9 May
2025
9 May
'25
2:46 a.m.
On May 8, 2025, at 6:15 PM, Steve Lewis via cctalk
<cctalk(a)classiccmp.org> wrote:
Makes sense, IPL (instruction program load?)
"Initial Program Load"
- how were the disk packs
initialized to indicate it was an IPL media? I assume the content to be
loaded had to be on a specific sector or of a file-type indicating it is to
be loaded on startup?
I don't know any system other than the 360 and that one only very vaguely. I think it
would load one record (a card, if IPL from the card reader; a sector, probably the first
one, if disk; don't know about tape) and execute that. And I think that the loaded
data is a channel program, so it's handled to the IPL channel to execute.
A special case is "Emulator IPL" on the 360 model 44, which loads the special
memory (inaccessible from normal execution) that contains the code to emulate SS class
instrutions. The emulator is provided as a card deck, in standard assembler object file
output format, one object data record per card. And the loader is a self-modifying
channel program that reads in the entire emulator deck by an impressive mix of data and
command chaining, picking up the data load address and length fields of each object card
to modify the CCW that then reads the data bytes into the correct memory location.
paul