參數(shù)資料
型號(hào)： 21326
英文描述： Implementation of Write Allocate Application Note
中文描述： 在寫(xiě)分配應(yīng)用執(zhí)行情況的說(shuō)明
文件頁(yè)數(shù)： 19/22頁(yè)
文件大?。?/td> 241K
代理商： 21326
Programming Details
11
21326F/0—February 1999
Implementation of Write Allocate in the K86 Processors
This programmable memory range exists because a small
number of uncommon memory-mapped I/O adapters are
mapped to physical RAM locations. If a card like this exists in
the system configuration, it is recommended that the BIOS
program the
‘memory hole’ for the adapter into this
non-write-allocatable range.
Top of Memory.
The other programmable memory range is
defined by the ‘top-of-memory’ field. The top of memory is
equal to zzzz_0000h, where zzzz is defined using bits 15–0 of
WATMCR. Addresses above zzzz_0000h are protected from
write allocate when bit 18 of WATMCR is enabled.
Once the BIOS determines the size of RAM installed in the
system, this size should also be used to program the top of
memory. For example, a system with 32 Mbytes of RAM
requires that the top-of-memory field be programmed with a
value of 0200h, which enables protection from write allocate for
memory above that value. Set bit 18 of WATMCR to enable
protection of this range.
Caching and write allocate are generally not performed for the
memory above the amount of physical RAM in the system.
Video frame buffers are usually mapped above physical RAM.
If write allocate were attempted in that memory area, there
could be performance degradation or compatibility problems.
Bits 18–16 of WATMCR control the enabling or disabling of the
three memory ranges as follows:
I
Bit 18: Top-of-Memory Enable bit
0 = disabled (default)
1 = enabled (write allocate can not be performed above Top
of Memory)
Bit 17: Programmable Range Enable bit
0 = disabled (default)
1 = enabled (write allocate can not be performed in this
range)
Bit 16: Fixed Range Enable bit
0 = disabled (default)
1 = enabled (write allocate can not be performed in this
range)
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