Microprograms

A microprogram is a sequence of elemental operations that a computer's control unit executes to perform a more complex instruction. Essentially, it's a low-level set of commands that translate higher-level machine code instructions into the actual electrical signals needed to control the processor's components like the ALU, registers, and memory. Microprograms are stored in a special memory called microprogram memory, often ROM (Read-Only Memory) or a similar non-volatile technology.

The concept of microprogramming was introduced by Maurice Wilkes in 1951. It offers a flexible way to design and implement the control unit of a processor. Instead of using hardwired logic gates, which can be complex and difficult to modify, a microprogram provides a more software-like approach. This allows for easier modification or extension of the instruction set, and it can simplify the design of complex instruction sets.

Each instruction in the machine's instruction set is represented by a unique microprogram. When the processor fetches an instruction from main memory, the control unit decodes it and then fetches and executes the corresponding microprogram. This process involves fetching a sequence of microinstructions, each of which specifies the operations to be performed by the processor's datapath in a single clock cycle. The last microinstruction in a sequence typically signals the completion of the instruction and initiates the fetch cycle for the next instruction. This layered approach effectively bridges the gap between the user-visible machine instructions and the underlying hardware operations.