IRET Pseudo-code Bug

[Philip_S_4]

Hi,

I believe that there is a documentation bug in the pseudo-code for the IRET instruction in the current edition of Volume 2A of the Architectures Software Developers' Manual.

The case we're looking at is using IRET to switch from Ring-0 to Ring-3.

The prose for protected mode states:

If the return is to another privilege level, the IRET
instruction also pops the stack pointer and SS from the stack, before resuming program execution. If the return is
to virtual-8086 mode, the processor also pops the data segment registers from the stack.

However, the flow through the pseudo-code is:

1. GOTO PROTECTED-MODE (because we're in protected mode, not in long mode, and not in virtual 8086 mode)
2. GOTO PROTECTED-MODE-RETURN (because we're not in a nested task, and not switching to virtual 8086 mode)
3. GOTO RETURN-OUTER-PRIVILEGE-LEVEL (because we're switching to a higher privilege level) - note, this appears to be a typo, it should be GOTO RETURN-TO-OUTER-PRIVILEGE-LEVEL

None of the code on this path restores SS or ESP, despite the prose indicating that it should.

I believe that the prose is correct in this case because that would mirror the behaviour of the INT instruction.

[MarkC_Intel]

The IRET pseudo code changed in rev 54. Rev 55 is current. Can you take a quick look and see if your question / comment is still valid? The change log doc for rev 54 highlights the changes if that is helpful.

http://www.intel.com/sdm

[Philip_S_4]

It is the Rev 55 (June 2015) manual that I've been reading.

Is the change log you are referring to the one at https://www-ssl.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developers-manual.pdf? Because I had a scan through that for IRET and can't see any updated pseudo code. So I assume I'm looking in the wrong place.

[MarkC_Intel]

I was referring to the rev 54 change log. Not sure that is still available externally. I always download the changes doc when I get each revision.

Okay, now that I know you are referring to the latest doc, I'll try to discuss this with the relevant parties internally.  

[Philip_S_4]

Hi Mark,

Was anyone internally able to look at this?

Kind regards,

Philip

[MarkC_Intel]

yes. It should be fixed within the next revision or two. Not sure if it'll make the next update due to timing. Thanks for reporting this issue.

[Philip_S_4]

Thanks!

[MarkC_Intel]

revision 56 of the SDM is out with this change. http://www.intel.com/sdm