AMD-K6 datasheet(148/346 Pages) AMD

148 / 346 page

130

Bus Cycles

Chapter 6

AMD-K6® Processor Data Sheet

20695H/0—March 1998

Preliminary Information

Burst Reads and

Pipelined Burst Reads

Figure 47 on page 131 shows normal burst read cycles and a

pipelined burst read cycle. The AMD-K6 processor drives

CACHE# and ADS# together to specify that the current bus

cycle is a burst cycle. If the processor samples KEN# asserted

with the first BRDY#, it performs burst transfers. During the

burst transfers, the system logic must ignore BE[7:0]# and must

return all eight bytes beginning at the starting address the

processor asserts on A[31:3]. Depending on the starting

address, the system logic must determine the successive

quadword addresses (A[4:3]) for each transfer in a burst, as

shown in Table 21. The processor expects the second, third, and

fourth quadwords to occur in the sequences shown in Table 21.

In Figure 47, the processor drives CACHE# throughout all burst

read cycles. In the first burst read cycle, the processor drives

ADS# and CACHE#, then samples BRDY# on every clock edge

starting with the clock edge after the clock edge that negates

ADS#. The processor samples KEN# asserted on the clock edge

on which the first BRDY# is sampled asserted, executes a

32-byte burst read cycle, and expects to sample BRDY# a total

of four times. An ideal no-wait state access is shown in Figure

47, whereas most system logic solutions add wait states between

the transfers.

The second burst read cycle illustrates a similar sequence, but

the processor samples NA# asserted on the same clock edge

that the first BRDY# is sampled asserted. NA# assertion

indicates the system logic is requesting the processor to output

the next address early (also known as a pipeline transfer

request). Without waiting for the current cycle to complete, the

processor drives ADS# and related signals for the next burst

cycle. Pipelining can reduce CPU cycle-to-cycle idle times.

Table 21. A[4:3] Address-Generation Sequence During Bursts

Address Driven By

Processor on A[4:3]

A[4:3] Addresses of Subsequent

Quadwords* Generated By System Logic

Quadword 1

Quadword 2

Quadword 3

Quadword 4

00b

01b

10b

11b

01b

00b

11b

10b

11b

00b

01b

11b

10b

01b

00b

Note:

quadword = 8 bytes

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