AM486DX2 datasheet(31/67 Pages) AMD

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Am5X86 Microprocessor

AMD

PRELIMINARY

Step 10 In the same clock cycle, the snooping cache

drives HITM back to 1.

Step 11 The write of data A is finished if BRDY transi-

tions to 0 (BLAST = 0), because it is a single

word.

The software write sequence was first data A and then

data B. But on the external bus the data appear first as

data B and then data A. The order of writes is changed.

In most cases, it is unnecessary to strictly maintain the

ordering of writes. However, some cases (for example,

writing to hardware control registers) require writes to

be observed externally in the same order as pro-

grammed. There are two options to ensure serialization

of writes, both of which drive the cache to Write-through

mode:

Set the PWT bit in the page table entries.

Drive the WB/WT signal Low when accessing these

memory locations.

Option 1 is an operating-system-level solution not di-

rectly implemented by user-level code. Option 2, the

hardware solution, is implemented at the system level.

BLAST

Data

BRDY

EADS

ADS

HITM

Cached Data

AHOLD

CLK

Write Buffer

B original

B modified

B+4

B+8

B+12

Ignored

XXX

Note:

The circled numbers in this figure represent the steps in section 4.8.7.1.

Figure 14. Write Cycle Reordering Due to Buffering

Step 3 The next write of the processor hits the cache

and the line is non-shared. Therefore, data B is

written into the cache. The cache line transits

to the modified state.

Step 4 In the same clock cycle, a snoop request to the

same address where data B resides is started

because EADS = 0. The snoop hits a modified

line. EADS is ignored due to the hit of a modified

line, but is detected again as early as in step 10.

Step 5 Two clock cycles after EADS asserts, HITM be-

comes valid.

Step 6 Because the processor-initiated access cannot

be finished (AHOLD is still 1), the BIU gives

priority to a write-back access that does not re-

quire the use of the address bus. Therefore, in

the clock cycle, the cache starts the write-back

sequence indicated by ADS = 0 and W/R = 0.

Step 7 During the write-back sequence, AHOLD is

deasserted.

Step 8 The write-back access is finished when BLAST

and BRDY transition to 0.

Step 9 After the last write-back access, the BIU starts

writing data A from the write buffers. This is

indicated by ADS = 0 and W/R = 0.

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