參數(shù)資料
型號(hào)： 21326
英文描述： Implementation of Write Allocate Application Note
中文描述： 在寫分配應(yīng)用執(zhí)行情況的說明
文件頁(yè)數(shù)： 15/22頁(yè)
文件大小： 241K
代理商： 21326
Programming Details
7
21326F/0—February 1999
Implementation of Write Allocate in the K86 Processors
means of one of the other write allocate mechanisms—“Write
to a Cacheable Page” and “Write to a Sector” (for more
information, see the “Cache Organization” chapter in the
AMD-K6
-2 Processor Data Sheet
, order# 21850 or
AMD-K6
-III
Processor Data Sheet
, order# 21918). The maximum value of this
limit is ((2
10
–1) · 4 Mbytes) = 4092 Mbytes. When all the bits in
this field are set to 0, all memory is above this limit and the
write allocate mechanism is disabled (even if all bits in the
WAELIM field are set to 0, write allocates can still occur due to
the “Write to a Cacheable Page” and “Write to a Sector”
mechanisms).
Once the BIOS determines the amount of RAM installed in the
system, this number should also be used to program the
WAELIM field. For example, a system with 32 Mbytes of RAM
would program the WAELIM field with the value
00_0000_1000b. This value (8), when multiplied by 4 Mbytes,
yields 32 Mbytes as the write allocate limit.
Write Allocate Enable 15-to-16-Mbyte.
The WAE15M bit is used to
enable write allocations for memory write cycles that address
the 1 Mbyte of memory between 15 Mbytes and 16 Mbytes. This
bit must be set to 1 to allow write allocates in this memory area.
This sub-mechanism of the WAELIM provides a memory hole to
prevent write allocates. This memory hole is provided to
account for a small number of uncommon memory-mapped I/O
adapters that use this particular memory address space. If the
system contains one of these peripherals, the bit should be set
to 0 (even if the WAE15M bit is set to 0, write allocates can still
occur between 15 Mbytes and 16 Mbytes due to the “Write to a
Cacheable Page” and “Write to a Sector” mechanisms). The
WAE15M bit is ignored if the value in the WAELIM field is set
to less than 16 Mbytes.
By definition, write allocations are not performed in the
memory area between 640 Kbytes and 1 Mbyte unless the
processor determines a pending write cycle is cacheable by
means of “Write to a Cacheable Page” or “Write to a Sector.” It
is not safe to perform write allocations between 640 Kbytes and
1 Mbyte (000A_0000h to 000F_FFFFh) because it is considered
a noncacheable region of memory. Additionally, if a memory
region is defined as write-combinable or uncacheable by a
Memory Type Range Register (MTRR), write allocates are not
performed in that region.
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