The Fetch Sequence

The fetch sequence is a sequence of microinstructions that begins the execution of every machine cycle. A typical fetch sequence loads into an "instruction" register the contents of the memory location whose address is in a "program counter" register. Then it increments the program counter and decodes the instruction in the instruction register. (An instruction register is specified as a parameter of each Decode microinstruction.) It is typically the case that the last microinstruction in the fetch sequence is a Decode microinstruction, because the execution of a Decode microinstruction causes the machine to (a) cease executing the fetch sequence, (b) decode the instruction register, and (c) begin executing the execute sequence of the machine instruction that was just decoded. In other words, no microinstructions of the fetch sequence get executed after a Decode microinstruction has been executed.