IBM 7074 and then some: "Systems we love" conference

Chuck Guzis cclist at
Tue Jan 24 12:38:16 CST 2017

On 01/24/2017 09:25 AM, Jon Elson wrote:

> I wonder how late IBM still supported the 7074 microcode emulation?
> And, of course, anybody could write a software-level emulation for
> the 7074, in IBM or other hardware.  One reason maybe to not run simh
> on a PC is if the data comes in on old mag tapes (gasp, maybe even
> 556 BPI NRZI half-inch tapes)?

That would seem to me to be an insane decision also.  How many 2400' 556
 bpi 7-track tapes can you fit on a 1TB PC drive?  Why fool with
maintaining a bank of drives in that light?

The biquinary coding used on the 7070 different from that of the 650.  A
two-out of 5 bit scheme was used (01236, with 0 being represented as
12).   IIRC, the 650 used 7 bits.  A word was 10 digits plus sign; a
reference to 55 bit length is made, which would seem to devote a whole
digit to the sign--it's not clear why this was done, as the sign appears
to have only 2 values.

Has a good detailed run-down.  There are some interesting details; for
example, although it employed a digit-sequential ALU, circuitry
apparently did leading nonsignificant zero detection, so that only the
number of significant digits was operated upon.

Another interesting aspect was the scatter/gather tape I/O facility.
One normally thinks of this as a feature on later gear; to see it in
1959 is a bit surprising.  Also interesting is hardware prioritizing of
I/O operations.

Was the 7070 IBM's first machine with a wire-wrapped backplane?


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