SH7641 Datasheet, PDF (152/1036 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer SuperH™ RISC engine Family / SH7641 Series

Section 3 DSP Operation
Negative Value Mode: CS[2:0] = 001: The DC flag indicates the same state as the MSB of the
operation result. When the result is a negative number, the DC bit shows 1. When it is 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 3.4.
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 3.4 DC Bit Generation Examples in Negative Value Mode
Zero Value Mode: CS[2:0] = 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.
Overflow Mode: CS[2:0] = 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, the DC bit always indicates
the result of when no guard bits are provided. So, the DC bit is always set if the guard-bit parts are
used for large number representation. Some DC bit generation examples in overflow mode are
shown in figure 3.5.
Example 1
Guard bits
1111 1111 1111 1111 1111 1111
+) _{1111 1111 1000 0000 0000 0000}
1111 1111 0111 1111 1111 1111
Overflow detecting field
Overflow case
Example 2
Guard bits
1111 1111 1111 1111 1111 1111
+) _{1111 1111 1000 0000 0000 0001}
1111 1111 1000 0000 0000 0000
Overflow detecting field
Non overflow case
Figure 3.5 DC Bit Generation Examples in Overflow Mode
Rev. 4.00 Sep. 14, 2005 Page 102 of 982
REJ09B0023-0400