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Motorola DSP96002 User Manual

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C-24

DSP96002 USER’S MANUAL

MOTOROLA

positions (3 bit shift).

The exponent comparator and update unit consists of an 11 bit subtracter, which compares the two expo-

nents of the floating point operands, and delivers the difference to the barrel shifter for mantissa alignment.

The largest of the two exponents is delivered to the exponent update unit. The exponent update unit may

update this exponent for normalization of the result, after which the exponent (biased) is stored in the high

portion of the destination register. This is depicted in Figure C-22.

For example, if the mantissa of the first operand in a floating point addition is 1.010...0, with biased exponent

of 10, and the mantissa of the second operand is 1.000...0000, with biased exponent of 13, the exponent

comparator simply delivers the difference (=3) to the barrel shifter, the first operand’s mantissa is aligned to

0.001010...0, the two mantissas are added to deliver 1.001010...0, and the result (biased) exponent equals

13. The postnormalization unit does not need to postnormalize the result in this case.

If the first operand’s mantissa is 1.010...0 with biased exponent of 13 and the second operand’s mantissa

is 1.000...0 with biased exponent of 13, the exponent difference is zero and the barrel shifter does not need

to realign the mantissas. The result after addition is now equal to 10.010...0, which needs to be postnormal-

ized by adding one to the result exponent. The exponent update unit sets the result exponent (biased) equal

to 14 and the result mantissa is 1.0010...0.

Finally, if the first operand’s mantissa in a floating point subtraction is 1.010...0 with biased exponent of 10,

and the second operand’s mantissa is 1.00...0 with a biased exponent of 10, the result mantissa after sub-

traction is -0.010...0. This is not normalized, and the postnormalization unit subtracts two from the exponent.

The result mantissa is -1.000...0 with a biased exponent equal to 8.

Exponent Adder

Multiplier Array

Control

ES1

ES2

MS1

MS2

ED

MD