x86 - double condition checking in assembly

Question

i'm beginning assembly, i'm using nasm for assembling the code, i'm trying to process a string residing in memory and change it, i want to check if a byte is in a certain range (ascii) so i can decide what to do with it, i can't seem to figure how to check if a value is in a certain range, i know all about the different kind of jump flags but how can i combine 2 cmp statements ?

my question is : how do i produce something similiar to this in assembly ?

if (x>=20 && x<=100)
     do something

thanks alot !

Answer 1

There is a way to express a range check like this using only a single conditional jump:

     sub  eax,  20
     cmp  eax,  80
     ja   END
     // do something
END: ret

This is a very common optimization trick when working with integer ranges. The initial subtract maps the range [20,100] to [0,80]; membership in that range is then be checked with a single unsigned comparison.

Note also that the same thing can be done in C:

unsigned int upperBound = 100;
unsigned int lowerBound = 20;
if (yourValue - lowerBound <= upperBound - lowerBound) {
    // do something
}