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參數(shù)資料
| 型號： | AM486DE2 |
| 英文描述： | Am486DE2 Microprocessor Data Sheet |
| 中文描述： | Am486DE2微處理器數(shù)據(jù)表 |
| 文件頁數(shù)： | 33/52頁 |
| 文件大?。?/td> | 1242K |
| 代理商： | AM486DE2 |
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Am486DE2 Microprocessor
33
A20M Pin
Systems based on the MS-DOS operating system con-
tain a feature that enables the CPU address bit A20 to
be forced to 0. This limits physical memory to a maxi-
mum of 1 Mbyte, and is provided to ensure compatibility
with those programs that relied on the physical address
wraparound functionality of the original IBM PC. The
A20M pin on Am486DE2 CPUs provides this function.
When A20M is active, all external bus cycles drive A20
Low, and all internal cache accesses are performed with
A20 Low.
The A20M pin is recognized while the CPU is in SMM.
The functionality of the A20M input must be recognized
in two instances:
1. If the SMI handler needs to access system memory
space above 1 Mbyte (for example, when saving
memory to disk for a zero-volt suspend), the A20M
pin must be deasserted before the memory above 1
Mbyte is addressed.
2. If SMRAM has been relocated to address space
above 1 Mbyte and A20M is active upon entering
SMM, the CPU attempts to access SMRAM at the
relocated address, but with A20 Low. This could
cause the system to crash, because there would be
no valid SMM interrupt handler at the accessed
location.
Figure 16. SMM Timing in Systems Using Overlaid Memory Space and Write-Through Mode with Caching
EnabledDuring SMM
Figure 17. SMM Timing in Systems Using Overlaid Memory Space and Write-Through Mode with Caching
DisabledDuring SMM
State
Save
SMI Handler
State
Resume
Normal
Cycle
RSM
SMI
SMIACT
FLUSH
SMI
Instruction x
Instruction x+1
Cache contents
invalidated
Cache contents
invalidated
State
Save
SMI Handler
State
Resume
Normal
Cycle
RSM
SMI
SMIACT
FLUSH
SMI
Instruction x
Instruction x+1
Cache contents
invalidated
KEN
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