SH7721 datasheet(203/1524 Pages) RENESAS

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Section 3 DSP Operating Unit

Rev. 3.00 Jan. 18, 2008 Page 141 of 1458

REJ09B0033-0300

3.5.9

Most Significant Bit Detection Operation

The PDMSB, most significant bit detection operation, is used to calculate the shift amount for

normalization. Figure 3.19 shows the PDMSB operation flow and table 3.28 shows the operation

definition. Table 3.29 shows the possible variations of this type of operation. The correspondence

between each operand and registers is the same as for ALU fixed-point operations, as shown in

table 3.21.

Note:

The result of the MSB detection operation is basically 24 bits as well as ALU integer

operation, the upper 16 bits of the base precision and eight bits of the guard-bit parts.

When a register not providing the guard-bit parts is specified as a destination operand, the

upper word of the operation result is input into the destination register.

As shown in figure 3.19, the PDMSB operation uses all bits as a source operand, but the

destination operand is treated as an integer operation result because shift amount data for

normalization should be integer data as described in section 3.5.8, Shift Operations. These

operations are executed in the DSP stage, as shown in figure 3.10. The DSP stage is the same

stage as the MA stage in which memory access is performed.

Every time a PDMSB operation is executed, the DC, N, Z, V, and GT bits in DSR are basically

updated in accordance with the operation result. In case of a conditional operation, they are not

updated, even though the specified condition is true, and the operation is executed. In case of an

unconditional operation, they are always updated with the operation result.

Cleared to 0

Priority encoder

Source 1 or 2

DSR

Guard

Figure 3.19 PDMSB Operation Flow

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