What does 0fh mean?

assembly binary bitwise-and
user2149780 picture user2149780 · Jul 9, 2013 · Viewed 9.6k times · Source

I am trying to do 

and eax, 0fh

and I know 0fh means 15. From 

and eax, 0fh

I am supposed to clear all but the last 4 bits of eax. I don't understand how you get (32-4)=28 bits of 1 and 4 0s.

Answer

Matteo Italia picture Matteo Italia · Jul 9, 2013

15 in binary is 1111 (in facts 15=8+4+2+1); adding the padding zeroes to reach 32 bits (the width of eax) we get

00000000000000000000000000001111

Now, if you do a binary and with the content of eax, obviously you clear all the bits of eax but the last 4, since the zeroes "kill" whatever digit is in the corresponding position, while the ones let it "survive".

  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx AND
  00000000000000000000000000001111
----------------------------------------
  0000000000000000000000000000xxxx

Related questions

Converting very simple ARM instructions to binary/hex

I've been trying to use this page as well as various other guides to figure out how to express very simple ARM instructions as binary and hex. It seems like it should be a straightforward process to me, but I …

Read More
Floating Point Modulo Operation

I am trying to implement the range reduction operation for trigonometry. But instead I think it might be better to just perform a modulo pi/2 operation on incoming data. I was wondering what algorithms exist and are efficient for this …

Read More
How does binary translate to hardware?

I understand how the code is compiled to assembly, and that assembly is a 1:1 replacement with binary codes. Can somebody help me understand how binary is connected to the hardware? How is the binary physically read and run? How does …

Read More