SH7720 Datasheet, PDF (198/1524 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer SuperHTM RISC engine Family / SH7700 Series

Section 3 DSP Operating Unit
Table 3.26 Correspondence between Operands and Registers
Register
Se
Sf
Dg
A0


Yes
A1
Yes
Yes
Yes
M0


Yes
M1


Yes
X0
Yes
Yes

X1
Yes


Y0
Yes
Yes

Y1

Yes

Note: The multiply operations basically generate 32-bit operation results. So when a register
providing the guard-bit parts are specified as a destination operand, the guard-bit parts will
copy bit 31 of the operation result.
The multiply operation of the DSP unit side is not integer but fixed-point arithmetic operation. So,
the upper words of each multiplier and multiplicand are input into a MAC unit as shown in figure
3.16. In the SH’s standard multiply operations, the lower words of both source operands are input
into a MAC unit. The operation result is also different from the SH’s case. The SH’s multiply
operation result is aligned to the LSB of the destination, but the fixed-point multiply operation
result is aligned to the MSB, so that the LSB of the fixed-point multiply operation result is always
0.
The fixed-point multiply operation is executed in one cycle. Multiply is always unconditional, but
does not affect any condition code bits, DC, N, Z, V, and GT , in DSR.
• Overflow Protection
The S bit in SR is effective for this multiply operation in the DSP unit. See section 3.5.11,
Overflow Protection, for details.
If the S bit is 0, overflow occurs only when H'8000*H'8000 ((-1.0)*(-1.0)) operation is
executed as signed fixed-point multiply. The result is H'00 8000 0000 but it does not mean
(+1.0). If the S bit is 1, overflow is prevented and the result is H'00 7FFF FFFF.
Rev. 3.00 Jan. 18, 2008 Page 136 of 1458
REJ09B0033-0300