Re: input from 0x21 during interrupt handling (2.0.* and 2.1.*)

Meelis Roos (mroos@tartu.cyber.ee)
Wed, 10 Jun 1998 22:29:50 +0300

PG> There is a a dummy input instruction in the interrupt handlers
PG> that reads a byte from 0x21 (or 0xa1). There is a comment in the
PG> 2.0.* tree that says if it is missing then older hard disks don't
PG> work. In 2.1.* there is just a DUMMY comment.

This seems to be for PIC (programmable interrupt controller).
0x20 is the IO base for primary 8259A and 0xA0 for the secondary (nowadays
included in chipsets' VLSI probably).

Don't have a handbook ready to look up what does command 0x21 mean. 0x20
was "End of Interrupt" AFAIR, and this was needed for the PIC to allow
next interrupts and interrupts with lower priority to occur.

Anybody have a handbook handy or knows 0x21 without it?

-- 
Meelis Roos (mroos@tartu.cyber.ee)


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