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Re: Objdump pop (8f opcode) misrepresentation? or "Objdump's trying too

From: Alan Modra
Subject: Re: Objdump pop (8f opcode) misrepresentation? or "Objdump's trying too hard"
Date: Thu, 19 Jun 2008 23:07:03 +0930
User-agent: Mutt/1.5.9i

On Thu, Jun 19, 2008 at 08:37:17AM -0400, gdb guy wrote:
> >From the intel spec:
> 8F /0           POP r/m32          N.E.  Valid        Pop top of stack
> into m32; increment stack pointer.(Vol 2B page 4-130)
> Now, the /0 I interpret from Vol 2A page 3-2
> /digit — A digit between 0 and 7 indicates that the ModR/M byte of the
> instruction uses only the r/m (register or memory) operand. The reg
> field contains the digit that provides an extension to the
> instruction's opcode.
> which in this case, since it's /0 would mean it's all the various
> forms which deal with eax on Vol 2A page 2-7.

Nope.  Read over the description again.  /0 is part of the opcode.
The rest of the modrm byte can select registers as well as memory.

Alan Modra
Australia Development Lab, IBM

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