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So far our programs have been quite boring. There are few interesting things to be done with unconditional jumps. To start doing some control flow, we need to learn about flags.

Flags

The register RFlags that we have seen earlier is used to store boolean flags. Each of its bits represent a certain flag. Most arithmetic and logical instructions executed by the CPU alter those flags. Some important flags are:

  • Carry flag (CF), bit 0

  • Parity flag (PF), bit 2

  • Zero flag (ZF), bit 6

  • Sign flag (SF), bit 7

  • Direction flag (DF), bit 10

  • Overflow flag (OF), bit 11

Comparisons and Conditional Jumps

If we use the cmp instruction, for example, like below:

cmp rax, rdx

The subtraction RAX - RDX is executed. If the result is zero, the zero flag ZF is set to 1, otherwise to 0. If the result is negative, the sign flag SF is set to 1, otherwise to 0. The instruction sub (for subtraction) does this as well, but the main difference is that cmp does not alter the operands (RAX and RDX in our example).

Based on these flags, we can issue a conditional jump instruction. For example:

cmp rax, 10
je location_to_jump

The opcode je means "jumps if equal". This code will jump to location_to_jump if RAX has value 10. What je actually does is to check the ZF flag, and jump if ZF is 1. Some other conditional jumps are: jne (not equal), jg (greater), jge (greater or equal), jl (less), jle (less or equal).

But there are many more flag-dependent suffixes that can be used for conditional jumps and other conditional instructions. See also the example with the overflow flag (overflow.asm). Some more information on this can be found here.