S/360 channel

Brian> Pretty much plug and go. It's very much like a SCSI interface.... Brian> But from the programmer's standpoint, it's very simple. An IO Brian> command points to a "channel program" which is usually just a Brian> device command word and a data transfer address. The channel Brian> takes it from there, and an interrupt is issued when the Brian> operation is complete (usually). SCSI is a good analogy. Some channel programs could be quite hairy. An extreme example is the boot loader for the 360/44 emulator facility -- that's a chunk of magic hidden memory and logic that allows the 44 to execute SS (character/decimal type) instructions. It doesn't natively do that, so it traps to the emulator, which does the work. So you can run OS/360 and COBOL or PL/I applications (very slowly). The emulator is loaded from a card deck, via the "Emulator IPL" button on the console. The deck is a plain assembler binary deck, exactly as you could get punched out for any binary, with address and data length fields on every card. The channel program is self-modifying; it uses data and command chaining to read the address and length fields of each card into another channel word, which then transfers the right number of bytes to the right place -- repeating until the deck is finished. Amazing... paul