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(nasm.info)Section B.4.95

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B.4.95. `FPREM', `FPREM1': Floating-Point Partial Remainder
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     FPREM                         ; D9 F8                [8086,FPU]
     FPREM1                        ; D9 F5                [386,FPU]

   These instructions both produce the remainder obtained by dividing
`ST0' by `ST1'. This is calculated, notionally, by dividing `ST0' by
`ST1', rounding the result to an integer, multiplying by `ST1' again,
and computing the value which would need to be added back on to the
result to get back to the original value in `ST0'.

   The two instructions differ in the way the notional round-to-integer
operation is performed. `FPREM' does it by rounding towards zero, so
that the remainder it returns always has the same sign as the original
value in `ST0'; `FPREM1' does it by rounding to the nearest integer, so
that the remainder always has at most half the magnitude of `ST1'.

   Both instructions calculate _partial_ remainders, meaning that they
may not manage to provide the final result, but might leave intermediate
results in `ST0' instead. If this happens, they will set the C2 flag in
the FPU status word; therefore, to calculate a remainder, you should
repeatedly execute `FPREM' or `FPREM1' until C2 becomes clear.