| 摘要 |
A microcomputer comprises a processor (15), on-board ROM (14) and an external store (13). The external store (13) is connected to the processor (15) by address and data lines (16, 17), the data line (17) being connected to earth via a resistance (30) which in the absence of the external store (13) causes a reference potential to be supplied to the processor 15. When an external store (13) is present, addressing of the store (13) causes the reference potential to disappear and hence the processor will use the external store (13) rather than the on-board ROM (14).
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