Central Processor Unit (CPU) Registers
Table 4-2. Status Register Bit Definitions (Continued)
Bit Number
16
Bit Name
FV
Reset Value
0
DO FOREVER Flag
Description
Set when a DO FOREVER loop executes. The FV flag, like the LF flag, is
restored from the stack when a DO FOREVER loop terminates. Stacking
and restoring the FV flag when initiating and exiting a DO FOREVER loop,
respectively, allow program loops to be nested. When returning from the
long interrupt with an RTI instruction, the system stack is pulled and the
value of the FV bit is restored.
15
LF
0
Do Loop Flag
When a program loop is in progress, enables the detection of the end of the
loop. The LF is restored from stack when a program loop terminates.
Stacking and restoring the LF when initiating and exiting a program loop,
respectively, allow program loops to be nested. When returning from the
long interrupt with an RTI instruction, the System Stack is pulled and the LF
bit value is restored.
14
DM
0
Double-Precision Multiply Mode
Enables four multiply/MAC operations to implement a double-precision
algorithm that multiplies two 48-bit operands with a 96-bit result. Clearing
the DM bit disables the mode.
Note:
The Double-Precision Multiply mode is supported to maintain
object code compatibility with devices in the DSP56000 family. For
a more efficient way of executing double precision multiply, refer
to the chapter on the Data Arithmetic Logic Unit in the DSP56300
Family Manual.
In Double-Precision Multiply mode, the behavior of the four specific
operations listed in the double-precision algorithm is modified. Therefore,
do not use these operations (with those specific register combinations) in
Double-Precision Multiply mode for any purpose other than the double
precision multiply algorithm. All other Data ALU operations (or the four
listed operations, but with other register combinations) can be used.
The double-precision multiply algorithm uses the Y0 Register at all stages.
Therefore, do not change Y0 when running the double-precision multiply
algorithm. If the Data ALU must be used in an interrupt service routine, Y0
should be saved with other Data ALU registers to be used and restored
before the interrupt routine terminates.
DSP56311 User’s Manual, Rev. 2
Freescale Semiconductor
4-7
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