GB1433076A - Data processing systems - Google Patents

Abstract

1433076 Microprogram controlled processor INTERNATIONAL BUSINESS MACHINES CORP 16 May 1974 [19 July 1973] 21728/74 Heading G4A In a computer in which each of a group of macroinstructions is implemented by control signals derived from a respective sequence of microinstructions, each such sequence including microinstrustions common to all the sequences, a single common microinstruction sequence is stored. The microinstruction(s) in the stored sequence where the microinstructions in the various implementing sequences differ include a field which, when logically combined with a part of the current macroinstruction, produces the required control signals. A number of macroinstructions, e.g. relating to AND, OR, and exclusive-OR operations between the contents of two registers are each implemented by a sequence of microinstructions, all the sequences having common parts, e.g. S1-S3 relating to accessing the macroinstruction, accessing the operands &c., and in general differing only in the actual operation performed. To reduce the amount of microinstruction storage required a single sequence, Fig. 2, only is stored. The microinstructions are accessed in sequence from microprogram store 4 by incrementing an address and loaded in register 5. An additional bit 50 of the first instruction of the sequence causes part of the macroinstruction op-code to be transferred from register 2 to register 7. In the example shown the implementing microinstruction sequences differ only at instruction S4b which has an additional bit 51, which causes the contents of register 7 to be exclusive-ORed with a field 52 in the microinstruction. The field 52 of the microinstruction, 1100, is such that when exclusive-ORed with the 4-bit op-code part of a macroinstruction control signals respective to that macroinstruction are generated and fed to the ALU 6. The macroinstruction op-code may consist of eight bits, the first four defining sixteen groups of operations each with a single stored implementing microinstruction sequence and the second four defining individual operations in a group and being used as above to modify the appropriate stored microinstuction sequence to produce the appropriate control signals.