ABSTRACT
This chapter discusses the basic principles of microcode and the microprogrammed central processing unit (CPU) that executes a subset of 8086 instructions. It shows that a complex target instruction, such as multiply, divide, or floating point, can be interpreted and executed by microcode. The microprogrammed CPU has the obvious advantage of being flexible. The trade-off is slower speed because more overhead is required to fetch, interpret, and execute microinstructions. Thus, the microprogrammed CPU is slower than its counterpart using hardwired logic. A microprogrammed CPU has its own merit for many reasons. When a new computer is developed, it is preferably backward-compatible so that all the instructions of an old processor can execute on the new processor. Because the adder generates the signed sum of two signed numbers, the CPU can interpret each bit in the multiplier from right to left and add the multiplicand accordingly.
