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From: Anne & Lynn Wheeler <l...@garlic.com>
Newsgroups: comp.lang.pl1,alt.folklore.computers
Subject: Re: PDP-10 system calls, was 1132 printer history
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Quadibloc <jsav...@ecn.ab.ca> writes:
> All the early IBM 360 computers featured, in one corner of the front
> panel, three knobs with hex digits on them with a pattern of buttons
> above and below.
>
> That was one of the few parts of the front panel that was for the use
> of machine operators instead of IBM service personnel. You turned the
> three knobs to choose the boot device, and then you pressed the IPL
> (Initial Program Load) to start the computer - you might boot it from
> cards, or tape, or disk. It didn't try scanning through them in an
> order set from the console screen, the way a modern PC does, but it
> did have a boot program of sorts in ROM, so that never had to be
> toggled in from the front panel.

aka the IPL button would invoke a "02" channel command read for 24 bytes
into location zero ... and assumed that the first 16 bytes (at location)
zero were additional channel i/o commands and do a channel program
transfer/tic to location zero. when the channel program ended, it would
do a load psw from location 16 (the 3rd double word read).

a three card loader program was first card with 24 bytes that had two
read ccws of 80bytes each (160 bytes) followed by a psw that started
executing at the first byte of the card image read. the 160 bytes was
enough for 40 4byte instructions which were a program to read additional
information.

a more complex disk ipl would be have the first ccw (loaded at location
zero) a read from the disk of additional channel program and the 2nd ccw
a transfer/tic to the channel I/O commands just read ... which could
read a much large program from disk before finishing ... leading to the
LPSW at location 16 ... starting the program.

-- 
virtualization experience starting Jan1968, online at home since Mar1970