Intel 460GX manual Devices View of the System Memory Map

System Address Map

4.3Devices View of the System Memory Map

Figure 4-1shows an Expander Bridge device’s view of system memory. The goal is to prevent invalid accesses at the expander bridge level, since different expander bridge devices are allowed to access different regions. For example, PXBs are allowed to access other logical PCI segments and GXBs are not. The SAC does not perform special checking to prevent this, and therefore the expander bridges must be set up by firmware accordingly. An exception to this rule can be made if the request is being routed to the system bus rather than directly to memory. For instance, accesses above the Top of Memory are not blocked at the PXB level, but instead cause a BINIT# in the SAC because they are guaranteed to go through the SAC’s decode logic since the PXB routes these accesses to the system bus

Note: Figure 4-1shows that parallel segment peer-to-peer accesses are only supported when initiated by a PXB (GXBs are only allowed to initiate accesses directed to system memory) and directed to one of the nx32M logical PCI segments (this segment can be anywhere except below the compatibility PCI bus). Parallel segment peer accesses are not permitted anywhere below 1 MB, not even to the VGA region. This means that if VGA is relocated below a GXB, a server management card on one of the PCI buses in the system can no longer access the VGA range.

Figure 4-4. System Memory Address Space as Viewed from an Expander Bridge (PXB/GXB)

FFF_FFFF_FFFF

System

Memory

1_0000_0000

Firmware,

Processor & 32

Chipset MB

Specific

System

Memory

10_0000

System

Firmware

F_0000

C, D, and E

Segments

C_0000

VGA

Memory

A_0000

512K-640K

DOS

Region

0

Notes:

	Top of Memory			PXB accesses above the Top of Memory
				are routed as if to peer and therefore
				go through the SAC decode and cause
				a BINIT#; GXB must cause a BINIT#
FFFF_FFFF				after the GART
		High		PXB must ignore
			GXB must BINIT# after GART
		System Firmware
FF00_0000			PXB must ignore; GXB must BINIT# after GART
		4G-16M to 4G-17M
		4G-17M to 4G-18M		PXB/GXB allow to interrupt delivery area
		4G-18M to 4G-19M
FEC0_0000		4G-19M to 4G-20M		PXB defaults to memory so SAC must trap;
				GXB must BINIT# after GART
		Chipset
			PXB must ignore; GXB must BINIT# after GART
		Specific
FE00_0000				PXB allows (only recommended to use
			from compatibility bus for server management)
		n x 32M		GXB must BINIT# after GART
		PCI Gaps	PXBs may support peer-to-peer; accesses
FE00_0000 -
(n x 32M)			to the PXB’s own PCI bus must be ignored;
			accesses to other PCI buses can be directed
			to the system bus so the SAC forwards
			them to the targeted peer bus
			GXBs can not allow peer-to-peer; accesses
			to the GXB’s own PCI bus must be ignored
			(allows AGP card to “talk to itself” in that)
			region); accesses to any PCI bus that are found
			after the GART must cause a BINIT#

if enabled as memory GAP (directed to a logical PCI segment), all PXB’s must ignore and all GXBs must BINIT# after GART

if enabled as memory GAP (directed to the compatibility PCI bus), all PXB’s must ignore and all GXBs must BINIT# after GART

if enabled as memory GAP (directed to a logical PCI segment), all PXB’s must ignore and all GXBs must ignore in the PCI stream

but BINIT# after GART (allows AGP card to “talk to itself” in that region)

if enabled as a memory GAP (directed to the compatibility PCI bus), all PXB’s must ignore and all GXB’s must BINIT# after GART

The term “ignore” from a PCI perspective means do not assert DEVSEL#; instead allow request to master abort

All remaining regions are mapped to main memory and are always forwarded inbound using the “memory” route encoding

Intel® 460GX Chipset Software Developer’s Manual 4-7