DE2555963

1533770 Data processing INTERNATIONAL BUSINESS MACHINES CORP 26 Oct 1976 [12 Dec 1975] 44531/76 Heading G4A In a multiprogrammed system in which time slices are allocated to programs by chained index words, the functions of specified types of instructions in specified programs are varied if certain external and/or internal non-program conditions are satisfied. The index words are held in a store 34, each word comprising a pointer which identifies which of a number of microprograms in a control store 20 is allocated the current time slice and a link address which points to a location in a store 36 holding the address of the next index word in store 34. The number of time slices allocated to any given micro-program by the index words in store 34 may be varied. The pointer from the current word accesses a micro-program pointer from a store 38 and the micro-program pointer in turn accesses the address of the next instruction for the relevant micro-program from a store 32. The current micro-program pointer from store 38 is compared with a set of micro-program identifiers and the OP code of the next microinstruction read from the control store 20 is compared with a set of micro-instruction identifiers. If equality is detected from both comparisons and if predetermined processor internal and/or peripheral external status signals are present a function change control signal on line 11 is produced and modification information pointed to by micro-program (or possibly microinstruction) pointer register 25 is read out from a buffer 29. The decoded modification information may be used, for example, to modify the result from the ALU 51 by +1. The processor also includes a data local store 46 for A and B operands and ALU results. The store 46 may also be used for loading micro-program and micro-instruction identifiers into the comparison registers and modification information into buffer 29 under micro-instruction control. The modification arrangements allow for (micro)programming flexibility without increasing the size of the (micro-)instruction set.