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Index > Compiler Internals > cmp rax, 0xffffffff

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RIxRIpt



Joined: 18 Apr 2013
Posts: 50
RIxRIpt
Code:
flat assembler  version 1.71.39  (1048576 kilobytes memory)
myfilename.asm [321]:
                                cmp rax, 0xffffffff
error: value out of range.
    

Why?
It's possible to assemble this in x64 dbg and the hexcode is:
Code:
48 3D FF FF FF FF        | cmp rax,FFFFFFFF
    

Bug?

Update: stepping the instruction above in the debugger results in ZF=0 when RAX=00000000FFFFFFFF.
What's wrong with it?

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Post 10 Jun 2015, 20:40
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l_inc



Joined: 23 Oct 2009
Posts: 881
l_inc
RIxRIpt
Quote:
It's possible to assemble this in x64 dbg and the hexcode is

x64dbg assembles this incorrectly. The immediate is sign-extended and the comparison is therefore made with 0xffffffffffffffff, not with 0xffffffff.

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Post 10 Jun 2015, 23:39
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17271
Location: In your JS exploiting you and your system
revolution
Values between -2^31 (0xffffffff80000000, stored as 0x80000000) and +2^31-1 (0x000000007fffffff, stored as 0x7fffffff) are encodable. Your number is 2^32-1 (0x00000000ffffffff) and cannot be encoded, because, as l_inc says, sign extending a stored value of 0xffffffff gives an absolute value 2^64-1 (0xffffffffffffffff).
Post 11 Jun 2015, 00:56
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shutdownall



Joined: 02 Apr 2010
Posts: 518
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shutdownall
When you use explicitly declaration you will get a more detailed error message. Wink

Code:
cmp rax, qword 0xffffffff    


throws

"Error: not encodable with long immediate"

instead of just

"Error: value out of range"

It is possible to use

Code:
mov rbx,0xffffffff
cmp rax,rbx    


instead.
Post 11 Jun 2015, 08:44
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17271
Location: In your JS exploiting you and your system
revolution
shutdownall wrote:
When you use explicitly declaration you will get a more detailed error message. Wink

Code:
cmp rax, qword 0xffffffff    


throws

"Error: not encodable with long immediate"

instead of just

"Error: value out of range".
Well actually that error is for all values:
Code:
cmp rax,qword 0
error: not encodable with long immediate.    
Post 11 Jun 2015, 08:48
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shutdownall



Joined: 02 Apr 2010
Posts: 518
Location: Munich
shutdownall
revolution wrote:
Well actually that error is for all values:
Code:
cmp rax,qword 0
error: not encodable with long immediate.    


Yes, but if you omit the manual type conversion it is just "value out of range" for all values.
I think the "not encodable" message is more clear. Cool
Post 11 Jun 2015, 08:50
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17271
Location: In your JS exploiting you and your system
revolution
shutdownall wrote:
Yes, but if you omit the manual type conversion it is just "value out of range" for all values.
Not quite. It is only "value out of range" for anything outside -2^31 to +2^31-1.
Post 11 Jun 2015, 08:53
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RIxRIpt



Joined: 18 Apr 2013
Posts: 50
RIxRIpt
Thanks for the answers, I think I got it.
That's what I've found:
Code:
;page 622 of Intel's manual
Opcode           Instruction       Op/  64-Bit Compat/   Description
                                   En   Mode   Leg Mode
REX.W + 3D id    CMP RAX, imm32    I    Valid    N.E.    Compare imm32 sign-extended to 64-bits with RAX.
    

and there's no cmp instruction with imm64 operand Confused.
Post 11 Jun 2015, 10:24
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17271
Location: In your JS exploiting you and your system
revolution
RIxRIpt wrote:
and there's no cmp instruction with imm64 operand Confused.
The only imm64 available is MOV reg,imm64.
Post 11 Jun 2015, 10:42
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