Programming Forums - View Single Post - What's the difference between mov and movl?

niteice · Nov 23rd, 2006, 9:36 AM

In GAS syntax, instruction sizes are usually explicitly defined. You use movl here because you're dealing with 32-bit values:

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movl %eax, %ebx

However, you would use movw for a different size:

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movw %ax, %bx

Generally, the instruction size can be inferred, but suppose you have a variable:

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mov $someVar, %edx

How does GAS know the size?

In NASM syntax, however, instruction size is inferred by the operands, so you deal with a syntax that is a lot closer to the machine code.

Nov 23rd, 2006, 9:36 AM	#4
niteice Programmer Join Date: Aug 2005 Posts: 98 Rep Power: 4	In GAS syntax, instruction sizes are usually explicitly defined. You use movl here because you're dealing with 32-bit values: (Toggle Plain Text) movl %eax, %ebx movl %eax, %ebx However, you would use movw for a different size: (Toggle Plain Text) movw %ax, %bx movw %ax, %bx Generally, the instruction size can be inferred, but suppose you have a variable: (Toggle Plain Text) mov $someVar, %edx mov $someVar, %edx How does GAS know the size? In NASM syntax, however, instruction size is inferred by the operands, so you deal with a syntax that is a lot closer to the machine code.