Microcode - The Reason For Microprogramming

The Reason For Microprogramming

Microcode was originally developed as a simpler method of developing the control logic for a computer. Initially CPU instruction sets were "hardwired". Each step needed to fetch, decode and execute the machine instructions (including any operand address calculations, reads and writes) was controlled directly by combinational logic and rather minimal sequential state machine circuitry. While very efficient, the need for powerful instruction sets with multi-step addressing and complex operations (see below) made such "hard-wired" processors difficult to design and debug; highly encoded and varied-length instructions can contribute to this as well, especially when very irregular encodings are used.

Microcode simplified the job by allowing much of the processor's behaviour and programming model to be defined via microprogram routines rather than by dedicated circuitry. Even late in the design process, microcode could easily be changed, whereas hard wired CPU designs were very cumbersome to change, so this greatly facilitated CPU design.

From the 1940s to the late 1970s, much programming was done in assembly language; higher level instructions meant greater programmer productivity, so an important advantage of microcode was the relative ease by which powerful machine instructions could be defined. During the 1970s, CPU speeds grew more quickly than memory speeds and numerous techniques such as memory block transfer, memory pre-fetch and multi-level caches were used to alleviate this. High level machine instructions, made possible by microcode, helped further, as fewer more complex machine instructions require less memory bandwidth. For example, an operation on a character string could be done as a single machine instruction, thus avoiding multiple instruction fetches.

Architectures with instruction sets implemented by complex microprograms included the IBM System/360 and Digital Equipment Corporation VAX. The approach of increasingly complex microcode-implemented instruction sets was later called CISC. An alternate approach, used in many microprocessors, is to use PLAs or ROMs (instead of combinational logic) mainly for instruction decoding, and let a simple state machine (without much, or any, microcode) do most of the sequencing. The various practical uses of microcode and related techniques (such as PLAs) have been numerous over the years, as well as approaches to where, and to which extent, it should be used. It is still used in modern CPU designs.

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