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| mov command with GAS? https://forum.osdev.org/viewtopic.php?f=13&t=16605 |
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| Author: | junkoi [ Tue Mar 25, 2008 2:52 am ] |
| Post subject: | mov command with GAS? |
Hello, I am writing a simple ASM code with GNU GAS. I am having some problems with mov data. Please could anybody tell me why 2 lines (*) and (**) are not equivalent?? (currently my code works as expected with (**), but not with (*) (program is in 16bit mode) --- var: .long 0xf1234 ... .code16gcc movl var, %ecx // (*) movl $0xf1234, %ecx // (**) Many thanks, J |
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| Author: | JamesM [ Tue Mar 25, 2008 2:58 am ] |
| Post subject: | |
Hi, Code: movl var, %ecx Actually means "take the address of the label 'var', and move it into %ecx." Code: movl $0xf1234, %ecx Actually means "take the constant 0xf1234, and move it into %ecx." To make the first equivalent to the second, you really want a dereference, so Code: movl 0(var), %ecx
Hope this helps. James |
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| Author: | junkoi [ Wed Mar 26, 2008 1:29 am ] |
| Post subject: | |
Hi james, Unfortunately your code is incorrect here, because with GAS "movl var, %ecx" and "movl (var), %ecx" do absolutely the same thing, that is copy the value in var into %ecx. I found the error in my code: that is "var" should be referred to using %cs, not %ds by default. So I fixed my code to smt like this: movl %cs:var, %ecx and it works. Many thanks, J |
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| Author: | JamesM [ Wed Mar 26, 2008 2:46 am ] |
| Post subject: | |
Ah yes, quite. I suggest you put your 'var' variable in the .data section to avoid having to change segment selector. |
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