IBM 701 IO Instructions

To complement this valuable asset I record a few details concerning 701 I/O that I remember from the IBM 701 computer.

There were read select and write select commands whose address field nominated an I/O device. The devices were, card reader, card punch, printer, drum (perhaps several), and tape (usually several). There was a sense command, and its twin (minus-sense) that, depending in its address field did one of the following:

turn on or off one of the four ‘sense lights’ which were installed on the operator’s console,
test one of said sense lights, by skipping if on.
test one of the 6 sense switches on the operator’s console,
send an ‘out of band’ signal to the printer to do such tricks as printing on the right side of the print line,
switch to or from a mode where 12 bit addresses of 18 bit operands were caused to refer to the ‘second frame’ for those 701’s with 4096 36 bit words of Williams tube memory,
switch to or from a similar mode for fetching instructions.

The printer could receive information for only 72 print columns per 400 ms print cycle. If more information were needed on one print line two such print cycles were required. Plug board logic was required to determine which of the 120 print positions printed on each cycle.

There were commands to write ‘end of file’ on magnetic tapes and variations on read select to read backwards. There was some way to test for a hardware detected magnetic tape parity error after reading a tape record. I do not remember the details. There was a separate opcode: LDA for ‘load drum address’ which specified, after drum select and before copy instructions, where on the drum the operation was to perform at.

Here are more details on 701 IO logic. There was a prominent button on the operator’s console labeled “CLEAR” which caused the machine to stop and set every memory cell to all 1’s. 1’s were less stressful than 0’s for the Williams tubes. A wild branch would after cause the machine to execute such a cleared location which would cause the machine to stop and a ‘copy-check’ light to come on because there was, typically, no I/O operation in progress. The binary op-code for the copy instruction was all ones.