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

Philip Gladstone (philip@raptor.com)
Wed, 10 Jun 1998 13:21:56 -0400

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

Does anyone know why this is required -- and for which systems?
[The reason is that the cost of an I/O instruction is large
and unneccessary ones should be removed. On a 350MHz PII, this
inb(0x21) takes around 600ns. This may not seem like a long time, but
it is around 200 instruction times.]

philip 

-- 
Philip Gladstone                           +1 781 530 2461
Raptor Systems / Axent Technologies 
Waltham, MA         		    http://www.raptor.com/


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