AMD-K6-2ESLASH233AMZ datasheet(224/332 Pages) AMD

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206

Write Merge Buffer

Chapter 9

AMD-K6™-2E Processor Data Sheet

22529B/0—January 2000

Preliminary Information

The EWBE control (EWBEC) bits in the EFER register provide

a mechanism for enforcing three different levels of write

ordering in the presence of the write merge buffer:

EFER[3]

defined

the

Global

EWBE#

Disable

(GEWBED). When GEWBED equals 1, the processor does

not attempt to enforce any write ordering internally or

externally (the EWBE# signal is ignored). This is the

maximum performance setting.

EFER[2] is defined as the Speculative EWBE# Disable

(SEWBED). SEWBED only affects the processor when

GEWBED equals 0. If GEWBED equals 0 and SEWBED

equals 1, the processor enforces strong ordering for all

internal write cycles with the exception of write cycles

addressed to a range of memory defined as uncacheable

(UC) or write-combining (WC) by the MTRRs. In addition,

the processor samples the EWBE# signal. If EWBE# is

sampled negated, the processor delays the commitment of

write cycles to processor cache lines in the Modified state or

Exclusive state until EWBE# is sampled asserted.

This setting provides performance comparable to, but

slightly

less

than,

the

performance

obtained

when

GEWBED equals 1 because some degree of write ordering is

maintained.

If GEWBED equals 0 and SEWBED equals 0, the processor

enforces strong ordering for all internal and external write

cycles. In this setting, the processor assumes, or speculates,

that strong order must be maintained between writes to the

merge buffer and writes that hit the processor’s cache. Once

the merge buffer is written out to the processor’s bus, the

EWBE# signal is sampled. If EWBE# is sampled negated, the

processor delays the commitment of write cycles to

processor cache lines in the Modified state or Exclusive

state until EWBE# is sampled asserted.

This setting is the default after RESET and provides the

lowest performance of the three settings because full write

ordering is maintained.

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