To add the imacn instruction

[djipi]

Signed Integer Multiply/Accumulate, no Wrtie-Back

16-bit signed integer multiply and accumulate, like IMULT, except that the 32-bit product is added to the result of the previous arithmetic operation, and the result is not written back to the destination register. Intended to be user after IMULTN to give a multiply/accumulate group.
*Refer to the section on Multiply and Accumulate instructions.

Code/Case: 20

Flags
ZNC – unaffected

Register Usage
Cycle 1: Source and Destination register read