Question

I am struggling with a problem in assembly, where I have to take the first byte (FF) of the hex code and copy it over the entire value:

0x045893FF      input
0xFFFFFFFF      output

What I did is:

movl $0x04580393FF, %eax
shl $24, %eax     # to get only the last byte 0xFF000000

Now I want to copy this byte into the rest of the register.

Answer 1

You could do it for instance like this:

mov %al, %ah    #0x0458FFFF
mov %ax, %bx    #0xFFFF
shl $16, %eax   #0xFFFF0000
mov %bx, %ax    #0xFFFFFFFF

Another way would be:

movzx %al, %eax
imul $0x1010101, %eax

The last one is possibly faster on modern architectures.