History
Until the late 1970s, most minicomputers did not have a multiply instruction, and so programmers used a "multiply routine" which repeatedly shifts and accumulates partial results, often written using loop unwinding. Mainframe computers had multiply instructions, but they did the same sorts of shifts and adds as a "multiply routine".
Early microprocessors also had no multiply instruction. The Motorola 6809, introduced in 1978, was one of the earliest microprocessors with a dedicated hardware multiply instruction. It did the same sorts of shifts and adds as a "multiply routine", but implemented in the microcode of the MUL instruction.
As more transistors per chip became available due to larger-scale integration, it became possible to put enough adders on a single chip to sum all the partial products at once, rather than reuse a single adder to handle each partial product one at a time.
Because some common digital signal processing algorithms spend most of their time multiplying, digital signal processor designers sacrifice a lot of chip area in order to make the multiply as fast as possible; a single-cycle multiply–accumulate unit often used up most of the chip area of early DSPs.
Read more about this topic: Binary Multiplier
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