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(nasm.info)Section 4.5

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4.5. Preprocessor Loops: `%rep'
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   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
     %endrep

   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:

     fibonacci:
     %assign i 0
     %assign j 1
     %rep 100
     %if j > 65535
         %exitrep
     %endif
             dw j
     %assign k j+i
     %assign i j
     %assign j k
     %endrep
     
     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.