1. An apparatus for performing selected tasks comprising: main memory means for storing instructions in the form of digital signals specifying tasks to be performed, said stored digital signals including at least one of a high performance instruction specifying a task and a plurality of low performance instructions each specifying a subtask, said task specified by said high performance instruction performable by performing the subtasks specified by at least one of said plurality of low performance instructions, each of said low performance instructions having a predetermined format; first control storing means for storing a first microprogram comprising a plurality of microinstructions specifying subtasks corresponding to said subtasks specified by said plurality of low performance instructions, said microinstructions of said first microprogram having said predetermined format; second control storing means for storing a second microprogram including a plurality of microinstructions each specifying a microtask, each of said subtasks performable by performing at least one of said microtasks; first reading means for reading an instruction stored in said main memory means; means for determining whether the instruction read by said first reading means is said high performance instruction; second reading means for reading at least part of said first microprogram stored in said first control storing means in response to a determination by said determining means that said instruction read by said first reading means is said high performance instruction; third reading means for alternately (1) reading microinstructions from said second control storing means specifying the microtasks which, when performed, perform the subtasks specified by said first microprogram read by said second reading means and (2) reading microinstructions from said second control storing means specifying microtasks which, when performed, perform said subtasks specified by said instruction read by said first reading means; and means for performing the microtasks specified by the microinstructions read by said third reading means.