Dragon-s-Lair-X68k/tools/analysis/26_sector_align.py

"""A record is not a sector: what the mismatch costs, three ways (P4b, 58.3).

src/player/ring.i asks the transport for a RECORD -- a byte offset into the
scene's frame stream and a length, both 4-byte aligned because that is what
`move.l (a0)+` needs (28.3) and neither of them a multiple of 512.  A SCSI
target answers in 512 B BLOCKS.  On the gate container 117 of 120 records start
part way into a sector, so something has to reconcile the two, and the three
ways of doing it are not close.

WHY IT IS NOT AN IMPLEMENTATION DETAIL.  The bytes on either side of a record in
the stream belong to OTHER records -- ones the decoder may still be reading --
and the block loop walks a0 with no bounds check at all (49.2).  So a transport
that reads whole sectors straight into the ring does not waste 500 bytes, it
CORRUPTS the neighbours, and the symptom is wrong pixels rather than a fault.

  A. WINDOWED PIO.  Read the sectors the record lies in, store only the record.
     src/player/scsi.i does this and it is what FINDINGS 58 measured.  It costs
     nothing in clocks -- the CPU is touching every byte anyway -- and it costs
     the extra sectors on the wire.  It CANNOT be done by a DMAC: a channel
     writes a contiguous run to a contiguous address and cannot be told to drop
     the first 300 bytes.
  B. BOUNCE BUFFER.  Let the DMAC write whole sectors somewhere else, then copy
     the record into the ring.  Works under DMA, and costs a copy of every
     delivered byte -- which is precisely the cost `aligned` was chosen over
     `split` to avoid (49.3, 19_ring_stream.py).
  C. SECTOR-ALIGNED RECORDS.  Pad each record up to 512 in the container
     instead of up to 4.  Costs bytes on the disc and in every delivery, and
     nothing else at all; the transport becomes a whole-sector read into the
     ring with no window and no copy.  It is a CONTAINER change -- a re-encode
     and a re-measurement of every constant fitted to the gate container, which
     is the class of change ROADMAP already has bundled with P2's other half.

    python3 tools/analysis/26_sector_align.py <in.dlx> [--ring KB]

No rate is taken and none is needed: every figure here is a fraction of the
delivered bytes or a count of clocks, and both are rate-free.  What a given
delivery rate does with them is 15_bus_occupancy.py's question.
"""
import sys, os, argparse
sys.path.insert(0, "tools/encoder")
from dlx import DLX

SECTOR = 512
CPUHZ = 10_000_000
# 5.0 clocks/byte, and it is 19_ring_stream.py's constant rather than a new one:
# a 68000 `move.l (a0)+,(a1)+` moves 4 bytes in 20 clocks on a 16-bit bus.  It
# is the OPTIMISTIC figure there and it is the optimistic figure here.
COPY_CLK_PER_BYTE = 5.0
# The windowed PIO loop in src/player/scsi.i, from the 68000's cycle table:
#   12  move.l #SC_PATIENCE,d3      patience reload
#   16  move.b SC_SSTS,d0           (xxx).L -> Dn
#   10  btst #0,d0
#   10  beq.s taken
#   20  move.b SC_DREG,(a1)+        (xxx).L -> (An)+
#    8  subq.l #1,d7
#   10  bne.s taken
# FINDINGS 58.2 measured 87.28 clocks per delivered byte against this loop's 86
# plus 1.15 for the dropped window bytes -- 0.2% apart, which is what says the
# cost is the instruction stream and not MAME's device model.
PIO_CLK_PER_BYTE = 86.0

ap = argparse.ArgumentParser()
ap.add_argument("container")
ap.add_argument("--ring", type=int, default=256, help="ring size in KB")
a = ap.parse_args()

d = DLX(a.container)

# The disc layout the 68000 walks: [u32 len][body], each record padded up to 4.
# Exactly tools/bench/prep_stream.py's, and it is rebuilt here rather than read
# from tmp/ so this tool works on any container.
off, recs = 0, []
for (o, n) in d.frames:
    ln = 4 + n
    ln += (-ln) % 4
    recs.append((off, ln))
    off += ln
payload = sum(ln for _, ln in recs)
nfr = len(recs)
budget = CPUHZ / d.fps

print(f"{a.container}: {nfr} records, {payload:,} B, {d.fps} fps")
print(f"  mean record {payload/nfr:,.0f} B; a {d.fps} fps frame is "
      f"{budget:,.0f} clocks")
aligned0 = sum(1 for o, _ in recs if o % SECTOR == 0)
print(f"  records that already start on a sector boundary: {aligned0}/{nfr}")
print()

# ---- A. windowed PIO: the sectors the record lies in, and only the record kept
wire_a = sum(((o % SECTOR) + ln + SECTOR - 1) // SECTOR for o, ln in recs) * SECTOR
drop_a = wire_a - payload
print("A. WINDOWED PIO (src/player/scsi.i, what FINDINGS 58 ran)")
print(f"     wire {wire_a:,} B for {payload:,} B of record "
      f"= +{100*drop_a/payload:.2f}%")
print(f"     clocks {PIO_CLK_PER_BYTE:.0f}/B on EVERY byte off the FIFO, "
      f"dropped ones included:")
print(f"       {PIO_CLK_PER_BYTE*wire_a/nfr:,.0f} clk/frame "
      f"= {100*PIO_CLK_PER_BYTE*wire_a/nfr/budget:.0f}% of the frame")
print( "     and it does not survive the move to the DMAC at all: a channel "
       "cannot drop bytes.")
print()

# ---- B. bounce buffer: DMA whole sectors elsewhere, copy the record in
print("B. BOUNCE BUFFER (whole sectors by DMA, then a copy)")
print(f"     wire {wire_a:,} B, the same +{100*drop_a/payload:.2f}% -- the "
      f"command is identical")
print(f"     clocks {COPY_CLK_PER_BYTE:g}/B of copy on every DELIVERED byte, "
      f"on top of whatever W the")
print(f"       channel steals: {COPY_CLK_PER_BYTE*payload/nfr:,.0f} clk/frame "
      f"= {100*COPY_CLK_PER_BYTE*payload/nfr/budget:.1f}% of the frame")
print( "     which is the cost `aligned` was chosen over `split` to avoid "
       "(49.3), arriving")
print( "     by a different door and on every byte instead of on a wrap.")
print()

# ---- C. sector-aligned records in the container
cur, pad = 0, 0
for _, ln in recs:
    if cur % SECTOR:
        pad += SECTOR - (cur % SECTOR)
        cur += SECTOR - (cur % SECTOR)
    cur += ln
print("C. SECTOR-ALIGNED RECORDS (a container change; a re-encode)")
print(f"     wire {cur:,} B for {payload:,} B of record = +{100*pad/payload:.2f}%")
print( "     clocks ZERO: the read is a whole-sector read straight into the "
       "ring, no window,")
print( "     no copy, and the DMAC can do it.")
print()

ringsz = a.ring * 1024
print(f"  VERDICT, in the currency this project prices delivery in.  C is "
      f"cheaper on the wire")
print(f"  than A and B by {100*(drop_a-pad)/payload:.2f} points of the payload "
      f"({drop_a-pad:,} B on this scene),")
print(f"  and it is the only one of the three a DMA channel can run without a "
      f"copy. What it")
print(f"  costs is a container revision and the re-measurement that comes with "
      f"one.")
maxrec = max(ln for _, ln in recs)
maxpad = maxrec + (-maxrec) % SECTOR
print(f"  It also grows the largest record from {maxrec:,} to {maxpad:,} B, "
      f"which a {a.ring} KB")
print(f"  ring still holds {ringsz//maxpad} times over.")