AM486DX2 datasheet(21/67 Pages) AMD

21 / 67 page

Am5X86 Microprocessor

AMD

PRELIMINARY

If the PWT signal is 0, the external WB/WT signal de-

termines the new state of the line. If the WB/WT signal

was asserted to 1 during reload, the line transits to the

exclusive state. If the WB/WT signal was 0, the line

transits to the shared state. If the PWT signal is 1, it

overrides the WB/WT signal, forcing the line into the

shared state. Therefore, if paging is enabled, the soft-

ware programmed PWT bit can override the hardware

signal WB/WT.

Until the line is reallocated, a write is the only processor

action that can change the state of the line. If the write

occurs to a line in the exclusive state, the data is simply

written into the cache and the line state is changed to

modified. The modified state indicates that the contents

of the line require copy-back to the main memory before

the line is reallocated.

If the write occurs to a line in the shared state, the cache

performs a write of the data on the external bus to update

the external memory. The line remains in the shared

state until it is replaced with a new cache line or until it

is flushed. In the modified state, the processor continues

to write the line without any further external actions or

state transitions.

If the PWT or PCD bits are changed for a specified mem-

ory location, the tag bits in the cache are assumed to

be correct. To avoid memory inconsistencies with re-

spect to cacheability and write status, a cache copy-

back and invalidation should be invoked either by using

the WBINVD instruction or asserting the FLUSH signal.

4.8.2

Snooping Actions and State Transitions

To maintain cache coherency, the CPU must allow

snooping by the current bus master. The bus master

initiates a snoop cycle to check whether an address is

cached in the internal cache of the microprocessor. A

snoop cycle differs from any other cycle in that it is ini-

tiated externally to the microprocessor, and the signal

for beginning the cycle is EADS instead of ADS. The

address bus of the microprocessor is bidirectional to

allow the address of the snoop to be driven by the sys-

tem. A snoop access can begin during any hold state:

s While HOLD and HLDA are asserted

s While BOFF is asserted

s While AHOLD is asserted

In the clock in which EADS is asserted, the micropro-

cessor samples the INV input to qualify the type of in-

quiry. INV specifies whether the line (if found) must be

invalidated (i.e., the MESI status changes to Invalid or

I). A line is invalidated if the snoop access was generated

due to a write of another bus master. This is indicated

by INV set to 1. In the case of a read, the line does not

have to be invalidated, which is indicated by INV set to 0.

The core system logic can generate EADS by watching

the ADS from the current bus master, and INV by watch-

ing the W/R signal. The microprocessor compares the

address of the snoop request with addresses of lines in

the cache and of any line in the copy-back buffer waiting

to be transferred on the bus. It does not, however, com-

pare with the address of write-miss data in the write

buffers. Two clock cycles after sampling EADS, the mi-

croprocessor drives the results of the snoop on the HITM

pin. If HITM is active, the line was found in the modified

state; if inactive, the line was in the exclusive or shared

state, or was not found.

Figure 2 shows a diagram of the state transitions in-

duced by snooping accesses.

4.8.2.1

Difference between Snooping

Access Cases

Snooping accesses are external accesses to the micro-

processor. As described earlier, the snooping logic has

a set of signals independent from the processor-related

signals. Those signals are:

s EADS

s INV

s HITM

In addition to these signals, the address bus is required

as an input. This is achieved by setting AHOLD, HOLD,

or BOFF active.

Snooping can occur in parallel with a processor-initiated

access that has already been started. The two accesses

depend on each other only when a modified line is writ-

ten back. In this case, the snoop requires the use of the

cycle control signals and the data bus. The following

sections describe the scenarios for the HOLD, AHOLD,

and BOFF implementations.

Figure 2. Snooping State Transitions

Invalid

Modified

Exclusive

Shared

(HITM asserted

+ write-back)

(EADS = 0 * INV = 1)

+ FLUSH = 0

(EADS = 0 * INV = 1)

+ FLUSH = 0

EADS = 0 * INV = 0

* FLUSH = 1

EADS = 0 * INV = 0

* FLUSH = 1

(HITM asserted

+ write-back)

EADS = 0 * INV = 0

* FLUSH = 1

EADS = 0 * INV = 1

+ FLUSH = 0

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