[Info-vax] TK50 - this is annoying...

[glen herrmannsfeldt]

Johnny Billquist <bqt at softjar.se> wrote:

(snip, someone wrote)

>> If I recall correctly, KM-9 (the sort-of OS DEC offered for the PDP-9)
>> wrote alternate blocks to DECtape, and wrote the skipped blocks
>> backwards.  Thus, if you imagine a ten-block tape it might be written in
>> this order: 0 2 4 6 8 9 7 5 4 3 2 1.  The odd-numbered blocks would be
>> processed backwards.

> So if you wanted to read block 0,1,2, you would read block 0, then 2, 
> then turn around to read block 1? And the controller would handle the 
> DMA for this??? As well as all the bit fiddling that is required? Are 
> you serious?

Think of it as a mapping between logical blocks and physical blocks.

Physical blocks 0 2 4 6 8 9 7 5 3 1 are logical blocks 0 1 2 3 4 5 6 7 8 9

(As in the case of floppy disks, this can be done either by renumbering
the block headers or with an external look-up table.)

-- glen