Chapter 9: Comparison

Overview

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 1=True.

Since the logical AND operation has already been discussed, let us start with the TEST instruction, which has a similar functionality.