[TUHS] Sector interleaving (was: Clever code)

[Greg 'groggy' Lehey]

On Tuesday, 13 December 2022 at  7:52:49 -0800, Bakul Shah wrote:
> On Dec 12, 2022, at 7:30 PM, Rudi Blom <rudi.j.blom at gmail.com> wrote:
>>
>> I vaguely remember having read here about 'clever code' which took
>> into account the time a magnetic drum needed to rotate in order to
>> optimise access.
>
> Similar consideration applied in the early days of unix workstations.
> Fortune 32:16 was a 5.6Mhz machine and couldn't process 1020KB/sec
> (17 sectors/track of early ST412/ST506 disks) fast enough. As Warner
> said, one dealt with it by formatting the disk so that the logical
> blocks N & N+1 (from the OS PoV) were physically more than 1 sector
> apart. No clever coding needed!

CP/M did something similar with floppy disks.  It imposed a 6 fold
software interleave between sectors (logical sectors 1, 2, 3.. were
"physical" sectors 1, 7, 13...)

On soft-sectored floppies, the "physical" sectors were really just the
numbers in the sector header.  By the time I got involved, computers
were far fast enough that they spent a lot of time just waiting for
the next sector.  I wrote a format program that positioned the
"physical" sectors so that there was only one sector between
"physical" 1, 7, 13 and so.  It made an amazing difference to the disk
speed.

Greg
--
Sent from my desktop computer.
Finger grog at lemis.com for PGP public key.
See complete headers for address and phone numbers.
This message is digitally signed.  If your Microsoft mail program
reports problems, please read http://lemis.com/broken-MUA.php
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 163 bytes
Desc: not available
URL: <http://minnie.tuhs.org/pipermail/tuhs/attachments/20221215/51bbdf31/attachment.sig>