "models are functional, not transfer-timing accurate, so the rate it"
+    echo "DOES deliver at is not a measurement either (docs/BENCHMARK.md)."
+    exit 2; }
+  command -v chdman > /dev/null || { echo "DLX_XFER=scsi needs chdman (ships"
+    echo "with mame-tools) to build the volume."; exit 2; }
+  bash tools/bench/mkvol.sh "$DLX"
+fi
 if [ "$OWN" = 1 ] && [ "$ITERS" != 1 ] && [ "$PACE" != 2 ]; then
   echo "DLX_ITER>1 needs DLX_PACE=2: the frame clock is rebased per pass by"
   echo "src/player/stream.s, and a host-written tick would go on counting"
@@ -54,6 +85,11 @@ TAG="r${RING}_k${KBPS}${CUT_AT:+_cut${CUT_AT}x${CUT_FR}}"
 if [ "$PACE" != 1 ]; then TAG="${TAG}_p$PACE"; fi
 if [ "$OWN" = 1 ]; then TAG="${TAG}_own"; fi
 if [ "$ITERS" != 1 ]; then TAG="${TAG}_x$ITERS"; fi
+if [ "$XFER" != model ]; then TAG="${TAG}_$XFER"; fi
+MAMEX=()
+if [ "$XFER" = scsi ]; then
+  MAMEX=(-exp1 cz6bs1 -rompath "$HOME/mame/roms;./p4roms" -hard dlxdisk.chd)
+fi
 
 tools/vasm/vasmm68k_mot -Fbin -o tmp/stream.bin src/player/stream.s > /dev/null
 [ -f tmp/stream_disk.bin ] || python3 tools/bench/prep_stream.py "$DLX" > tmp/prep_stream.log
@@ -63,13 +99,15 @@ mkdir -p "tmp/snap_pace_$TAG"; rm -f "tmp/snap_pace_$TAG/x68000"/*.png
 # command and the run dies with "SDL_VIDEODRIVER=dummy: command not found".
 CUTENV=(); [ -n "$CUT_AT" ] && CUTENV=(DLX_CUT_AT="$CUT_AT" DLX_CUT_FR="$CUT_FR")
 ( cd tmp && env DLX_PACE=$PACE DLX_RING_KB=$RING DLX_STREAM_KBPS=$KBPS \
-    DLX_RINGOWN=$OWN DLX_ITER=$ITERS \
+    DLX_RINGOWN=$OWN DLX_ITER=$ITERS DLX_XFER=$XFER \
     ${DLX_PREFILL_FR:+DLX_PREFILL_FR=$DLX_PREFILL_FR} \
     "${CUTENV[@]}" DLX_SLACK_CSV="slack_$TAG.csv" \
     SDL_VIDEODRIVER=dummy stdbuf -oL timeout -k 5 900 \
-    mame x68000 -bios ipl10 -ramsize 2M -video soft -window -sound none \
+    mame x68000 -bios ipl10 "${MAMEX[@]}" -ramsize 2M -video soft -window \
+    -sound none \
     -nothrottle -plugins -autoboot_script ../tools/bench/stream.lua \
-    -snapshot_directory "./snap_pace_$TAG" -snapview native -seconds_to_run 90 \
+    -snapshot_directory "./snap_pace_$TAG" -snapview native \
+    -seconds_to_run $SECS \
     > "pace_$TAG.log" 2>&1 )
 # The completion marker is not optional: a run killed mid-decode compares a
 # half-drawn screen and reads as a wrap bug rather than as a truncated run.
@@ -77,6 +115,6 @@ grep -q "snapshot taken" "tmp/pace_$TAG.log" || {
   echo "FAIL($TAG): no snapshot marker -- the pass did not complete."
   tail -6 "tmp/pace_$TAG.log"; exit 1; }
 echo "=== $TAG"
-grep -aE "decoder (SELF-PACED|PACED|FREE)|FRAME CLOCK|ring: |UNDERRUNS|NO IDLE|SEEK SLACK|RING-BOUND|RATE-BOUND|BUILD TIME|PIPE CUT|DEADLINE|REQUIRED|MACHINE-OWNED|PREFILL:|CHANNEL IDLE|MISPLACED|SEEK PASS" \
+grep -aE "decoder (SELF-PACED|PACED|FREE)|FRAME CLOCK|ring: |UNDERRUNS|NO IDLE|SEEK SLACK|RING-BOUND|RATE-BOUND|BUILD TIME|PIPE CUT|DEADLINE|REQUIRED|MACHINE-OWNED|PREFILL:|CHANNEL IDLE|MISPLACED|SEEK PASS|REAL TRANSPORT|SECTOR OVERHEAD|TRANSPORT FAILED|IS VACUOUS" \
   "tmp/pace_$TAG.log" | sed "s/\[STR\] /  /"
 python3 tools/bench/verify_decode.py "$DLX" --snap "tmp/snap_pace_$TAG" | tail -2