PURPOSE:To improve a bit rate in an instruction buffer by providing a replacing direction bit and inverting the replacing order indicated by the replacing information bit of an instruction block by means of an output in a replacing direction. CONSTITUTION:A replacing direction bit 3 is provided and the replacing direction information of the bit 3 is sent to a replacing order preparing means 2 through a replacing direction information line 301. At the means 2 the exclusive OR of the output of a replacing information bit 21 and each bit is taken from the replacing order information by means of exclusive OR (XOR) gates 23 and 24. By leading output signals of the XOR gates 23 and 24 to T/C gates 25 and 26 and AND gates 27-30, replacing instruction blocks are decided and the replacement of instruction blocks is designated to respective instruction blocks 11-14 through instruction block replacement designating lines 201-204. Therefore, the bit rate of an instruction buffer is improved and the instruction execution processing speed can be improved.
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