Syntax : MULWF f
Description : An unsigned multiplication is carried out between the
contents of the W register and the register file location 'f'.
The 16-bit result is stored in PRODH:PRODL register pair.
Both W reg and 'f' are unchanged. PRODH contains the MSB.
None of the status flag are affected.
| 0011 | 0100 | ffff | ffff |