Motorola DSP56301 - Manual (Page 82)

Central Processor Unit (CPU) Registers
4 -8 DSP56301 User’s Manual
20 SM 0 Arithmetic Saturation Mode
Selects automatic saturation on 48 bits for the results going to the
accumulator. This saturation is performed by a special circuit inside the
MAC unit. The purpose of this bit is to provide an Arithmetic Saturation
mode for algorithms that do not recognize or cannot take advantage of the
extension accumulator.
19 CE 0 Cache Enable
Enables/disables the instruction cache controller. If CE is set, the cache is
enabled, and instructions are cached into and fetched from the internal
Program RAM. If CE is cleared, the cache is disabled and the DSP56300
core fetches instructions from external or internal program memory,
according to the memory space table of the specific DSP56300 core-based
device.
Note: To ensure proper operation, do not clear Cache Enable mode
while Burst mode is enabled (OMR[BE] is set).
18 0 Reserved. Write to zero for future compatibility.
17 SA 0 Sixteen-Bit Arithmetic Mode
Affects data width functionality, enabling the Sixteen-bit Arithmetic mode of
operation. When SA is set, the core uses 16-bit operations instead of 24-bit
operations. In this mode, 16-bit data is right-aligned in the 24-bit memory
locations, registers, and 24-bit register portions. Shifting, limiting, rounding,
arithmetic instructions, and moves are performed accordingly. For details
on Sixteen-Bit Arithmetic mode, consult the
DSP56300 Family Manual.
16 FV 0 DO FOREVER Flag
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.
Table 4-3. Status Register Bit Definitions (Continued)
Bit Number Bit Name Reset Value Description