| 摘要 |
PURPOSE:To easily alterate a decode output, by installing a register that can perform an easy rewriting independently from an ROM and then extracting the contents of this register in the form of a decode output for a specified instruction code. CONSTITUTION:The instruction code 20 to be decoded has, for instance, 3 bits, and 8 pieces of output (selection) line 22 are given from the decoder 21. Then one of the 8 lines 22 is turned to logic ''1''. Only 7 pieces of line 22 excepting a specified instruction code (such as the selection line 22' corresponding to ''111'') of the code 20 are connected to the ROM23, and at the same time the line 22' corresponding to the above-mentioned specified instruction code is connected to the rewritable register 25. Accordingly, a corresponding decode output is obtained from the ROM23 in case the code other than the specified code is supplied as the code 20. Then a decode output set at the register 25 is obtained when the specified instruction code is supplied. |
