82801BA Datasheet, PDF (300/498 Pages) Intel Corporation – Intel 82801BA I/O Controller Hub 2 (ICH2) and Intel 82801BAM I/O Controller Hub 2 Mobile

LPC Interface Bridge Registers (D31:F0)
9.4.9
OCW3—Operational Control Word 3 Register
Offset Address:
Default Value:
Master Controller–020h
Attribute:
Slave Controller–0A0h
Size:
Bit[6,0]=0, Bit[7,4:2]=undefined,
Bit[5,1]=1
WO
8 bits
Bit
Description
7 Reserved. Must be 0.
Special Mask Mode (SMM)—WO.
6
1 = The Special Mask Mode can be used by an interrupt service routine to dynamically alter the
system priority structure while the routine is executing, through selective enabling/disabling of
the other channel's mask bits. Bit 5, the ESMM bit, must be set for this bit to have any meaning.
Enable Special Mask Mode (ESMM)—WO.
5 _{0 = Disable. The SMM bit becomes a "don't care".}
1 = Enable the SMM bit to set or reset the Special Mask Mode.
4:3 OCW3 Select—WO. When selecting OCW3, bits 4:3 = “01”
Poll Mode Command—WO.
0 = Disable. Poll Command is not issued.
2 _{1 = Enable. 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.
Register Read Command—WO. These bits provide control for reading the In-Service Register (ISR)
and the Interrupt Request Register (IRR). When bit 1=0, bit 0 will 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
1:0 ^{reprogramming OCW3. To select a new status register, OCW3 must be reprogrammed prior to}
attempting the read.
00 = No Action
01 = No Action
10 = Read IRQ Register
11 = Read IS Register
9-38
82801BA ICH2 and 82801BAM ICH2-M Datasheet