Browse Prior Art Database

Means To Utilize PCI Level-Sensitive Interrupts within an ISA-PC Edge-Sensitive Environment

IP.com Disclosure Number: IPCOM000115570D
Original Publication Date: 1995-May-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 4 page(s) / 103K

Publishing Venue

IBM

Related People

Dean, ME: AUTHOR [+3]

Abstract

Disclosed is a means to utilize PCI level-sensitive interrupts within ISA-based edge-sensitive systems.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Means To Utilize PCI Level-Sensitive Interrupts within an ISA-PC
Edge-Sensitive Environment

      Disclosed is a means to utilize PCI level-sensitive interrupts
within ISA-based edge-sensitive systems.

      The PowerPC* desktop system relies on the use of an ISA
expansion bus to access a vast array of peripheral cards.  The system
also uses a PCI bus that defines interrupts as level-sensitive, which
is incompatible in an ISA-based environment.

      In the PowerPC system, this problem is resolved with an
interrupt system based on a chip containing two interrupt
controllers.  The system assigns devices to interrupts in an
industry-compatible manner in order to simplify porting available
code to the PowerPC processor.

      These assignments result in a certain number of interrupts
being available for use on both expansion slot types.  PCI defines up
to four interrupts (A, B, C, D) per slot, which are far too many.

      When a PCI slot is not occupied, the corresponding interrupt is
available to ISA cards.  Auto configuration cards will see a low
level and not automatically utilize these interrupts.

      ISA devices connected to IRQ9 (interrupt request) or IRQ11 are
not affected by this mechanism.  The software may or may not issue
EOI9 (end of interrupt) or EOI11 when servicing an ISA card connected
to one of these levels, and the only effect may be to delay the next
interrupt by approximately 500 ns.

      The invention achieves its objectives of allowing PCI cards to
operate in an edge-sensitive environment and allowing ISA devices to
use interrupts to unoccupied slots.

      The Figure illustrates the interrupt connections for one PCI
slot.  The dotted lines indicate a variation on the invention.  With
the variation, the pull-down resistor...