82801DB Datasheet, PDF (333/587 Pages) Intel Corporation – Intel 82801DB I/O Controller Hub 4 (ICH4)

LPC Interface Bridge Registers (D31:F0)
9.5.5 EOIR—EOI Register
Memory Address FEC0_0040h
Default Value: N/A
Attribute:
Size:
WO
32 bits
The EOI register is present to provide a mechanism to maintain the level triggered semantics for
level-triggered interrupts issued on the parallel bus.
When a write is issued to this register, the I/O APIC will check the lower 8 bits written to this
register, and compare it with the vector field for each entry in the I/O Redirection Table. When a
match is found, the Remote_IRR bit for that I/O Redirection Entry will be cleared.
Note:
This is similar to what already occurs when the APIC sees the EIO message on the serial bus. Note
that if multiple I/O Redirection entries, for any reason, assign the same vector for more than one
interrupt input, each of those entries will have the Remote_IRR bit reset to 0. The interrupt which
was prematurely reset will not be lost because if its input remained active when the Remote_IRR
bit is cleared, the interrupt will be reissued and serviced at a later time. Note: Only bits 7:0 are
actually used. Bits 31:8 are ignored by the ICH4.
Note: To provide for future expansion, the CPU should always write a value of 0 to Bits 31:8.
Bit
Description
31:8
Reserved. To provide for future expansion, the processor should always write a value of 0 to Bits
31:8.
Redirection Entry Clear — WO. When a write is issued to this register, the I/O APIC will check
7:0 this field, and compare it with the vector field for each entry in the I/O Redirection Table. When a
match is found, the Remote_IRR bit for that I/O Redirection Entry will be cleared.
9.5.6
ID—Identification Register
Index Offset:
Default Value:
00h
00000000h
Attribute:
Size:
R/W
32 bits
The APIC ID serves as a physical name of the APIC. The APIC bus arbitration ID for the APIC is
derived from its I/O APIC ID. This register is reset to zero on power up reset
Bit
Description
31:28
27:24
23:16
Reserved
APIC ID — R/W. Software must program this value before using the APIC.
Reserved
15 Scratchpad bit.
14:0 Reserved
Intel^® 82801DB ICH4 Datasheet
333