Question about the PUSH instruction

Question about the PUSH instruction

Portrait de millersd
millersd
mer, 30/04/2008 - 15:55

Looking through 3 versions of the references, I can't seem to find a clear description of the semantics that covers all forms of the instruction. Vol. 2B says that "the operand-size attribute of the current code segment determines the amount the stack pointer is decremented (2, 4, or 8 bytes)." Yet, the description of pushing immediate values states, "If the source operand is an immediate and its size is less than the address size of the stack, a sign-extended value is pushed on the stack."


Does this imply that the address-size attributeof the stack segment cannot be larger than the operand-size attribute of the code segment?


The computation of the operand sizes for segment registers other than FS and GS seems a bit ambiguous as well. I have heard that there are quite a few errors in the documentation of PUSH, but have never verified this, nor have I found the corrections anywhere.


Working on a code analyzer, so I now need to hammer this out. Does anyone know of a complete, consistent, and correct description of PUSH?


Thanks for any help.

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Portrait de Shih Kuo (Intel)
Shih Kuo (Intel)
ven, 02/05/2008 - 16:04

Perhaps you were expecting the elements that influence the behavior differences between different flavors of PUSH should be intuitive and do not have branchy behavior.


Basically, the operand size of a given instruction (on a per opcode basis, vs. per mnemonic) can be opcode-specific. However, the organization of the instruction reference pages are either per-mnemonic or grouping several mnemonic with the same general processing capabilities. It is quite often that the operand size of a given instruction is associated with the default osize attribute of the current code segment but there are several nuances that deviates from that generality and making things a bit complicated.


If you look at the operation section, you'll see the somewhat complicated if/else structure illustrating the mode-specific behavior with a common pattern, the size granularity of data that are push/pop'ed either follow the default or at a different size via the use of Osize override prefix 66H. So in 64-bit segment, you have 8 byte (default) vs. 2 byte; in 32-bit segment you have 4 bytes (default) vs. 2 byte; in 16-bit segment, you have 2 byte (default) vs. 4 byte.


The immediate flavor of PUSH adds a small twist (the operand size of the instruction can be different from the number bytes that are encoded as a immediate) to these behavior so that PUSH and POP can work together with either 64-bit, 32-bit, or 16-bit segments.Since the number of bytes being encoded in a PUSH instruction can be smaller than the data size that gets push on the stack, Sign-extension is applied to make up the difference is size.

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Portrait de millersd
millersd
mar, 13/05/2008 - 07:56

Could you clarify the role of the address-size attribute of the stack segment in the operation of the push-immediate flavor?

The reference says that immediates are sign-extended if they are smaller than the stack-segment address-size attribute...does this imply that they are sign-extended to the stack-segment address size?

The reference says that the stack pointer is decremented based on the operand size attribute of the code segment, so I hope that either the above is false, or there is a mechanism to prevent the stack-segment address size from being larger than the code segment operand size...

Thanks in advance

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