TMPR4955AF datasheet(79/168 Pages) TOSHIBA

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Chapter 6 TX4955A System Interface

6-7

6.2.6.2

Processor requests

The term “processor request” refers to either a single request or a series of requests

issued via the system interface in order to access external resources. As illustrated in

Figure 6.2.7, there are two types of processor request: read and write.

Figure 6.2.7 Processor Requests

Read requests are requests that read data from main memory or other memory

resources in block double word, partial double word, word, and partial word units.

Write requests are requests that write data to main memory or other system

resources in block, double word, partial double word, word, and partial word units.

Processor requests are managed by the TX4955A processor in the same manner as

the R4000/R4400 non-secondary cache mode.

The processor issues requests strictly according to a sequential method. In other

words, the processor cannot issue the next request while a previous request is on hold.

For example, after issuing a read request, the processor waits for a read response

before issuing the next request. The processor only issues write requests when there

are no read requests on hold.

When using processor input signals RdRdy* and WrRdy*, the external agent can

control the processor request flow. RdRdy* is the signal that controls the processor

read flow, and WrRdy* controls the processor write request flow. Figure 6.2.8

illustrates the sequence of the processor request cycle.

Figure 6.2.8 Processor Requests

6.2.6.3

Processor read requests

When the processor issues a read request, the external agent must access the

specified resource and return the requested data.

The external agent returns response data for processor read requests so they can be

executed separately from the requests. In other words, the external agent can start an

external request before returning response data for the processor read request. A

processor read request is complete when the final word of the response data is received

from the external agent.

Depending on the data identifier combined with the response data, an error in the

response data may be pointed out. The processor would then treat this error as a bus

error.

If data have not been returned to the issued processor read request, the applicable

request is said to be “on hold.” This state continues until the requested read data are

returned.

Processor Request

・ Read

・ Write

TX4955A

External agent

1. Processor issues read/

write requests.

TX4955A

External agent

2. External system controls

acceptance of requests

by asserting either

RdRdy* or WrRdy*.

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