1. A digital computer system, comprising: a central processing unit having a machine cycle; a main storage unit; a buffer storage unit connected between said central processing unit and said main storage unit, receiving data from said central processing unit and storing the data in said main storage unit, reading data from said main storage unit and transmitting the data to said central processing unit, and functioning as an internal memory device for said central processing unit and operating at a high speed access time; and a store buffer device connected between said central processing unit and said main storage unit, having a higher operating speed than and operating independently of said buffer storage unit, receiving data identical to that stored in said buffer storage unit and control information, from said central processing unit, for the received data from said central processing unit in response to requests from said central processing unit and transferring the received data and control information to said main storage unit, the transfer operation from said central processing unit to said store buffer device and the transfer operation from said store buffer device to said main storage unit being effected during the machine cycle of said central processing unit, and said store buffer device including: a controller connected between said central processing unit and said main storage unit, and controlling the transfer operations; data register sets connected to said central processing unit, said controller and said main storage unit, each set including data registers for receiving data to be stored in said main storage unit from said central processing unit; a byte mark register set connected between said central processing unit and said main storage unit, and including byte mark registers for receiving, from said central processing unit, information designating data to be stored in said data registers; and an address register set connected between said central processing unit and said main storage unit, and including address registers for receiving a starting storage address in said main storage unit for the data to be stored in said main storage unit from the data registers from said central processing unit.