Found in some versions such as the MSP430F1611
C compilers do not necessarily do a good job of using it. Below is an example using pure C vs the multiplier accumulator equivalent.
typedef unsigned short UINT16; ///< unsigned 16-bit typedef unsigned long UINT32; ///< unsigned 32-bit UINT32 u32Answer; UINT16 uX,uY,uZ; uX = 10; uY = 2; uZ = 40000; The line below translates to lots of code (even with the "use multiplier" option checked in the compiller): u32Answer = (UINT32)uX*3 + (UINT32)uY*4 + (UINT32)uZ*5 + (UINT32)uX*6; Efficientcy by using multiply accumulate: *(INT32 *)RESLO_ = 0L; // Clear accumulator MAC = 3; OP2 = uX; MAC = 4; OP2 = uY; MAC = 5; OP2 = uZ; MAC = 6; OP2 = uX; NOP(); u32Answer = *(INT32 *)RESLO_; Code in CPU looks like: 94 *(INT32 *)RESLO_ = 0L; \ 000072 82433a01 MOV.W #0x0, &0x13a \ 000076 82433c01 MOV.W #0x0, &0x13c 95 MAC = 3; \ 00007A B24003003401 MOV.W #0x3, &0x134 96 OP2 = uX; \ 000080 824E3801 MOV.W R14, &0x138 97 MAC = 4; \ 000084 A2423401 MOV.W #0x4, &0x134 98 OP2 = uY; \ 000088 824C3801 MOV.W R12, &0x138 99 MAC = 5; \ 00008C B24005003401 MOV.W #0x5, &0x134 100 OP2 = uZ; \ 000092 824F3801 MOV.W R15, &0x138 101 MAC = 6; \ 000096 B24006003401 MOV.W #0x6, &0x134 102 OP2 = uX; \ 00009C 824E3801 MOV.W R14, &0x138 103 NOP(); \ 0000A0 0343 NOP 104 u32Answer = *(INT32 *)RESLO_; \ 0000A2 92423A01.... MOV.W &0x13a, &u32Answer \ 0000A8 92423C01.... MOV.W &0x13c, &u32Answer + 2
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