SH7720 Datasheet, PDF (191/1524 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer SuperHTM RISC engine Family / SH7700 Series

Section 3 DSP Operating Unit
(2) Negative Value Mode: CS[2:0] = B'001
The DC flag indicates the same value as the MSB of the operation result. When the result is a
negative number, the DC bit shows 1. When it is 0 or a positive number, the DC bit shows 0. The
ALU always executes 40-bit arithmetic operation, so the sign bit to detect whether positive or
negative is always got from the MSB of the operation result regardless of the destination operand.
Some examples are shown in figure 3.12.
Example 1
Guard bits
1100 0000 0000 0000 0000 0000
+) 0000 0000 0000 0000 0000 0001
1100 0000 0000 0000 0000 0001
Sign bit
Negative value
Example 2
Guard bits
0011 0000 0000 0000 0000 0000
+) 0000 0000 1000 0000 0000 0001
0011 0000 1000 0000 0000 0001
Sign bit
Positive value
Figure 3.12 DC Bit Generation Examples in Negative Value Mode
(3) Zero Value Mode: CS[2:0] = B'010
The DC flag indicates whether the operation result is 0 or not. When the result is 0, the DC bit
shows 1. When it is not 0, the DC bit shows 0.
(4) Overflow Mode: CS[2:0] = B'011
The DC bit indicates whether or not overflow occurs in the result. When an operation yields a
result beyond the range of the destination register, except the guard-bit parts, the DC bit is set.
Even though guard bits are provided in the destination register, the DC bit always indicates the
result of when no guard bits are provided. So, the DC bit is always set to 1 if the guard-bit parts
are used for large number representation. Some DC bit generation examples in overflow mode are
shown in figure 3.13.
Example 1
Guard bits
111111111111111111111111
+) 111111111000 0000 0000 0000
111111110111111111111111
Overflow detecting field
Overflow case
Example 2
Guard bits
111111111111111111111111
+) 111111111000 0000 0000 0001
111111111000 0000 0000 0000
Overflow detecting field
Non overflow case
Figure 3.13 DC Bit Generation Examples in Overflow Mode
Rev. 3.00 Jan. 18, 2008 Page 129 of 1458
REJ09B0033-0300