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[/] [zipcpu/] [trunk/] [sw/] [lib/] [mpy32s.S] - Rev 52
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; We could build mul32s (32-bit signed multiply) as
mpy32s:
PUSH R2
CLR R2 ; Keep track of resulting sign in R2
TST R0 ; Is R0 negative?
XOR.LT #1,R2 ; If so, resulting sign will be negative, and
NEG.NZ R0 ; then we negate R0 (R0 = ABS(R0))
TST R1 ; Is R1 negative?
XOR.LT #1,R2 ; If so, result will be opposite sign of before
TST R1 ; Need to retest since xor modified flags
NEG.LT R1 ; Now we get R1=ABS(R1)
JSR mpy32u ; Do our unsigned multiply
CMP R2 ; Check resulting sign
BZ ret_mul32s ; If positive, do nothing more
NOT R0 ; If negative, negate the result
NOT R1
ADD $1,R1
ADD.C $1,R0
ret_mul32s:
POP R2
RET
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