How do we move 40-bit data in the accumulator into 16-bit data memory space?
Two methods:

  • MOV from memory mapped accumulator register
    • Contents of specified portion of the accumulator are stored
    • Does not allow for:
      • Rounding
      • Data Write Saturation
    • Not recommended for most applications
      • OK for integer multiplies
  • SAC (Store Accumulator)
    • Specialized command for storing accumulators
    • Maintains sign bit
    • Automatically rounds if desired
    • Can saturate destination register if desired (data write saturation)
    • Can shift during store process
      • Recommended way of retrieving data from accumulators
Click image to enlarge.

Since the six accumulator registers are memory mapped, one can also store the contents of an accumulator register using the MOV instruction. However, it is best to use the SAC instructions to take advantage of the built in rounding and saturation that this command provides.

Store Accumulator

The SAC and SAC.R instructions provide the most convenient and most straightforward methods of saving the accumulator. These instructions store the contents of an accumulator to a location in memory, using either direct or indirect addressing. Accumulator High is the accumulator target source register, and one may optionally shift the data as it is being stored.

* Store Accumulator
* Store truncated ACCxH into 16-bit memory, with optional pre-shift.

* Store Rounded Accumulator
* Store rounded ACCxH into 16-bit memory, with optional pre-shift
* Rounding type determined by RND bit, CORCON<1>

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