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Computer organization Notes

ROM Chips

A ROM chip is organized externally in a similar manner. However, since a ROM can only read, the data bus can only be in an output mode. The block diagram of a ROM chip is shown in Figure below. For the same-size chip, it is possible to have more bits of ROM than of RAM, because the internal binary cells in ROM occupy less space than in RAM. For this reason, the diagram specifies a 512-byte ROM, while the RAM has only 128 bytes.

The nine address lines in the ROM chip specify any one of the 512 bytes stored in it. The two chip select inputs must be CS1 = 1 and CS2’ = 0 for the unit to operate. Otherwise, the data bus is in a high-impedance state. There is no need for a read or write control because the unit can only read. Thus when the chip is enabled by the two select inputs, the byte selected by the address lines appears on the data bus.

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