| 摘要 |
PURPOSE:To make an ROM system complete in a short time with low cost, by adding an EPROM only storing a different content, when a part of a content of a mask ROM is different and the most parts of the contents are the same. CONSTITUTION:Most parts 12 except a part 13 of a mask ROM 11 are written with information common to each user. Each address of an EPROM 14 having the same storage capacity as the part 13 is the same as that of the part 13. The most parts 12 of the mask ROM 11 are read out in response to the address information from an address buffer 15 and outputted to a data bus line 18 via an opened gate circuit 17. On the other hand, address information corresponding to the part 13 is obtained from the address buffer 15, then an address discriminating circuit 16 outputs a discrimination signal, closes the gate circuit 17, inhibits the readout from the mask ROM 11 and the content of the EPROM 14 accessed according to the address information in this case is read out to the data bus line 18. |
