82374EB Datasheet, PDF (86/208 Pages) Intel Corporation – SYSTEM COMPONENT (ESC)

82374EB 82374SB
Third OCW3 provides control for reading the In-Service Register (ISR) and the Interrupt Request Register
(IRR) Either the ISR or IRR is selected for reading with a write to OCW3 Bits 0 and 1 carry the encoded
command to select either register The next I O read to the OCW3 port address will return the register status
specified during the previous write The register specified for a status read is retained by the interrupt control-
ler Therefore a write to OCW3 prior to every status read command is unnecessary provided the status read
desired is from the register selected with the last OCW3 write
Bit
Description
7 Reserved Must be 0
6 SMM If ESMM^e1 and SMM^e1 the Interrupt Controller will enter Special Mask Mode If ESMM^e1
and SMM^e0 the Interrupt Controller will revert to normal mask mode When ESMM^e0 SMM has no
effect
5 Enable Special Mask Mode When this bit is set to 1 it enables the SMM bit to set or reset the Special
Mask Mode When ESMM^e0 the SMM bit becomes a ‘‘don’t care’’
4 3 OCW3 Select When selecting OCW3 bit 3 must be a 1 and bit 4 must be 0 If bit 4 is a 1 the Interrupt
Controller interprets the write to this port as an ICW1 Therefore always ensure that bits 4 3 are
‘‘01b’’ when writing an OCW3
2 Poll Mode Command When bit 2 is a 0 the Poll command is not issued When bit 2 is a 1 the next
I O read to the Interrupt Controller is treated as an Interrupt Acknowledge cycle An encoded byte is
driven onto the data bus representing the highest priority level requesting service
1 0 Register Read Command Bits 1 0 provide control for reading the In-Service Register (ISR) and the
Interrupt Request Register (IRR) When bit 1^e0 bit 0 will not effect the register read selection When
bit 1^e1 bit 0 selects the register status returned following an OCW3 read If bit 0^e0 the IRR will be
read If bit 0^e1 the ISR will be read Following ICW initialization the default OCW3 port address read
will be ‘‘read IRR’’ To retain the current selection (read ISR or read IRR) always write a 0 to bit 1
when programming this register The selected register can be read repeatedly without reprogramming
OCW3 To select a new status register OCW3 must be reprogrammed prior to attempting the read
Bits 1 0
00
01
10
11
Function
No Action
No Action
Read IRQ Register
Read IS Register
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