| 摘要 |
PURPOSE:To change the data outputted by a ROM without replacing the ROM by providing a second control means to prohibit the reading from a read-only memory. CONSTITUTION:By using a data bus 4 and a first control means 24, the address to be changed in a ROM 5 is written into a first register 12 and the contents are written in a second register 13. Consequently, when the ROM is read, an address from an address bus 3 and an address from a first register 12 are compared by an address comparator 25, and when the comparing result is in discrepancy, the contents of the ROM 5 are read as they are and outputted to the data bus 4 by a second control means 32 and when the result is coincident, the reading of the ROM 5 is prohibited, the contents of the second register 13 are read and outputted to the data bus. Thus, the data in the ROM 5 corresponding to the address designated by the first register 12 are replaced with the data of the second register 13. |
