MB86330 Datasheet, PDF (25/46 Pages) Fujitsu Component Limited. – 16-bit Fixed-point DSP

MB86330
s BASIC CONFIGURATION OF THE DATA OPERATION SECTION
1. Data Format
Integral type and fixed-point data can be handled regardless of whether the data is signed or unsigned. For
signed data, the most significant bit indicates a sign. A number of 1 indicates negative data, which is expressed
by 2’s complement. The decimal point for fixed-point type data is located between the sign bit (bit 14) and its
right bit (bit 15). When the accumulator value resulting from execution of a multiplication instruction is transferred
to a 16-bit length register or memory, the decimal point is returned to the original position (between bits 14 and 15).
• Fixed-point type, signed data
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
bit
S
Decimal part (2’s complement)
Sign Decimal point
• Fixed-point type, unsigned data
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
bit
Decimal part
Decimal point
• Integral-type, signed data
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
bit
S
Integral part (2’s complement)
Sign
• Integral-type, unsigned data
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
bit
Integral part
Since a value is handled in 2’s complement format for addition and subtraction, no distinction is made in the
result of an operation by the above four data format. Since, for multiplication, the result of an operation varies
with whether data is signed or unsigned, three combinations of “signed data x signed data,” “signed data ×
unsigned data,” and “unsigned data × unsigned data” exist for multiplication instructions.
For the fixed-point type and the integral type, the result of multiplication varies with the decimal point position.
When fixed-point type data is multiplied, the result of the operation is stored into the accumulator with one bit
shifted to the left in comparison with integral type data. The “ITG” bit in the status register is used to switch
between the fixed-point type and the integral type. With this bit set at 0, an operation is executed in the fixed-
point type format.
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