
Altera Corporation
6–15
January 2008
Stratix II Device Handbook, Volume 2
DSP Blocks in Stratix II and Stratix II GX Devices
Figure 6–6. Rounding and Saturation Bits
(1)
Both sign bits are the same. The design only uses one sign bit, and the other one is ignored.
If the design performs a multiply_accumulate or multiply_add
operation, the multiplier output is input to the
adder/subtractor/accumulator blocks as a 2.31 value, and the three LSBs
are 0.
1 Sign
Bit
2 LSBs
15 Bits
2 Sign
Bits (1)
3 LSBs
31 Bits
1 Sign
Bit
2 LSBs
15 Bits
19 LSBs
are Ignored
18 Bits
2 Sign
Bits (1)
3 LSBs
31 Bits
2 Sign
Bits (1)
2 Sign
Bits (1)
3 LSBs
31 Bits
18
36
11 1
1
10
0
00
0
00
0
00
00 0
0
01
0
00
0
00
00 0 0
0
00 0
0
18
× 18 Multiplication
Saturated Output Result
Rounded Output Result
+
=