1. In a microprogrammed computer having a main memory adapted to store microinstructions, sets of microinstructions forming microprograms; a main memory address register for storing the address of a location in main memory; and a main memory output register for storing data read out of a location in main memory corresponding to the address stored in the main memory address register; the improvements comprising: a microprogram memory for storing a plurality of microprograms; a microinstruction address register for receiving and storing the address of a microinstruction, the address including data identifying the microprogram memory or the main memory as the memory in which each microinstruction is stored; a microinstruction register for storing microinstructions; auxiliary register means; means for applying the data identifying the memory in which a microinstruction is stored to the auxiliary register, the auxiliary register providing an output identifying the memory in which the microinstruction in the microinstruction address register is stored; first address channel means including gate means for selectively connecting the microinstruction address register to the microprogram memory to apply the address stored therein to the microprogram memory; second address channel means including gate means for selectively connecting the microinstruction address register to the main memory address register to apply the address stored therein to the main memory address register, whereby data stored at the addressed location in main memory is written into the main memory output register; first output channel means, including gate means, for selectively connecting the microprogram memory to the microinstruction register to transfer data stored at an addressed location in the microprogram memory to the microinstruction memory register; an intermediate register; second output channel means, including gate means, for selectively connecting the main memory output register to the intermediate register, said intermediate register storing the contents of the main memory output register therein and for connecting the intermediate register to the microinstruction register to transfer data stored in the intermediate register to the microinstruction register; a logic sequencer circuit; circuit means for applying the output of the auxiliary register means to the logic sequencer; a decoder network connected to the intermediate register to test instructions stored therein to determine if the instruction is a jump microinstruction and to produce outputs identifying the presence or absence of a jump microinstruction; circuit means for applying the output of the decoder network to the logic sequencer circuit, said logic sequencer circuit, if the output signal from the auxiliary register means identifies main memory as the location of the microinstruction whose address is in the microinstruction address register and if the microinstruction is not a jump instruction, generating a sequence of microcommands; a conditioning network connected to the logic sequencer and responsive to microcommands produced by the sequencer circuit for applying appropriate control signals to the gates of the channels for enabling and inhibiting selectively said channels to cause the reading from main memory of a microinstruction during the execution of the preceding microinstruction.