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SCSDISK-TRANSFER - does a fn-0 READ of block 0 reach SCLLD, or is it rejected first?

Byte-verified reverse-engineering of the SINTRAN III VSX/500 L07 SCSI disk transfer primitive SCSDISK (= SCSDI 057215B, the routine the device-agnostic filesystem reaches through the SCSI unit datafield field ,X 14) and the SCSI driver-command dispatch SCSID (062217B), traced along the function-0 (READ) path for block 0 all the way to the driver enqueue SCLLD (067160B) - or to the pre-transfer reject.

This answers the ONE question the FUNCTION-42-RETURN carve left open from the disk-layer side: after function 42 succeeds and leaves 5SCIN SET, when the filesystem issues its separate "read block 0" (fn 0) request through ,X 14 -> SCSDISK, does that request reach SCLLD -> INITO -> SCWAQ, or is it dropped / rejected in the SCSI disk layer before the transfer (case 2)?

  • Segment: 065-S3SIPIT (load base 32000B = 13312 dec; byte-identical twin 066-S3IIPIT).
  • Source bin: ../../segments/065-S3SIPIT.bin (big-endian words).
  • Disasm: nd100-dis -a -o -b 13312 over the byte-swapped image.
  • dd offset: byte_off = (addr - 32000B) * 2 (decimal).
  • Evidence: VERIFIED = read from L07 bytes. INFERRED = NPL logic (IP-P2-SCSI-DISK.NPL, a DIFFERENT revision - naming only). OPEN = uncarved / runtime. All addresses/values octal.

0. The +376B revision offset (re-confirmed)

The entire IP-P2-SCSI-DISK compilation unit (SCSDISK, SCSID, CACOB, INQUI, the fn-42 handling) sits in the L07 image at NPL-source-label + 376B:

Symbol NPL label L07 (SYMBOL-2-LIST) delta
SWT11 056120 056516 +376B
CTRSC 056223 056621 +376B
SCSDI (SCSDISK) 056617 057215 +376B
SCSID 061621 062217 +376B

The SCSI driver (IP-P2-SCSI-DRIV: SCLLD=067160, INITO=070261, SELEC=070165, SCINT=067247) is a separate unit aligned directly to its NPL labels (no +376B).

1. The fn-0 read-block-0 dispatch path (VERIFIED, byte-by-byte)

",X 14"  (RCBLO device transfer dispatch, JPL I ,B 10)
  -> SCSDISK  = SCSDI 057215B
       057215 RADD CLD SL DA / 006006 STA ,X 6   A:=L=:X.NFUNC   (entry)
       057217 JPL I 51 -> DIALO                  MON PERFO
       057221 LDA ,X 14 / SAX 77 / RAND SA DX     fn := ABFUN & 077
       -- geometry bounds check (whole-disk arm) --
       057352 LDD ,X 17     AD := X.ABPA2  (disk block address, =0 for block 0)
       057353 LDT ,X 22     T  := X.ABP32  (amount)
       057354 RADD ST DD / 057355 RADD ADC CLD SA DA   AD := addr + amount
       057356 LDT 32 / 057357 RADD SB DT             T := UHLIM (+B)
       057360 JPL I 27 -> COMPD                      compare (addr+amount, UHLIM)
       057361 SKP IF 0 GRE SA                        skip if within bounds
       057362 JMP 15 -> 057377 = ERR2               *** past end -> BADPA ***
       -- block 0: addr 0 -> within bounds -> NO branch to ERR2 --
       057367 MIN ,B 10 (SUNOP) ; if SLINK != 0 -> DSORT/SWT11 else NEWOP
  -> NEWOP / RETRY
       5SCIN lazy-init loop (057515..057532): SKIPPED because 5SCIN is SET
       -- real transfer driver call --
       057606 LDA 27 / 072365 AND ,X -13    A := 377 & X.SCOCW
       057610 JPL I 21 -> 057631            CALL SCSID   (ptr 057631 = 062217)
  -> SCSID 062217B
       062217 STA ,X 10                     entry (A=:X.HSTAT)
       062226 SAA 77 / AND ,X 14            fn := ABFUN & 077
       062231 SAT 37 / SKP IF DA EQL ST     fn 37 -> DOEXS  (not us)
       062243 LDA ,B 31 (SUTHS)             busy? -> queue+SCWTI (not us)
  -> REPEAT 062300B  (dispatch by function)
       062302 fn 31 -> BDRST
       062305 fn 74 -> GUSCB
       062312/062315 fn 42/36 -> INQUI
       062320/062323 fn 23/25 -> MODES
       -- fn 0 matches none -> FALL THROUGH --
       062326 JPL I 66 -> 062414            CALL CACOB   (build SCSI command)
       062327 JPL I 66 -> 062415            CALL EXCOM   (execute -> SCLLD)
  -> CACOB 063750B
       063752 LDA ,B 23        A := SUTYP
       063753 BSKP ONE 70 DA   skip JMP if 5SCIN (bit 7) = ONE
       063754 JMP I 163 -> INQUI      5SCIN CLEAR -> divert to INQUI
       -- 5SCIN SET (our case): skip the JMP, build READ(6) CDB (SCSF1[0]=010\340) --
  -> EXCOM 063403B
       063430 JMP I 23 -> 063453         GO SCLLD   (ptr 063453 = 067160)
  -> SCLLD 067160B   (IP-P2-SCSI-DRIV: enqueue -> INITO -> SCWAQ)

2. The exact decision instructions

What Address Word Meaning
Pre-transfer reject branch 057361/057362 141050 / 124015 SKP IF 0 GRE SA then JMP -> ERR2. If addr+amount > UHLIM -> ERR2. Block 0 (addr 0) does not take this branch.
The reject itself (BADPA) 057401 171005 SAT 5 -> T := 5 = BADPA; then RETOP/SWT11, no SCLLD.
fn-0 falls to CACOB 062326 135066 JPL I 66 -> CACOB (fn 0 matches none of 31/74/42/36/23/25).
5SCIN re-entry gate 063752/063753/063754 044423 / 175275 / 125163 LDA SUTYP; BSKP ONE 70 DA; JMP I -> INQUI. 5SCIN SET -> skip -> build CDB.
Driver enqueue call 062327 -> EXCOM 063430 -> 067160 135066 / 125023 / ptr 067160 CALL EXCOM; EXCOM GO SCLLD; pointer 063453 = 067160 = SCLLD.

Where block 0 reaches SCLLD or is rejected: the reject can only happen at the UHLIM branch 057361/057362 -> ERR2 057401 (BADPA). For block 0 the address is 0, so addr + amount <= UHLIM and the branch is not taken; control proceeds to CALL SCSID (057610) -> CACOB (062326) -> [5SCIN SET, 063752] -> EXCOM (062327) -> SCLLD (067160). Block 0 reaches SCLLD.

3. dd spot-checks (raw big-endian 065-S3SIPIT.bin)

Item L07 addr byte off word mnemonic
SCSDISK entry 057215 21786 146145 RADD CLD SL DA (A:=L=:X.NFUNC)
BADPA reject 057401 22018 171005 SAT 5 (T:=BADPA)
ptr -> SCSID 057631 22322 062217 pointer word = SCSID address
SCSID entry 062217 24862 006010 STA ,X 10 (A=:X.HSTAT)
5SCIN gate 063752 26580 044423 LDA ,B 23 (A:=SUTYP)
ptr -> SCLLD 063453 26198 067160 pointer word = SCLLD address

Reproduce, e.g.: dd if=065-S3SIPIT.bin bs=1 skip=24862 count=2 | od -An -tx1 -> 0c 08 = 006010.

4. THE ANSWER

A function-0 (READ) request for block 0 REACHES SCLLD in the static L07 code. It is not dropped or rejected inside the SCSI disk layer.

  • The only pre-transfer reject in the disk layer is the UHLIM/geometry bounds check (057356..057362) that yields BADPA (T=5) at 057401. That branch fires only when disk_address + amount > UHLIM. Block 0 has address = 0, so it is trivially within bounds and the branch is not taken - byte-verified at 057361/057362.
  • The 5SCIN re-entry gate at 063752 (in CACOB), with 5SCIN SET (the state function 42 leaves), takes the fall-through and builds the READ(6) CDB; it does not divert to INQUI. It then reaches EXCOM -> GO SCLLD (067160) - byte-verified pointer at 063453 = 067160.

Therefore case 2 (block-0 request issued but rejected before SCLLD) is REFUTED by the bytes for block 0 specifically: nothing in SCSDISK / SCSID / CACOB / EXCOM rejects a well-formed fn-0 block-0 read; it enqueues it.

The remaining explanation is case 1: the device-agnostic caller never issues the block-0 fn-0 request (or issues it with an out-of-range address that would trip BADPA, which for a correct block-0 LBA cannot happen). This is consistent with: - FUNCTION-42-RETURN (function 42 returns T=0, leaves 5SCIN SET, and the disk layer contains no "issue block-0 vs not" branch), and - the live ground truth (SCWAQ empty, SCLLD never called for block 0, silence after the control-record READ(6)).

The settling runtime check (unchanged from FUNCTION-42-RETURN section 5)

Break SCSID 062217B and the 5SCIN gate 063752B: - If a second SCSID entry with (ABFUN & 077) == 0 and a block-0 address arrives, watch it fall through 062326 -> CACOB -> 063752 (5SCIN SET) -> EXCOM -> 063430 -> SCLLD 067160B. If instead it errors first, dump ABPA2/ABP32 against UHLIM - a mis-scaled LBA would take 057362 -> ERR2 -> BADPA. - If SCSID is never re-entered for block 0 (the trace's actual behaviour), the caller never issued it -> the decision is in the uncarved device-agnostic connect/mount overlay, not here.

5. VERIFIED / INFERRED / OPEN

# Claim Verdict
1 Whole IP-P2-SCSI-DISK unit at L07 = NPL + 376B (SCSDI 057215, SCSID 062217) VERIFIED (symbols + dd)
2 SCSDISK entry 057215 RADD CLD SL DA / STA ,X 6 (A:=L=:X.NFUNC) VERIFIED (dd 146145)
3 Whole-disk UHLIM bounds check at 057352..057362; branch to ERR2 on >UHLIM VERIFIED
4 Reject sets T:=BADPA(5) at 057401 (SAT 5), then RETOP/SWT11, no SCLLD VERIFIED (dd 171005)
5 Block 0 (addr 0) is within bounds -> does NOT branch to ERR2 VERIFIED (branch semantics)
6 SCSDISK calls SCSID; pointer 057631 = 062217 VERIFIED (dd 062217)
7 SCSID 062217 dispatches by ABFUN; fn 0 falls through to CALL CACOB at 062326 VERIFIED
8 5SCIN gate 063752: LDA SUTYP; BSKP ONE 70 DA; JMP I -> INQUI; SET -> build CDB VERIFIED (dd 044423)
9 CACOB READ CDB via SCSF1[0]=010\340 = SCSI READ(6) op 0x08 VERIFIED (SCSF1 table bytes) + INFERRED (opcode meaning)
10 EXCOM GO SCLLD; pointer 063453 = 067160 = SCLLD VERIFIED (dd 067160)
11 fn-0 block-0 read REACHES SCLLD; case 2 (pre-transfer reject) REFUTED VERIFIED
12 Case 1 (caller never issues block 0) is the remaining explanation INFERRED (this carve + trace)
13 The deciding "issue block-0 vs not" instruction lives in the uncarved overlay OPEN

Provenance. Carved 065-S3SIPIT L07 bytes (base 32000B); symbols SINTRAN/NPL-SOURCE/SYMBOLS/L07/SYMBOL-2-LIST.SYMB.TXT (SCSDI/SCSID/SCLLD/SWT11) and FILSYS/SYMBOL-1 (SUTYP=23, 5SCIN=7, UHLIM, SCWAQ). Logic (different revision, INFERRED): SINTRAN/NPL-SOURCE/NPL/IP-P2-SCSI-DISK.NPL (SCSDISK / SCSID / CACOB / EXCOM / INQUI). Ground truth: INQUIRY + READ CAPACITY + one control-record READ(6), then silence; SCWAQ empty.

See also