82374EB Datasheet, PDF (81/208 Pages) Intel Corporation – SYSTEM COMPONENT (ESC)

82374EB 82374SB
3 4 2 ICW2 INITIALIZATION COMMAND WORD 2
Register Location
Default Value
Attribute
Size
021h INT CNTRL-1
0A1h INT CNTRL-2I
xxh
Write Only
8 Bits per controller
ICW2 is used to initialize the interrupt controller with the five most significant bits of the interrupt vector
address The value programmed for bits 7 3 is used by the CPU to define the base address in the interrupt
vector table for the interrupt routines associated with each IRQ on the controller Typical ISA ICW2 values are
04h for CNTRL-1 and 70h for CNTRL-2 Section 9 8 1 of the Interrupt Unit Functional Description contains a
table detailing the interrupt vectors for each interrupt request level as they would appear when the vector is
driven onto the data bus
Bit
Description
7 3 Interrupt Vector Base Address Bits 7 3 define the base address in the interrupt vector table for the
interrupt routines associated with each interrupt request level input For CNTRL-1 a typical value is
00001b and for CNTRL-2 10000b
The interrupt controller combines a binary code representing the interrupt level to receive service with
this base address to form the interrupt vector that is driven out onto the bus For example the
complete interrupt vector for IRQ 0 (CNTRL-1) would be 0000 1000b (CNTRL-1 7 3 ^e00001b and
000b representing IRQ 0 ) This vector is used by the CPU to point to the address information that
defines the start of the interrupt routine
2 0 Interrupt Request Level When writing ICW2 these bits should all be 0 During an interrupt
acknowledge cycle these bits will be programmed by the interrupt controller with the interrupt code
representing the interrupt level to be serviced This interrupt code is combined with bits 7 3 to form
the complete interrupt vector driven onto the data bus during the second INTA cycle The table in
Section 9 8 1 outlines each of these codes The code is a simple three bit binary code 000b
represents IRQ0 (IRQ8) 001b IRQ1 (IRQ9) 010b IRQ2 (IRQ10) and so on until 111b IRQ7 (IRQ15)
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