Extended-info block (page-0 words 1750B-1757B)¶
The extended-info block is a 16-byte / 8-word structure at page-0 byte
offset 2000 (word 1750B, hex 0x07D0), immediately preceding the 32-byte
master block at byte 2016. It carries a checksum, a flag
word, the number of the system that has the directory entered, and the
directory's page capacity. On floppy (FLOMON) devices the 16 bytes are boot-code
remnants, not a valid extended-info block.
This document is grounded in the producing kernel code carved from the real
SINTRAN L image (006-S3FS), cross-checked against the real disk SMD0.IMG.
Where the kernel settles a question it is marked VERIFIED (kernel); where only
the real disk bytes prove it, VERIFIED (disk); guesses that the kernel does
not settle are OPEN.
Evidence base:
- Writer WXDIR = 37702B in 006-S3FS - computes the checksum and writes
the block back.
- Validator / enter-directory CHDSI = 37763B - reads the block
(via RXDIR = 37643B), re-computes the checksum, compares, and on mismatch
rebuilds the block; then stamps the flag word + system number.
- Release REENB = 40162B - clears the "entered" flag bit and rewrites.
- Real disk ~/repos/nd100x/SMD0.IMG (volume PACK-ONE), page-0 bytes 0x07D0.
- All addresses octal; on-disk multi-byte values are big-endian words.
1. Byte layout¶
Real bytes from SMD0.IMG (xxd -s 0x7D0 -l 16):
000007d0: 10b7 0000 0000 0000 8000 0066 0000 9051
| Word # | Page word | Byte off | Field | PACK-ONE value | Verdict |
|---|---|---|---|---|---|
| 0 | 1750B | 0x07D0 | checksum | 10B7 |
VERIFIED (kernel + disk) |
| 1 | 1751B | 0x07D2 | reserved 1 | 0000 |
VERIFIED (disk); purpose OPEN |
| 2 | 1752B | 0x07D4 | reserved 2 | 0000 |
VERIFIED (disk); purpose OPEN |
| 3 | 1753B | 0x07D6 | reserved 3 | 0000 |
VERIFIED (disk); purpose OPEN |
| 4 | 1754B | 0x07D8 | flag word | 8000 |
VERIFIED (kernel + disk) |
| 5 | 1755B | 0x07DA | system number | 0066 (102) |
VERIFIED (kernel + disk) |
| 6 | 1756B | 0x07DC | pages-available (high word) | 0000 |
VERIFIED (kernel + disk) |
| 7 | 1757B | 0x07DE | pages-available (low word) | 9051 |
VERIFIED (kernel + disk) |
Words 6-7 together form a 32-bit big-endian pages_available = 0x00009051 =
110121B (36945).
2. Checksum - a 16-bit ADDITIVE SUM (kernel-corrected)¶
VERIFIED (kernel). The checksum in word 0 is the 16-bit additive sum of the seven words that follow it (words 1-7), truncated to 16 bits:
checksum = (reserved1 + reserved2 + reserved3
+ flag_word + system_number
+ pages_hi + pages_lo) mod 2^16
Both the writer (WXDIR) and the validator (CHDSI) use the identical summation
loop, so the algorithm is proven from both the producing and the consuming side.
2.1 Writer - WXDIR = 37702B¶
037707 170401 SAA 1 ; A = 1 (loop index start)
037710 146151 RADD CLD SA DD ; D = 1 (running counter)
037711 173401 AAX 1 ; X++ -> point past the checksum word to word 1
037712 146105 RADD CLD 0 DA ; A = 0 (sum accumulator)
037713 171010 SAT 10 ; T = 8
037714 143061 SKP IF DD LST ST ; while D < 8 ...
037715 124005 JMP 5 ; ... else exit -> 037722
037716 062000 ADD ,X 0 ; A += mem[X] (accumulate a word)
037717 173401 AAX 1 ; X++
037720 146401 RADD AD1 0 DD ; D++ (count = 1..7)
037721 124373 JMP -5 ; loop -> 037714
037722 054400 LDX ,B 0 ; X = block base
037723 006000 STA ,X 0 ; mem[base+0] = A ; store the checksum in word 0
The loop runs for D = 1..7 (seven iterations) and adds words 1-7; the sum is
stored in word 0. ADD is a plain two's-complement add, so overflow past bit 15
is discarded - a pure 16-bit additive checksum.
2.2 Validator - CHDSI = 37763B (enter-directory)¶
CHDSI reads the block (RXDIR), then re-runs the same add loop and compares the
result to the stored word 0:
040002 170401 SAA 1 ; same add loop as WXDIR ...
040003 146151 RADD CLD SA DD
040004 173401 AAX 1
040005 146105 RADD CLD 0 DA
040006 171010 SAT 10
040007 143061 SKP IF DD LST ST
040010 124005 JMP 5 ; -> 040015
040011 062000 ADD ,X 0 ; A += word
040012 173401 AAX 1
040013 146401 RADD AD1 0 DD
040014 124373 JMP -5 ; -> 040007
040015 173770 AAX -10 ; X -= 8 (back to block base)
040016 052000 LDT ,X 0 ; T = stored checksum (word 0)
040017 140065 SKP IF DA EQL ST ; if computed(A) == stored(T) -> accept
040020 124043 JMP 43 ; mismatch -> rebuild path (040063)
040021 131042 JAZ 42 ; sum == 0 -> also rebuild
2.3 Numeric proof on PACK-ONE¶
Sum of words 1-7 = 0 + 0 + 0 + 0x8000 + 0x0066 + 0x0000 + 0x9051
= 0x110B7 -> truncated to 16 bits = 0x10B7 = the stored checksum. Exact
match.
2.4 Why the earlier XOR-then-ADD guess also matched¶
A prior reconstruction (NDFS master_block.c) modelled the checksum as
(pages_lo XOR pages_hi XOR flag XOR res1 XOR res2 XOR res3) + system_number.
On PACK-ONE that also yields 0x10B7, purely by coincidence: the only two
summed words that share a set bit are flag = 0x8000 and pages_lo = 0x9051
(both bit 15). Under ADD the two bit-15s carry out past bit 15 and are lost;
under XOR they cancel. Both leave bit 15 clear in the low 16 bits, so a
single sample cannot distinguish the two. The kernel routine is unambiguously
ADD (see the ADD ,X 0 in both loops - no REXO). The additive form is the
correct one; the XOR form is a look-alike that fails as soon as any two summed
words share a lower set bit.
2.5 "Low-byte-only valid" form¶
Not a kernel concept. WXDIR writes and CHDSI compares a full 16-bit sum;
there is no path that accepts a match on the low byte alone. The
"ValidLowByteOnly" tolerance is a reader-side heuristic, not something SINTRAN
produces or checks.
3. Flag word (word 4, 1754B) - bit 15 = "directory entered"¶
VERIFIED (kernel + disk). CHDSI (enter) sets bit 15 of the flag word and
REENB (release) clears bit 15:
CHDSI, on enter:
040110 044004 LDA ,X 4 ; A = flag word
040111 175375 BSKP ONE 170 DA ; skip if bit 15 already set (already entered)
040112 124007 JMP 7 ; -> checks system number ...
...
040123 046004 LDA ,X 4 ; A = flag word
040124 174375 BSET ONE 170 DA ; A |= (1<<15) set the "entered" bit
040125 006004 STA ,X 4 ; store flag word back
REENB, on release:
040200 046004 LDA ,X 4 ; A = flag word
040201 174175 BSET ZRO 170 DA ; A &= ~(1<<15) clear the "entered" bit
040202 006004 STA ,X 4 ; store flag word back
(The disassembler prints the bit-number field pre-shifted: 170 octal = 120
decimal = 15 << 3, i.e. bit 15.)
| Bit | Meaning | Evidence |
|---|---|---|
15 (0x8000) |
Directory entered / in use by a system (set on enter, cleared on release) | VERIFIED (kernel CHDSI/REENB + PACK-ONE flag = 0x8000) |
| 0-14 | No meaning tested or set by these routines; 0 on the real disk |
OPEN (no kernel evidence they are used on this version) |
The real PACK-ONE flag = 0x8000 therefore means the volume was left entered by
system number 102 (below) - exactly what bit 15 + system number encode.
4. System number (word 5, 1755B)¶
VERIFIED (kernel). This is the number of the system that currently has the
directory entered. CHDSI compares it against the entering system and, on
success, stores the entering system's number:
040113 046005 LDA ,X 5 ; A = stored system number
040114 131005 JAZ 5 ; if 0 (no owner) -> proceed
040115 142065 SKP IF DA UEQ ST ; if stored == current system -> ok
040116 124003 JMP 3 ; else ...
040117 044037 LDA 37 ; error: entered by a DIFFERENT system
040120 124015 JMP 15
040121 045034 LDA I 34 ; current system number
040122 006005 STA ,X 5 ; store as the new owner (word 5)
So the field answers OPEN-question 4 directly: it is the "system that entered
this directory". On PACK-ONE = 0x0066 = 102. Combined with flag bit 15 set,
PACK-ONE was last entered - and not cleanly released - by system 102.
5. Pages-available (words 6-7, 1756B-1757B) - directory capacity¶
VERIFIED (kernel). A 32-bit big-endian count that CHDSI treats as the
directory's page capacity (total pages the directory spans), not a live
free-page count. On the good-checksum path it reads the stored value and
compares it against a value computed from device geometry; on the rebuild path it
writes the geometry-derived value:
040027 026006 LDD ,X 6 ; DD = stored pages-available (32-bit, words 6-7)
040030 140065 SKP IF DA EQL ST ; compare against computed capacity ...
...
040077 022006 STD ,X 6 ; (rebuild path) store computed capacity into words 6-7
On PACK-ONE pages_available = 36945, i.e. the directory's total usable page
count (the device geometry figure), distinct from the bitmap's live
free/used counts. See directory-label.md and
NDFS-VALIDATION.md
for how this differs from the master block's unreserved_pages.
6. Reserved words 1-3 (1751B-1753B)¶
VERIFIED (disk): 0 on PACK-ONE. The kernel routines carry these words only
as part of the checksummed range - WXDIR and CHDSI sum them but never test or
assign them individually. So on this SINTRAN L revision they are genuinely
reserved (zero, included in the checksum). Whether a different SINTRAN version
assigns them a version/type meaning is OPEN - the carved L image gives no such
meaning.
7. Enter / validate / release flow (kernel-proven)¶
flowchart TB
START["Enter directory<br/>CHDSI 37763B"] --> READ["RXDIR 37643B<br/>read 8-word ext-info block"]
READ --> SUM["re-compute additive sum<br/>of words 1-7"]
SUM --> CMP{"sum == stored<br/>word 0 ?"}
CMP -->|no / sum=0| REBUILD["zero the 8 words,<br/>rewrite pages capacity<br/>040063"]
CMP -->|yes| CHKCAP["compare stored capacity<br/>vs device geometry"]
REBUILD --> STAMP
CHKCAP --> OWN{"flag bit15 set AND<br/>system# != mine AND != 0 ?"}
OWN -->|yes| ERR["error:<br/>entered by another system"]
OWN -->|no| STAMP["store my system# word5,<br/>set flag bit15 word4"]
STAMP --> WRITE["WXDIR 37702B<br/>recompute checksum, write block"]
REL["Release directory<br/>REENB 40162B"] --> RREAD["RXDIR read"]
RREAD --> CLR["clear flag bit15 word4"]
CLR --> RWRITE["WXDIR recompute + write"]
classDef blue fill:#E3F2FD,stroke:#0D47A1,color:#0D47A1;
classDef teal fill:#E0F7FA,stroke:#00838F,color:#00838F;
classDef green fill:#E8F5E9,stroke:#2E7D32,color:#2E7D32;
classDef orange fill:#FFF3E0,stroke:#E65100,color:#E65100;
classDef red fill:#FFEBEE,stroke:#B71C1C,color:#B71C1C;
class START,READ,RREAD blue
class SUM,CMP,CHKCAP,OWN teal
class REBUILD,STAMP,CLR orange
class WRITE,RWRITE,RREAD,REL green
class ERR redBehaviour on a bad checksum (answers OPEN-question 7): the kernel does not
refuse the mount and does not merely warn. It treats a bad-or-zero checksum as
"extended info not yet initialised", zeroes the 8-word block and rebuilds it
(writes the capacity, then stamps system number + flag and recomputes the
checksum via WXDIR). The extended-info block is therefore a self-healing
convenience record, not a mount gate.
8. Field-by-field verdict¶
| Word | Field | Verdict | Basis |
|---|---|---|---|
| 0 (1750B) | checksum = 16-bit ADD of words 1-7 | VERIFIED | WXDIR writes, CHDSI validates; disk match |
| 1-3 (1751-1753B) | reserved, 0, summed only |
VERIFIED value / OPEN purpose | disk + kernel (summed, never tested) |
| 4 (1754B) | flag word; bit 15 = entered | VERIFIED (bit 15) / OPEN (other bits) | CHDSI set / REENB clear; disk |
| 5 (1755B) | system number = current owner | VERIFIED | CHDSI compare + store; disk = 102 |
| 6-7 (1756-1757B) | pages-available = directory capacity | VERIFIED | CHDSI read/compare/write; disk = 36945 |
Provenance: carved 006-S3FS SINTRAN L bytes (WXDIR 37702B, RXDIR 37643B,
CHDSI 37763B, REENB 40162B); real disk SMD0.IMG page-0 bytes 0x07D0-0x07DF.