START

Sign Check

If ( A > 0 ) If ( B > 0 )

If ( A < 0 ) If ( B < 0 )

CH = 0

CL = 1CH = 1

CL = 0

PUSH CX

The first bit of A and B is set to 0as if they were positive number

Original values are restoredat the end

15th bit check

To reduce the number ofiterations we round the number

up by adding 1 to the MSB of thenumber (if this doesn't cause

overflow)

Estimation of Results

Using the invariance propertywe compute an estimated result

A* = TMP_low*B_low + + (TMP_low*B_high + TMP_high*B_low)*2^16 +

+ TMP_high*B_high*2^32

Then we calculate A* by multiplying theobtained value with the second operand

A > A*

S = A - A*A = A* - B

S < BA* = A* + B

END

YN

Y

N

A = A* ENDY

N

R = S

Restoration

Finally we restore the sign of theoperands by popping CX andwe make final controls on the

residual