> On May 8, 2025, at 6:15 PM, Steve Lewis via cctalk <[email protected]>
> 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
