Information processing system having mode signal holding means and selecting operand access mode based on mode signal indicating type of currently executed instruction

Claim 1

In an information processing system operable according to a software program which defines instructions of a first and a second kind, each of the instructions of said first kind carried out by a microprogram which defines a plurality of microinstructions, each of the instructions of said second kind carried out by an instruction program which defines the instructions of said first kind, said information processing system comprising a main memory for memorizing the instructions of said first kind and said second kind, instruction decoding means connected to an instruction fetching means for decoding a current instruction into a decoded signal, microinstruction processing means connected to said instruction decoding means for successively processing said microinstructions to produce a processed signal in response to each of said microinstructions, operand accessing means connected to said main memory, said instruction fetching means, and said microinstruction processing means for accessing said main memory in response to said processed signal to read an operand for said current instruction, and an executing unit connected to said instruction decoding means, said microinstruction processing means, and said operand accessing means for executing said current instruction on said operand in response to said decoded signal and said processed signal, the improvement wherein said operand accessing means comprises: mode holding means connected to said instruction decoding means for holding a mode signal in response to said decoded signal, said mode holding means being set when said decoded signal indicates that the processing of said instruction program has started, said mode holding means being reset when said decoded signal indicates that processing of said instruction program has stopped, said mode signal thereby indicating whether or not one of the instructions of said second kind is executed by said executing unit as said current instruction; a memory access mode register connected to said microinstruction processing means for holding a memory access mode bit indicative of a memory access mode which defines an access mode for said main memory, said memory access mode register being controlled by said microinstruction processing means, said memory access mode bit thereby being renewed by said processed signal; a predetermined access mode register for holding a predetermined access mode bit indicative of a predetermined access mode; access mode selecting means connected to said mode holding means, to said memory access mode register, and to said predetermined access mode register for selecting one of said memory access mode bit and said predetermined access mode bit as a selected access mode bit indicative of a selected access mode, said access mode selecting means selecting said memory access mode bit as said selected access mode bit when said mode signal indicates that said one of the instructions of the second kind is not executed by said executing unit, said access mode selecting means selecting said predetermined access mode bit as said selected access mode bit when said mode signal indicates that said one of the instructions of the second kind is executed by said executing unit; and operand fetching means connected to said access mode selecting means, to said instruction fetching means, and to said main memory, for fetching said operand in accordance with the selected access mode indicated by said selected access mode bit.