4.5. Preprocessor Loops

NASM’s TIMES prefix, though useful, cannot be used to invoke a multi-line macro multiple times, because it is processed by NASM after macros have already been expanded. Therefore NASM provides another form of loop, this time at the preprocessor level: %rep.

The directives %rep and %endrep (%rep takes a numeric argument, which can be an expression; %endrep takes no arguments) can be used to enclose a chunk of code, which is then replicated as many times as specified by the preprocessor:

%assign i 0
%rep    64
        inc     word [table+2*i]
%assign i i+1

This will generate a sequence of 64 INC instructions, incrementing every word of memory from [table] to [table+126].

For more complex termination conditions, or to break out of a repeat loop part way along, you can use the %exitrep directive to terminate the loop, like this:

%assign i 0
%assign j 1
%rep 100
%if j > 65535
        dw j
%assign k j+i
%assign i j
%assign j k

fib_number equ ($-fibonacci)/2

This produces a list of all the Fibonacci numbers that will fit in 16 bits. Note that a maximum repeat count must still be given to %rep. This is to prevent the possibility of NASM getting into an infinite loop in the preprocessor, which (on multitasking or multi-user systems) would typically cause all the system memory to be gradually used up and other applications to start crashing.