rmmovl dynamic number of bits

Question

I know that rmmovl can be used in the following way:

rmmovl %ecx, 4(%edx)

But how can I dynamically set the number of bits to move down the stack (4 in this case)? I've tried setting a variable with the value I want to shift to, such as rmmovl %ecx, %edi(%edx), but this is not working.

Answer 1

You'll have to manually set %edx to contain the offset. We can save the value of %edx on the stack and restore it afterwards, so its original value is not affected.

pushl %edx             # save current value of %edx
addl %edi, %edx        # add %edi to %edx
rmmovl %ecx, (%edx)    # store value of %ecx into %edx with offset %edi
popl %edx              # restore old %edx