dsp56800 Freescale Semiconductor, Inc, dsp56800 Datasheet - Page 52

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

Data Arithmetic Logic Unit
In all cases in Table 3-1 where a MOVE operation is specified, it is understood that the function is
identical for parallel moves and bit-field operations.
3.2.1
The instruction set provides instructions for loading and storing one of the portions of an accumulator
register without affecting the other two portions. When an instructions uses the F1 or F0 notation instead of
F, the instruction only operates on the 16-bit portion specified without modifying the other two portions.
When an instruction specifies F2, then the instruction operates only on the 4-bit accumulator extension
register without modifying the F1 or F0 portions of the accumulator. Refer to Table 3-1 for a summary of
accessing the accumulator registers.
Data limiting, as outlined in Section 3.4, “Saturation and Data Limiting,” is enabled only when an entire
accumulator is being stored to memory. When only a portion of an accumulator is being stored (by using
an instruction which specifies F2, F1, or F0), limiting through the data limiter does not occur.
When F2 is written, the register receives the low-order portion of the word; the high-order portion is not
used. See Figure 3-4.
When F2 is read, the register contents occupy the low-order portion (bits 3–0) of the word; the high-order
portion (bits 15–4) is sign extended. See Figure 3-5.
3-8
Register
A1
B1
A0
B0
Register F2 Used
as a Destination
Accessing an Accumulator by Its Individual Portions
For a MOVE instruction:
The 16-bit F1 portion is read onto the CGDB bus.
When used in an arithmetic operation:
The F1 register is used as a 16-bit source operand
for an arithmetic operation.
F1 can be used in the following:
MOVE
Parallel Move
Several different arithmetic
For a MOVE instruction:
The 16-bit F0 register is read onto the CGDB bus.
Table 3-1. Accessing the Accumulator Registers (Continued)
Read of an Accumulator Register
Figure 3-4. Writing the Accumulator Extension Registers (F2)
15
15
DSP56800 Family Manual
No Bits Present
Not Used
4 3
4 3
4 LSB of
For a MOVE instruction:
The contents of the CGDB bus are written into the
16-bit F1 register.
The corresponding F2 and F0 portions are not
modified.
For a MOVE instruction:
The contents of the CGDB bus are written into the
16-bit F0 register.
The corresponding F2 and F1 portions are not
modified.
Word
F2
Write to an Accumulator Register
0
0
CGDB Bus Contents
Register F2
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