Art of Assembly Language, 1st Edition

5.3. 5.3 Saving the State of the Machine

Take a look at the program in Example 5-2. This section of code attempts to print 20 lines of 40 spaces and an asterisk. Unfortunately, there is a subtle bug that creates an infinite loop. The main program uses the repeat..until loop to call PrintSpaces 20 times. PrintSpaces uses ECX to count off the 40 spaces it prints. PrintSpaces returns with ECX containing zero. The main program then prints an asterisk, calls a newline, decrements ECX, and then repeats because ECX isn't zero (it will always contain $FFFF_FFFF at this point).

The problem here is that the PrintSpaces subroutine doesn't preserve the ECX register. Preserving a register means you save it upon entry into the subroutine and restore it before ...