dsp56800 Freescale Semiconductor, Inc, dsp56800 Datasheet - Page 128

• Manufacturer Part Number: dsp56800
• Description: 16-bit Digital Signal Controllers
• Manufacturer: Freescale Semiconductor, Inc

Program Controller
Saturation is performed by a dedicated circuit inside the MAC unit. The saturation logic operates by
checking 3 bits of the 36-bit result out of the MAC unit — EXT[3], EXT[0], and MSP[15]. When the SA
bit is set, these 3 bits determine if saturation is performed on the MAC unit’s output and whether to
saturate to the maximum positive or negative value, as shown in Table 5-4.
5.1.9.4
The rounding (R) bit (OMR bit 5) selects between convergent rounding and two’s-complement rounding.
When set, two’s-complement rounding (always round up) is used. The two rounding modes are discussed
in Section 3.5, “Rounding,” on page 3-30. This bit is cleared by processor reset.
5.1.9.5
The stop delay (SD) bit (OMR bit 6) is used to select the delay that the DSC needs to exit the stop mode.
When set, the processor exits quickly from stop mode. This bit is cleared by processor reset.
5.1.9.6
The condition code (CC) bit (OMR bit 8) selects whether condition codes are generated using a 36-bit
result from the MAC array or a 32-bit result. When this bit is set, the C, N, V, and Z condition codes are
generated based on bit 31 of the data ALU result. When this bit is cleared, the C, N, V, and Z condition
codes are generated based on bit 35 of the data ALU result. The generation of the L, E, and U condition
codes are not affected by the CC bit. This bit is cleared by processor reset.
5-12
Rounding Bit (R) — Bit 5
Stop Delay Bit (SD) — Bit 6
Condition Code Bit (CC) — Bit 8
EXT[3]
0
0
0
0
1
1
1
1
Saturation mode is always disabled during the execution of the following
instructions: ASLL, ASRR, LSLL, LSRR, ASRAC, LSRAC, IMPY16,
MPYSU, MACSU, AND, OR, EOR, NOT, LSL, LSR, ROL, and ROR.
For these instructions, no saturation is performed at the output of the MAC
unit.
The unsigned condition tests used when branching or jumping (HI, HS,
LO, and LS) can only be used when the condition codes are generated with
Table 5-4. MAC Unit Outputs With Saturation Mode Enabled (SA = 1)
EXT[0]
0
0
1
1
0
0
1
1
DSP56800 Family Manual
MSP[15]
0
1
0
1
0
1
0
1
NOTE:
NOTE:
Result Stored in Accumulator
$0 7FFF FFFF
$0 7FFF FFFF
$0 7FFF FFFF
$F 8000 0000
$F 8000 0000
$F 8000 0000
(Unchanged)
(Unchanged)
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