24 Jan
2017
24 Jan
'17
10:38 a.m.
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.
https://www.computer.org/csdl/proceedings/afips/1959/5054/00/50540222.pdf
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?
--Chuck