The example below implements the equation:

4660 292 – × 1360720 – =

MOV MAC0CF, #01h ; Use integer numbers, and multiply only mode (add to zero)
MOV MAC0AH, #12h ; Load MAC0A register with 1234 hex = 4660 decimal
MOV MAC0AL, #34h
MOV MAC0BH, #FEh ; Load MAC0B register with FEDC hex = -292 decimal
MOV MAC0BL, #DCh ; This line initiates the Multiply operation
NOP
NOP ; After this instruction, the Accumulator should be equal to
; FFFFEB3CB0 hex = -1360720 decimal. The MAC0STA register should
; be 0x01, indicating a negative result.
NOP ; After this instruction, the Rounding register is updated

assuming the values are in 4 DATA slots
DATA to SFR in a F120 takes 30ns, so to load the value will take 120ns, to unload it (20 bits = 3 SFRs) will take 90ns and the nop will take 10ns all in all the operation DATA memory to DATA memory will take 220ns. With a f124 it would take 440ns.

Erik

MAC0CF, of course, need only be set in the initialization and never again.

Erik