[Simh] DECtape format(s)

[Bob Supnik]

As was mentioned, DECtape formats are very much the same at the hardware 
level, but the block size does vary.

Block header format, all controllers except Type 55x, in 18b words:

word 0 - 0
word 1 - forward block number
words 2,3 - 0
word 4 - reverse checksum (077)

Block trailer format, all controllers except Type 550/555, in 18b words:

word 0 - forward checksum (one's complement of XOR of each 6b data frame)
words 1,2 - 0
word 3 - reverse block number (complement obverse of block # if read 
forward)
word 4 - 0

The ringer is the Type 55x controllers, which was used in the PDP-1, 4, 
5, 6, 7. The checksum format varied from unit to unit. Some used parity 
checking (XOR); others (like the 7) relied on software and used one's 
complement arithmetic. The reverse checksum was always 0777777 (-0). 
Further, the first five Type 550's had only four word headers.

So "modern" (TC series) DECtapes are all interchangeable. However, 
ancient DECtapes from before 1966 will require unique handling.

/Bob