SH7721 datasheet(208/1524 Pages) RENESAS

208 / 1524 page

Section 3 DSP Operating Unit

Rev. 3.00 Jan. 18, 2008 Page 146 of 1458

REJ09B0033-0300

3.5.11

Overflow Protection

The S bit in SR is effective for any arithmetic operations executed in the DSP unit, including the

SH’s standard multiply and MAC operations. The S bit in SR is used as the overflow protection

enable bit. The arithmetic operation overflows when the operation result exceeds the range of

two’s complement representation without guard-bit parts. Table 3.31 shows the definition of

overflow protection for fixed-point arithmetic operations, including fixed-point signed by signed

multiplication described in section 3.5.7, Fixed-Point Multiply Operation. Table 3.32 shows the

definition of overflow protection for integer arithmetic operations. The lower word of the

saturation value of the integer arithmetic operation is don’t care. Lower word value cannot be

guaranteed.

When the overflow protection is effective, overflow never occurs. So, the V bit is cleared, and the

DC bit is also cleared when the overflow mode is selected by the CS[2:0] bits.

Table 3.31

Definition of Overflow Protection for Fixed-Point Arithmetic Operations

Sign

Overflow Condition

Fixed Value

Hex Representation

Positive

Result > 1 – 2

–31

1 – 2

–31

H'00 7FFF FFFF

Negative

Result < –1

–1

H'FF 8000 0000

Table 3.32

Definition of Overflow Protection for Integer Arithmetic Operations

Sign

Overflow Condition

Fixed Value

Hex Representation

Positive

Result > 2

15 – 1

00 7FFF ****

Negative

Result < –2

–2

FF 8000 ****

Note:

means don’t care.

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