32/64-Bit 80x86 Assembly Language Architecture

Chapter 9. Comparison

Comparisons are typically related to decision making. A C programmer is typically familiar with one of the following:

• An index into a table is calculated by deciding between multiple factors:

  index = (a < b ) ? i : j;

• A comparison is used to decide which branch to take:

  if (a < b )
  {
      dothis();
  }
  else
  {
      dothat();
  }

In assembly code, comparisons actually take on multiple forms.

The general-purpose instructions set the EFLAGS/RFLAGS conditional flags to indicate their scalar result. Additional instructions would then utilize individual flags that are reset (cleared) or set, such as branching and bit shifting/rotation. The instructions utilizing packed data store the result of the comparison as packed Boolean values of 0=False and ...