idea for a universal disk interface

[Tom Gardner]

Got me - I was thinking of HDDs which from the beginning had at least 2
heads so, given an appropriately sized Gap1,  it was always faster to switch
heads. 
And FWIW is some later SyQuest cartridge drives employing sectored servos it
was faster to seek than to switch heads so they made a head switch into a
seek, but they masked that all in the drive firmware

In any event, it really doesn't matter for this device emulator if the end
of a track is followed by a seek command; any time there is a seek command
there is lots of time for the device's housekeeping to get ready for the
next read.

Tom

-----Original Message-----
From: Fred Cisin [mailto:cisin at xenosoft.com] 
Sent: Wednesday, April 20, 2022 11:55 AM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: RE: idea for a universal disk interface

On Wed, 20 Apr 2022, Tom Gardner via cctalk wrote:
> Likewise, I don't know it for certain, but I am pretty sure that it is 
> true that virtually all controllers switch heads sequentially when 
> transferring blocks beyond the end of the track,

Are you implying that data/file that is more than one track long has its
next data on a track that is a different head of the same cylinder?

If that is, indeed what you are saying, . . .
It would make sense, and is common.  Since it is obvious that switching
heads should take less time than stepping to the next cylinder.  BUT, it is
a choice by the file system, not by the controller.

As a simple example, when floppy disks went from single sided to double
sided, SOME OS programmers chose to switch heads before stepping to the 
next cylinder.   (Cylinder 0 side A, cylinder 0 side B, Cylinder 1 side A, 
cylinder 1 side B, etc.)

BUT, some chose to "keep what they had", and use the second side as an
"extension" of the first side, and chose to not switch heads until all
tracks on the first side were exhausted.  (Cylinder 0 side A, Cylinder 1
side A, Cylinder 2 side A . . . )  Of those, most "recalibrated" (seek to
zero) for the second side (cylinder 0 side A, Cylinder 1 side A . . .
Cylinder 75 side A, Cylinder 76 side A, Cylinde 0 side B, Cylinder 1 side B)
(that's for 77 track 8")  while others started using the second side
starting at the high end (to avoid the seek to zero delay).  (cylinder 0
side A, Cylinder 1 side A . . . Cylinder 75 side A, Cylinder 76 side A,
Cylinder 76 side B, cylinder 75 side B, . . .) There were a few more
variations, because it was the programmer making the decision, not the
controller, and we can come up with some amazing cockamamy ideas.

--
Grumpy Ol' Fred     		cisin at xenosoft.com