SB82371 Intel Corporation, SB82371 Datasheet - Page 89

• Manufacturer Part Number: SB82371
• Description: 82371FB (PIIX) AND 82371SB (PIIX3) PCI ISA IDE XCELERATOR
• Manufacturer: Intel Corporation

3.0.
This section describes each of the major functions on the PIIX/PIIX3 including the memory and I/O address
map, DMA controller, interrupt controller, timer/counter, and power management. The PCI, ISA, X-Bus, and
IDE interfaces.
3.1.
The PIIX/PIIX3 interfaces to two system buses—PCI and ISA Buses. The PIIX/PIIX3 provides positive
decode for certain I/O and memory space accesses on these buses as described in this section.
ISA masters and DMA devices have access to PCI memory and some of the internal PIIX/PIIX3 registers as
described in the Register Description section. ISA masters and DMA devices do not have access to host or
PCI I/O space.
3.1.1.
The PIIX/PIIX3 positively decodes accesses to the PCI configuration registers (PCI only), power
management registers (PCI only), and bus master IDE interface registers (PCI only). The PIIX/PIIX3
subtractively decodes memory accesses to the APIC Registers (PCI only), The PIIX/PIIX3 also positively
decodes the ISA-Compatible registers (PCI and ISA), except for the DMA register I/O space which is
subtractively decoded. For details concerning accessing these registers, see Register Description section.
The PIIX/PIIX3 also provides positive decode for BIOS, X-Bus, and system event decode for SMM support.
In addition, the PIIX/PIIX3 positively decodes PCI Bus accesses to registers located on the IDE device, when
enabled. For IDE port accesses, see PCI Local Bus IDE section.
3.1.2.
For PCI accesses to ISA memory, accesses below 16 Mbytes (including BIOS space) that are not claimed by
a PCI device (subtractive decode) are forwarded to ISA. For write accesses that are not claimed by an ISA
slave, the cycle completes normally (i.e., 8-bit, 6 SYSCLK cycle). For read accesses that are not claimed by
an ISA slave, the PIIX/PIIX3 returns data corresponding to the state of the ISA Bus and completes the cycle
normally (i.e., 8-bit, 6 SYSCLK cycle).
For ISA/DMA accesses to main memory, all accesses to memory locations 0–512 Kbytes (512–640 Kbytes, if
enabled), or accesses above 1 Mbyte and below the top of memory are forwarded to the PCI Bus (Table 12).
The Top of Memory is equal to the value programmed in the Top of Memory Register (bits [7:3]). The PIIX3
can also enable ISA/DMA accesses to the VGA frame buffer space from 640-768 Kbytes (A0000-BFFFFh).
All remaining ISA originated accesses are confined to the ISA Bus.
Top of main memory to 128 Mbytes
1 Mbyte to top of main memory
1 Mbyte minus 128 Kbytes to 1 Mbyte minus 64 Kbytes
640 Kbytes to 1 Mbyte minus 128 Kbytes
512–640 Kbytes
0–512 Kbytes
FUNCTIONAL DESCRIPTION
Memory and I/O Address Map
I/O Accesses
Memory Address Map
Table 12. DMA and ISA Master Accesses to Main Memory
Memory Space
82371FB (PIIX) AND 82371SB (PIIX3)
Confine to ISA
Forward to main memory 1
Confine to ISA 2
Confine to ISA
Confine to ISA 3
Forward to PCI
Response
89