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Register Description
R
90 Intel
®
82845 MCH for SDR Datasheet
3.6.17 MBASE1—Memory Base Address Register (Device 1)
Address Offset: 20–21h
Default Value: FFF0h
Access: R/W
Size: 16 bits
This register controls the host to AGP non-prefetchable memory accesses routing based on the
following formula:
MEMORY_BASE1 ≤ address ≤ MEMORY_LIMIT1
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 4 bits of this register are read-only and return 0s when
read. The configuration software must initialize this register. For the purpose of address decode,
address bits A[19:0] are assumed to be 0. Thus, the bottom of the defined memory address range
will be aligned to a 1 MB boundary.
Bit Description
15:4 Memory Address Base 1 (MEM_BASE1). Corresponds to A[31:20] of the memory address.
3:0 Reserved.
3.6.18 MLIMIT1—Memory Limit Address Register (Device 1)
Address Offset: 22–23h
Default Value: 0000h
Access: R/W
Size: 16 bits
This register controls the host to AGP non-prefetchable memory accesses routing based on the
following formula:
MEMORY_BASE1 ≤ address ≤ MEMORY_LIMIT1
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 4 bits of this register are read-only and return 0s when
read. The configuration software must initialize this register. 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.
Bit Description
15:4 Memory Address Limit 1(MEM_LIMIT1). Corresponds to A[31:20] of the memory address.
Default=0
3:0 Reserved.
Note: Memory range covered by MBASE1 and MLIMIT1 registers are used to map non-prefetchable
AGP address ranges (typically, where control/status memory-mapped I/O data structures of the
graphics controller will reside) and PMBASE 1and PMLIMIT1 Registers are used to map
prefetchable address ranges (typically, graphics local memory). This segregation allows
application of USWC space attributes to be performed in a true plug-and-play manner to the
prefetchable address range for improved host-AGP memory access performance.