82371AB (PIIX4)
E
56
4/9/97 2:23 PM PIIX4aDS
INTEL CONFIDENTIAL
(until publication date)
PRELIMINARY
4.1.6.
CLASSC—CLASS CODE REGISTER (FUNCTION 0)
Address Offset:
Default Value:
Attribute:
09
0Bh
060100h
Read Only
This register identifies the Base Class Code, Sub-Class Code, and Device Programming interface for PIIX4 PCI
function 0.
Bit
Description
23:16
Base Class Code (BASEC).
06h=Bridge device.
15:8
Sub-Class Code (SCC)
. 01h=PCI-to-ISA Bridge. 80h=Other bridge Device (PCI Positive Decode
Bridge). This value depends upon the programming of bit 1 of the General Configuration register
(PCI function 0, address B0h). If programmed as a Subtractive Decode bridge (default), this will read
01h. If programmed as a Positive Decode bridge, this will read 80h.
7:0
Programming Interface (PI).
00h=No register level programming interface defined.
4.1.7.
HEDT—HEADER TYPE REGISTER (FUNCTION 0)
Address Offset:
Default Value:
Attribute:
0Eh
80h
Read Only
The HEDT Register identifies PIIX4 as a multi-function device.
Bit
Description
7:0
Device Type (DEVICET).
80h=multi-function device.
4.1.8.
IORT—ISA I/O RECOVERY TIMER REGISTER (FUNCTION 0)
Address Offset:
Default Value:
Attribute:
4Ch
4Dh
Read/Write
The I/O recovery mechanism in PIIX4 is used to add additional recovery delay between CPU or PCI master
originated 8-bit and 16-bit I/O cycles to the ISA Bus. PIIX4 automatically forces a minimum delay of
3.5 SYSCLKs between back-to-back 8- and 16-bit I/O cycles to the ISA Bus. This delay is measured from the
rising edge of the I/O command (IOR# or IOW#) to the falling edge of the next I/O command. If a delay of greater
than 3.5 SYSCLKs is required, the ISA I/O Recovery Time Register can be programmed to increase the delay in
increments of SYSCLKs. No additional delay is inserted for back-to-back I/O “sub cycles” generated as a result
of byte assembly or disassembly. This register defaults to 8- and 16-bit recovery enabled with one SYSCLK
clock added to the standard I/O recovery.