2 Oct
2024
2 Oct
'24
1:39 p.m.
-----Original Message-----
From: Jon Elson <elson(a)pico-systems.com>
Sent: Tuesday, October 01, 2024 4:55 PM
To: Tom Gardner via cctalk <cctalk(a)classiccmp.org>
Subject: [cctalk] Re: Might be antique computer parts
On 10/1/24 18:29, Tom Gardner via cctalk wrote:
For the earlier 1311, lack of overlap made perfect sense. After all, the 1620 has no
interrupts, no parallelism of any kind: every I/O operation stalls the CPU until the
operation is finished. (That and the BB instruction are among the reasons why Dijkstra
rejected the 1620.)
Speaking of high profit margins: on the 1620, there was an extra cost option called
"direct seek". I don't know if involved a jumper cut or some actual
circuitry (an adder, most likely). We didn't have that, and the result is that a seek
from cylinder x to cylinder y was done by a full retract to cylinder 0, followed by a seek
out to y. It was amusing to watch the shaking resulting from a simple "incrementing
seek test" -- seek to cylinder i for i = 0 to 99. Those last few seeks would take
the better part of a second.
paul
I wouldn't call the 2314 low tech - it was the highest areal density at the time, a
breakthru with ferrite heads and very low cost to manufacture. Note I said cost, its
profit margin was enormous, in part by putting as much expensive electronics as possible
in the control unit. ??
Actually the 2314 did not ship with the first 360's in 1965; it was announced in
April 1965 about 1 year after the 360 announcement and AFAICT from Bitsavers document
dates it didn't ship until late 1966, which FWIW, at the Computer History Museum, 1966
is also the date for first shipment of the 2414 and its ferrite heads. BTW the hydraulic
actuator design goes back to the 1311 - more or less the same actuator in the 1311, 2311
and 2314.
Well, yes, and in the days of SLT logic, everything was expensive. So, putting as much
of the functions in the control unit rather than the drive was good. But, one thing that
this mindset caused was that they could not have one drive seeking while another drive was
transferring. The entire operation, cylinder seek, rotational seek and data transfer was
all one atomic operation. That really killed the throughput of the whole disk system.
The reason was that the IBM developers came from systems like 7070 and 7090 where all
permanent storage was on tape, and they didn't quite "get" how central disks
were going to be to the 360 systems. They had the CKD scheme, where you could search
several cylinders for a match of some arbitrary field in the DATA portion of a sector, but
this resulted in massive slowdown of the system, as it tied up not only the drive, but the
controller and the channel as well! Thus the need for the database system, which would
make selecting the desired record much faster.
I didn't mean that the 2314 DISK was low tech, just that the drive, itself, was quite
spartan.
Jon