3.3.12.3.4 Accumulator Write Back

The MAC and MSC instructions can optionally write a rounded version of the accumulator that is not the target of the current operation into data space memory. The write is performed across the X-bus into the combined X and Y address space. This accumulator write-back feature is beneficial in certain algorithms, such as FFT and LMS filters.

Two addressing modes are supported by the accumulator write-back hardware:

• W0, W1, W2, W3 or W13, Register Direct: The rounded contents of the non-target accumulator are written into the destination register as a 1.15 (Word mode) or 1.31 (Long Word mode) fractional result.
• [W13++] or [W15++], Register Indirect with Post-Increment: The rounded contents of the non-target accumulator are written into the address pointed to by W13 or W15 as a 1.15 (Word mode) or 1.31 (Long Word mode) fraction. W13 or W15 is then incremented by 2/4 depending on selected Word/Long Word mode. [W15++] is equivalent to a push onto the system stack.