PROGRAM SEQUENCE CONTROL

Program sequence control is described in connection with a computer having a main system program and one or more micro-order programs. The instructions in the system program are of two types: one type actually comprises a micro-order, and the other type designates an address where a sequence of micro-orders begins in a micro-order program. The sequence controller is able to load single micro-order instructions directly into its micro-order register for execution, or alternatively it addresses the micro-program to fetch a sequence of micro-orders which correspond to a multiple micro-order instruction. A special type of multiple micro-order instruction requires repetition of a particular micro-order any number of times up to a predetermined maximum. The system employs a single marked bit to distinguish single and multiple micro-order instructions, and also to identify the last micro-order in any multiple micro-order instruction, including the last repetition of a repeat cycle. A buffer register is also provided which permits more rapid access to the main system program through a "look ahead" feature, and provision is made for discarding the content of the buffer register when the "look ahead" assumption is invalidated by subsequent program contingencies. Provision is made for delaying the micro-program memory cycle when necessary to allow the system program memory to catch up.