The Art of Assembly Language, 2nd Edition

6.3 Logical (Boolean) Expressions

Consider the following expression from a Pascal program:

b := ((x = y) and (a <= c)) or ((z - a) <> 5);

b is a boolean variable and the remaining variables are all integers.

How do we represent boolean variables in assembly language? Although it takes only a single bit to represent a boolean value, most assembly language programmers allocate a whole byte or word for this purpose (thus, HLA also allocates a whole byte for a boolean variable). With a byte, there are 256 possible values we can use to represent the two values true and false. So which two values (or which two sets of values) do we use to represent these boolean values? Because of the machine's architecture, it's much easier to test for conditions like ...