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This article mainly introduces "what is the concept of Outstanding in AXI4 bus". In daily operation, I believe that many people have doubts about what the concept of Outstanding in AXI4 bus is. The editor consulted all kinds of data and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the doubt of "what is the concept of Outstanding in AXI4 bus"! Next, please follow the editor to study!

The concept of Outstanding is mentioned in the AXI4 bus specification, so how to understand it? We first give a basic request-response model, as shown in the following figure.

In the following figure, Master sends a read request to Slave. After receiving the request, Slave processes the request and returns the data to Master after processing.

The following introduces the concept of Outstanding combined with the above request-response model. Literally, Outstanding means ongoing and unfinished meaning, and figuratively means "on the road".

Let's first introduce the simplest case, that is, if the Outstanding of the Slave side is 1, the working process of the request-response model in the figure above is as follows:

Mater read request 0 → Slave returns data 0 → Mater read request 1 → Slave returns data 1 → Mater read request 2 → Slave returns data 2 →.

If the Outstanding of the Slave side is N (N > 1), then the Masterside can issue N requests continuously without returning read data from the Slave. If the Slave side returns m pieces of data during this period, the Master side can send m more requests. Figuratively speaking, the maximum number of requests "on the road" on the Masterside is N.

As shown in the following figure, the server contains a Buffer with a depth of 8 to cache the request command, and the Outstanding of the Slave is 8. For this Slave, even if none of the commands are processed, eight commands can be cached. Then the corresponding Master side can issue 8 requests in a row. If the Slave does not return data during this period, then the Master side cannot issue any new requests. If the server reads m request commands from the Buffer and returns the data to the Master during this period, then there is a new space in the Buffer to cache the commands, that is to say, the Master side can continue to send new request commands.

If the outstanding capability is N > 1, then:

1) read operation: you can issue N read address commands in a row. During this period, if the read data is not returned, you need to wait for the read data to be returned. If any read data is returned, several are returned, then you can still send several more. In other words, the read command (or read data) for "on the road" can be at most N. To say a little more, you can see that if the data is returned more slowly, then IP will have to wait and be less efficient, so in order to improve efficiency, it is necessary to improve outstanding capabilities to make up for the delay introduced by the "on the road" (bus). However, it can not be sent indefinitely, otherwise it may cause bus congestion and block other IP. The following figure is an example of a timing diagram for outstanding reading.

2) write operation: you can issue N groups of write address (write data) commands continuously. During this period, if the write response does not return, you must wait for the write response to return before sending the write address (write data) command. If a write response is returned, several are returned, so you can still send several groups. In other words, the write response of "on the road" can be N at most.

At this point, the study of "what is the concept of Outstanding in AXI4 bus" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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