HD6417641 RENESAS [Renesas Technology Corp], HD6417641 Datasheet - Page 152

• Manufacturer Part Number: HD6417641
• Manufacturer: RENESAS [Renesas Technology Corp]

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.
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.
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+)
+)
Figure 3.4 DC Bit Generation Examples in Negative Value Mode
1111
1111
1111 1111 0111 1111 1111 1111
1100
0000
1100 0000 0000 0000 0000 0001
Guard bits
Guard bits
Figure 3.5 DC Bit Generation Examples in Overflow Mode
1111
1111
0000
0000
Sign bit
Negative value
Overflow case
Example 1
Example 1
1111
1000
0000
0000
Overflow detecting field
1111
0000
0000
0000
1111
0000
0000
0000
1111
0000
0000
0001
+)
+)
1111
1111
1111 1111 1000 0000 0000 0000
0011
0000
0011 0000 1000 0000 0000 0001
Guard bits
Guard bits
1111
1111
0000
0000
Sign bit
Non overflow case
Positive value
Example 2
Example 2
1111
1000
0000
1000
Overflow detecting field
1111
0000
0000
0000
1111
0000
0000
0000
1111
0001
0000
0001