double condition checking in assembly

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 开发者_开发问答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 !

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
}

Depending on what syntax you're using, and assuming x is in the eax register, something like this:

cmp  eax, 20
jl   ELSE
cmp  eax, 100
jg   ELSE
#do something
jmp  END

ELSE:
#do else

END:

You can try compiling it from a higher level language (C/C++/ ...) with optimalisations at high (-O3 for gcc), and have a look at what the compiler generates (objdump). It should generate very efficient code.

Like that?

x<20
if false jump to ELSE
x>100
if false jump to ELSE
  do something
  jump to ENDIF
:ELSE
  do something else
:ENDIF

Or do you mean using only one assembly instruction?