| 摘要 |
PURPOSE:To simplify the procedure to write a microprogram into a read-only memory ROM by storing the correction information into a back-up memory BUM, comparing an address included in the BUM with an address to receive an access, and outputting the data out of the BUM when the coincidence is secured between both addresses. CONSTITUTION:When a microprogram stored in a ROM 2 has a defect and is corrected, the address and the data of the corrected area are written into a BUM 3 and a flag 8 is set at an H level. When the flag 8 of the microprogram is set at an H level, a multiplexer circuit 7 selects the side of an address counter 6 while a CPU 1 has an access to the ROM 2. Then a comparator 4 compares the address data shown by the counter 6 contained in the BUM 3 with the value of an address bus 9. When the coincidence is obtained from this comparison, a selection circuit 5 decides that the microprogram of the ROM 2 shuld be corrected and outputs the data on the BUM 3. Then the circuit 5 corrects the data of the address on the BUM 3 and advances the counter 6 to the next address. Thus it is possible to simplify the procedure where the microprogram is written into the ROM 2.
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