82371AB (PIIX4)
E
78
4/9/97 2:23 PM PIIX4aDS
INTEL CONFIDENTIAL
(until publication date)
PRELIMINARY
Bit
Description
2:0
Interrupt Level Select (L2, L1, L0).
L2, L1, and L0 determine the interrupt level acted upon when
the SL bit is active (bit 6). When the SL bit is inactive, bits [2:0] do not have a defined function;
programming L2, L1 and L0 to 0 is sufficient in this case.
Bit[2:0]
000
001
010
011
Interrupt Level
IRQ 0(8)
IRQ 1(9)
IRQ 2(10)
IRQ 3(11)
Bit[2:0]
100
101
110
111
Interrupt Level
IRQ 4(12)
IRQ 5(13)
IRQ 6(14)
IRQ 7(15)
4.2.2.8.
OCW3—Operational Control Word 3 Register (IO)
I/O Address:
Default Value:
Attribute:
INT CNTRL-1—020h; INT CNTRL-2—0A0h
Bit[6,0]=0; Bit[7,4:2]=Undefined; Bit[5,1]=1
Read/Write
OCW3 serves three important functions—Enable Special Mask Mode, Poll Mode control, and IRR/ISR register
read control.
Bit
Description
7
Reserved
.
Must be 0.
6
Special Mask Mode (SMM).
If ESMM=1 and SMM=1, the interrupt controller enters Special Mask
Mode. If ESMM=1 and SMM=0, the interrupt controller is in normal mask mode. When ESMM=0,
SMM has no effect.
5
Enable Special Mask Mode (ESMM).
1=Enable SMM bit; 0=Disable SMM bit.
4:3
OCW3 Select.
Must be programmed to 01 selecting OCW3.
2
Poll Mode Command.
0=Disable Poll Mode Command. When bit 2=1, the next I/O read to the
interrupt controller is treated as an interrupt acknowledge cycle indicating highest priority request.
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=0, bit 0 does not affect the register read selection.
When bit 1=1, bit 0 selects the register status returned following an OCW3 read. If bit 0=0, the IRR
will be read. If bit 0=1, 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.
Bit[1:0]
00
01
10
11
Function
No Action
No Action
Read IRQ Register
Read IS Register