8.10. Multiply and multiply accumulate

The multiply instructions make use of special hardware that implements integer multiplication. All cycles except the last are internal.

During the first (Execute) stage of a multiply instruction, the multiplier and multiplicand operands are read onto the A and B buses, on which the multiplier unit is connected. The first stage of the multiplier performs Booth recoding and partial product summation, using 16 bits of the multiplier operand each cycle.

During the second (Memory) stage of a multiply instruction, the partial product result from the Execute stage is added with an optional accumulate term (read onto the C bus) and a possible feedback term from a previous multiply step for multiplications that require additional cycles.


In Thumb state, only the MULS and MLAS operations are possible.

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