Abstract: A microprogram sequencer for generating in a proper sequence the addresses of the successive microinstructions used in executing a given machine instruction includes a PROM next address generator that produces the successive addresses. The successive addresses are utilized as the successive microinstructions. Each address produced includes a normal next address, but this normal next address may be alterable by address alteration signals that are generated in response to a number of sensed conditions within the computer and in response to predetermined machine instruction register bits. The address alteration of a normal next address, if required, is accomplished within the same clock period in which the normal next address is initially formed, permitting jump or branch instructions to be performed as rapidly as normal instructions. No mapping PROM, microsequencer counter, nor microsequencer incrementer are needed to implement the present invention.