[TUHS] Old mainframe I/O speed (was: core)

Paul Winalski paul.winalski at gmail.com
Fri Jun 22 00:00:04 AEST 2018

On 6/20/18, Peter Jeremy <peter at rulingia.com> wrote:
> Note that most S360 machines were microcoded with the native ALU size
> varying between 8 and 32 bits.  The model 25 was also the only S360 with
> writable microcode and there was a microcoded APL implementation for it so
> it "natively" executed APL.  I'm not sure if there were any other novel
> microcode sets for it.

Yes, the model 25's microcode was in core.  I remember we had to
reaload it from punch cards at one point when IBM issued an update.  I
didn't know about the custom APL microcode.  I do recall that the disk
controller logic, as well as the "selector channel", was in CPU
microcode.  After the model 25 was withdrawn, IBM released the sources
for the microcode to customers.  There were several hacks in there to
slow down the disk I/O so that it didn't outperform the model 30.

> [3] "selector" channels were used for high speed devices - tape, DASD[4]
> [4] Direct Access Storage Device - IBM speak for "disk"

They used the term DASD because it covered non-disk devices such as
drums and the 2321 data cell drive (aka "noodle picker").

-Paul W.

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