Accessing An Accumulator By Its Individual Portions; Figure 3-4 Writing The Accumulator Extension Registers (F2) - Motorola DSP56800 Manual

Data Arithmetic Logic Unit
Table 3-1. Accessing the Accumulator Registers (Continued)
Register Read of an Accumulator Register
A1 For a MOVE instruction:
B1 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
A0 For a MOVE instruction:
B0 The 16-bit F0 register is read onto the CGDB
bus.
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

Accessing an Accumulator by Its Individual Portions

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.
Register F2 Used
as a Destination
Figure 3-4. Writing the Accumulator Extension Registers (F2)
3-8
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.
15 4 3
LSB of
Not Used
Word
15 4 3
No Bits Present F2
DSP56800 Family Manual
Write to an Accumulator Register
0
CGDB Bus Contents
0
Register F2