Messages in this thread | | | Date | Wed, 10 Jun 1998 22:34:04 +0200 | From | OLK <> | Subject | Re: input from 0x21 during interrupt handling (2.0.* and 2.1.*) |
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Philip Gladstone wrote: > > 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 Port 0x21 and 0xa1 are the EOI Registers from the two 8259 Interrupt-controllers (master/slave) nowadays not separately found on motherboards ;-))
> Does anyone know why this is required -- and for which systems? To tell the 8259 that the InterruptServiceRoutine has finished and so the 8259 can accept new Interrupts. > for which systems? x86-based
> [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.] Count the executed ISR's per sec. and decide if it is worthwhile
bye OLK -- eMail : delise@online-club.de
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