Messages in this thread | | | Date | Wed, 10 Jun 1998 13:21:56 -0400 | From | Philip Gladstone <> | Subject | input from 0x21 during interrupt handling (2.0.* and 2.1.*) |
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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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