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Index > Compiler Internals > FASM sign-extends current address (why that's bad)

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PopeInnocent



Joined: 01 Jan 2004
Posts: 18
Location: USA
PopeInnocent
My apologies if this has already been addressed.

The following

Code:
org 80000000h - 8

dq $
dq $    


generates the following qwords:

Code:
000000007ffffff8
ffffffff80000000    


It appears that FASM sign-extended the current address. This could lead to subtle bugs, breaks code that was written for the lower 2GB of address space when that code is assembled for the top half of memory, and it seems like it wouldn't work well for x86-64.

For instance, the construct:

Code:
db foo shr 24    


Where foo is a label, works just fine when the foo is below 2GB, but generates an error when foo is above 2GB.

Priv, do you plan on changing this feature?
Post 09 Mar 2004, 03:24
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7532
Location: Kraków, Poland
Tomasz Grysztar
This is only because current fasm versions store "org" base address as 32-bit value. This has of course to be changed in the fasm version with 64-bit code generation. And with 32-bit code it shouldn't cause a problems, as you usually don't use 64-bit addresses in this mode.
Post 09 Mar 2004, 08:44
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