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

[Johnny Billquist]

On 2012-10-19 00:42, glen herrmannsfeldt wrote:
> 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.)

Right. In which case, if we want to read blocks 0,1,2, we actually need 
to read physical block 0,2,4, which means you can't set up the 
controller to read multiple blocks, but instead have to issue an 
individual read for each block. It then also becomes a seek between each 
read, and then you need to make sure that there isn't a stop-and-go 
effect between the read of one block, and the read of the next block.
Not to mention that you still need to know in which direction to read 
the blocks, so that you don't get that wrong. :-)

	Johnny