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
