Date   : Mon, 11 Feb 1985 09:23:00 PST
From   : Alan Bomberger <ACB.TYM@OFFICE-2.ARPA>
Subject: Ampro disk performance

My wife uses an Ampro Little Board computer to do her writing and was 
complaining about the poor disk performance.  I applied high powered diagnostic
techniques (an AM radio placed next to the computer) and discovered that the 48
TPI drives were only reading 1 sector per revolution.  Examination of the 
formatter disclosed a format that assumed that every other sector could be read
(skew of 2?).  It would take an extremely optimistic person to believe that you
could read every other 512 byte sector.  The 96 TPI drives use 1024 byte 
sectors and it is possible to read every other sector.  Changing the formatter 
to set the skew at 3 (reading every third sector  1,8,5,2,9,6,3,10,7,4) 
improved the disk performance by a factor of 3!

Further poking around with the multiformatter revealed that many 5 inch formats
actually use 256 byte sectors.  It takes 6 revolutions to read a full track 
with 256 byte sectors and only 2 to read a full track with 1024 byte sectors 
(due to controller and BIOS set up times).  No wonder a lot of people complain 
about 5 inch disks being SLOW!!

One final comment (somewhat unrelated) a recent DDJ article blamed CP/M for 
only having one buffer shared between reading and writing, thus causing a lot 
of prereading (and poor performance) on certain tasks that alternated reading 
and writing single 128 byte records.   BULL....  that is a BIOS fault!  BIOS's 
are implemented by the computer manufacturer and one would think that more of 
them would try harder to get the best performance from their system.  Only a 
little extra code is required to support separate buffers for read and write 
(deblocking buffers).

My Northstar Horizon with 512 byte sectors and separate read and write 
deblocking buffers (my own BIOS) really hums.  I have 8 inch disks as well and 
with 512 byte sectors there is still a 2 to 1 speed difference.  If you compare
8 inch disks to poorliy implemented (AMPRO's bad choice of skew or 256 byte 
sectors) 5 inch BIOS's you might see 6 or even 12 to 1 (256 byte sectors).  My 
heavens!!