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Journeyman III

What is meant by /0 in the AMD64 specification?

The AMD64 specification talks about /0 with regards to instruction encoding but I don't have a clue what is meant by that. For example, in Volume 3 one variant of the ADD instruction has three forms:

ADD reg/mem16, imm16 81 /0 iw Add imm16 to reg/mem16

ADD reg/mem32, imm32 81 /0 id Add imm32 to reg/mem32.

ADD reg/mem64, imm32 81 /0 id Add sign-extended imm32 to reg/mem64.

These all use the opcode 81 followed by /0 which I presume distinguishes between the three followed by the immediate value iw or id. But what does /0 mean?

8 Replies
Big Boss

Tapejara, I suspect most of us are not familiar with this.  Please post a document link.  Thanks and enjoy, John.


Found this PDF Document on AMD64 coding. Not sure if I found the correct switch in the document. But you can take a look at it if this isn't the document you have already looked at.

I have attached the PDF document to this comment so that you can download it and see if it helps you understand what that "/0" switch means.

I really don't have any experience in CPU coding, so not sure if this is the correct document that can help you.

You can always open an Online AMD Support Request and someone may be able to help you from here : Online Service Request | AMD 

Also not sure if this AMD Forum may be applicable: Devgurus 


Hello elstaci,

Yes. The document is AMD's 24594 and my copy has this appendix. However, the /0 is not Slash-Oh. It is Slash-Zero so it can't be the reference you site. Here is a more recent version than the one I am using. 


Try opening a AMD Support ticket or post your question at Devgurus‌. One of those might be able to answer your question.

Devgurus‌ would be a good start since that involves all types of coding and programming if AMD Support is unable to assist you. 

Meanwhile I will see if I find what the /0 switch means in the CPU codes.

I see they have a /3 switch and it does mention /0 switch in several of the codes in the PDF file. Maybe you can determine by looking at the explanation that involves the /0 switch coding what that switch entails.

Either way, unfortunately, this is as far as I can assist you since I have no experience or technical background in CPU coding.

Possibly another User(s) here will reply that does have experience in this particular coding and could direct you in finding out the information that you require.

Big Boss

I assume you have read page 499, Tapejara?  Is it REX.B (page 494)?  Just a guess.  This is a user forum and to contact AMD, please open a Ticket on AMD Online SUpport site.  Enjoy, John.


misterj: I think that explains it. It wasn't explained in the introductory chapters and I didn't expect the explanation to be hidden at the end in the Appendices. It's very difficult to read this document from beginning to end because the order of information needed seems to be backwards. I come from a Motorola processor background where documentation is so wonderful and find Intelisms very twisted and contorted. I'll try reading the Appendices first. These people have a lot to learn from Motorola about how to write technical documentation as well as how to support their technical users. I have found no places in AMD's forums for people like me. They seem to want me to either be a gamer or multi-$$ partner or just to go away.

elstaci: Thanks for the recommendations. I tried Stack-Exchange first since they have so many different forums. But users tend to be arrogant, dismissive and questioning of your right to even post. Even the forum system seems to pit one against another with their ranking system which doesn't allow you to create your own tags unless you have a high rank among the pack for their alpha wolves. Many questions are so specialized that users ridicule you for posting in the wrong place.


I can understand what you are saying. But Devgurus‌ Users are, I believe, more professional and are people such as yourself involve in a very technical field.

But it seems like you found your answer. Which is great.

Machine language programming is an art in itself and can be very complicated with the newer CPUs.


elstaci: The name Devgurus doesn't hint that it is an AMD forum and it seems to be exactly what I have been looking for. Thanks a bunch! I'll started looking there for future assistance. And thanks to misterj too!