ROM

A read only memory (ROM) is a memory device designed to hold information (program, data) in a permanent or semipermanent way - ROMs are non-volatile. They keep their data when power is removed.

PCs use ROMS to store the boot-up program and other routines (BIOS), and the SBC5206 microcomputer uses ROM to store its dBug monitor operating system.

Conceptually, a ROM is an addressed array of registers. Figure 124 shows a simple $4 \times 1$ ROM. The 2 address lines are used to address the 4 memory locations.

**Figure:** A $4 \times 1$ ROM.
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ROM technology has developed over the years and a variety of ROMs are available.

Mask-programmed ROM. Permanently programmed at the semiconductor factory using a photographic mask.
Programmable ROMS (PROM). Programmable by blowing a fusable link - one-time programmable. A special PROM programming device is used.
Erasable PROM (EPROM). An EPROM can be programmed and erased many times. A special EPROM programming device is used which programs the EPROM via an high voltage pulse, and erasing is achieved using UV light (through a window on the chip).
Electrically erasable PROM (EEPROM). Similar to an EPROM, except that it can be erased using an HV pulse.
Flash ROM. Similar to EEPROM, but with rapid erase and write times, and with higher densities. They can be erased/programmed in circuit.

The SBC5206 microcomputer system has 256KB of flash ROM (to stored the dBug monitor). This consists of two $128K \times 8$ flash ROM chips, in the address range $FFE0 0000 to $FFE3 FFFF.

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