[Info-vax] Question about checksum in BACKUP block headers

[Anton Lavrentiev]

Hi group,

Hope this message finds all of you well!

Does anybody know what formula is used in DEC's BACKUP block
headers (the last word in the 256-byte headers)?..  I tried the
standard ODS-1/2 file header two's complement sum, one's complement
sum, XOR sum, Fletcher-16, CRC-16 (0x8005), and even BSD, but none of
them match ...  Note that I'm not asking about the longword CRC for
the entire block (even though if anybody knows what poly is used,
that'd be much appreciated) -- that (optionally) resides somewhere in
the middle of the block headers.

Here's what I'm talking about:

00 01 04 00 01 00 01 00 01 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
01 01 01 00 00 00 00 00 F0 0F 00 00 01 00 00 00
0A 49 4E 53 54 41 4C 2E 42 43 4B 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 E1 A1

There, the last word 0xA1E1 is the checksum that I'm after.

Formatted output of the above:

BACKUP block header (256. byte(s))
        Operating system ID:   RSTS
        Subsystem ID:          BACKUP
        Application ID:        DATA BLOCK
        Block sequence number: 1.
        Block structure level: 0x0101
        Media volume number:   1.
        Block size:            4080. byte(s)
        Saveset name:          INSTAL.BCK
        Block header checksum: 0xA1E1

Many thanks,
Anton