microsequencer

A microsequencer is a digital circuit within a processor’s control unit that selects and issues microinstructions from a microinstruction store to govern the processor’s datapath. It operates at the microarchitectural level, orchestrating the sequence of micro-operations that implement machine instructions.

During operation, the microsequencer reads the current microinstruction, decodes its fields, and generates the control signals that drive datapath elements such as registers, ALUs, and buses. It also determines the address of the next microinstruction to fetch. The next-address decision can depend on the current microinstruction fields, condition codes, and external signals, enabling conditional branching, subroutine calls, and returns.

Key components typically include a microinstruction memory (ROM or RAM) that stores microcode, a microprogram counter that addresses this memory, and sequencing logic that computes the next address. A multiplexer or similar mechanism may select the next address from sequential progression, conditional branches, or other sources, and the circuit advances on each clock edge.

Microinstruction formats vary. In horizontal microcode, wide words may specify many micro-operations and a direct next-address field; in vertical microcode, compact fields encode fewer operations, with more complex decoding. The microsequencer thereby provides flexibility: updates to instruction behavior can be achieved by modifying microcode rather than hardware.

Historically, microsequencers were central to microprogrammed CPUs in complex instruction set architectures, offering easier implementation and updates. While some modern designs favor hardwired control for speed, microsequencers remain a fundamental concept in the study of processor control and microprogramming.