82845G Datasheet, PDF (88/193 Pages) Intel Corporation – Intel 82845G/82845GL/82845GV Graphics and Memory Controller Hub (GMCH)

Register Description
3.5.2.18
MLIMIT1—Memory Limit Address Register (Device 1)
Address Offset:
Default Value:
Access:
Size:
22–23h
0000h
RO, R/W
16 bits
This register controls the CPU to PCI_B non-prefetchable memory access routing based on the
following formula:
MEMORY_BASE ≤ address ≤ MEMORY_LIMIT
The upper 12 bits of the register are read/write and correspond to the upper 12 address bits
A[31:20] of the 32-bit address. The bottom four bits of this register are read-only and return zeros
when read. This register must be initialized by the configuration software. For the purpose of
address decode, address bits A[19:0] are assumed to be FFFFFh. Thus, the top of the defined
memory address range will be at the top of a 1-MB aligned memory block.
Note:
Memory range covered by MBASE and MLIMIT registers are used to map non-prefetchable
PCI_B/AGP address ranges (typically, where control/status memory-mapped I/O data structures of
the graphics controller will reside) and PMBASE and PMLIMIT are used to map prefetchable
address ranges (typically graphics local memory). This segregation allows application of USWC
space attribute to be performed in a true plug-and-play manner to the prefetchable address range
for improved processor-AGP memory access performance.
Note:
Configuration software is responsible for programming all address range registers (prefetchable,
non-prefetchable) with the values that provide exclusive address ranges (i.e., prevent overlap with
each other and/or with the ranges covered with the main memory). There is no provision in the
GMCH hardware to enforce prevention of overlap and operations of the system in the case of
overlap are not guaranteed.
Bit
Description
Memory Address Limit (MLIMIT). Corresponds to A[31:20] of the memory address that corresponds
15:4 to the upper limit of the range of memory accesses that will be passed by the Device 1 bridge to AGP/
PCI_B.
3:0 Reserved.
88
Intel^® 82845G/82845GL/82845GV GMCH Datasheet