ONCE MORE, I APOLOGIZE.
(details bottom posted)
This was the approach IBM used in it's first RAMAC RAID where I think
they had to buffer a whole cylinder but that was many generations ago
(my copy of the specs may not be exact):
Buffering a whole cylinder, or a whole surface, of the RAMAC was no big
deal.
One hundred surfaces (52 platters, but not using bottom of bottommost nor
top of topmost) totalling to 5 million 6 bit characters.
That's 50,000 characters per surface.
OR 50,000 characters per cylinder
("square geometry" :-)
100 tracks per side of a platter (at 20 tracks per inch) meant about 500
characters per track
Problematic in the CP/M days, but such a buffer is small in current usage.
On Sat, 16 Apr 2022, Tom Gardner via cctalk wrote:
Not the RAMAC of 1956 but the RAMAC Virtual Array of 1996,
https://www.ibm.com/common/ssi/cgi-bin/ssialias?htmlfid=897/ENUSC96-029&info
type=AN&subtype=CA&appname=skmwww
It emulated several different IBM DASD of varying CKD track lengths on fixed
block HDDs
The trick they used and the one I'm suggesting is they stored an entire
track, index to index including, gaps, headers, etc, in a concatenated set
of fixed blocks greater than the maximum length of the raw track.
For example, an SMD drive turning at 3600 RPM and with a data rate of 15
Mb/sec and a 5% speed variation has a maximum track length of 31,250 bytes
nominally but never more than 32,895 on the slowest drive. So allocating 65
sectors (512 byte) will fit the worst track. Of course since the emulator
doesn't have any speed variation only 62 sectors need be allocated per
track.
I poked around in some old Disk/Trends and it seems the largest ESDI/SMD
drive was on the order of 2.5 GB which is likely a formatted capacity so a
full drive emulation might require a maximum of 3.3 GB which is well within
the size of a modern PC and given the memory data rate I suspect an emulator
wouldn't have to buffer more than two memory words.
THAT is an fascinating project!
"Never mind" - Emily Litella (Gilda Radner)
I need to step back and BUFFER my replies for a period of time.
It seems that almost everything that I reply to, I misunderstand some part
of what I am replying to, and need to apologize.
As a mitigating factor, in this same thread, I had mentioned owning a
RAMAC platter (from 60 years ago), that was too damaged to even consider
trying to read from, so that I am making a patio table out of it.
I don't consider 1996 to be "many generations ago"
and, You had said buffer a whole CYLINDER. I wrote a reply assuming that
you meant TRACK. When I noticed the cylinder/track confusion, I edited
my reply to be numbers for a cylinder, but failed to completely edit out
my comment that buffering the size of an entire [EARLY 1960] RAMAC TRACK
was no big deal, and was promptly reminded that in the day, buffering 50K
WAS a big deal.
"Never mind"
--
Grumpy Ol' Fred ci...@xenosoft.com
