Read-only memory
ROM is a redirect to this article. For other meanings, see Rome (disambiguation).
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A read-only memory (ROM) is a data memory that can only be accessed for reading during normal operation, not for writing, and that is non-volatile. This means that it retains its data even when de-energized. It is used in embedded systems with unchangeable software, but nowadays flash memory is usually installed instead, the contents of which can be subsequently corrected/changed. On PCs, for example, the "hard-wired" BIOS was originally in ROM, which could only be changed by replacement or external reprogramming.
Before the development of semiconductor memories, read-only memories were implemented using core rope memory, for example, in which the content is determined by the type of wiring. This uses toroidal cores.
Fixed value core memory (Core Rope Memory), most likely from a Nixdorf System 820, memory capacity max. 256 wires per 16 lines, results in 4096 handwired instruction words of 18 bits each
Overview
Originally, semiconductor ROMs were also "hard-wired" during production. Since this wiring is directly exposed onto the chip with a "mask" (a kind of film negative), this is referred to as mask-programmed ROM or mask ROM for short. Since this process is only economical in large-scale production, a - constantly growing - family of further memory chips was developed which can also be filled with information after production, the so-called PROMs (Programmable ROM).
The writing of data into a ROM is called programming of the device and is not comparable with the write accesses in a random access memory (hard disk). A distinction must be made between devices with reversible and irreversible programming. However, like RAM, ROM allows random access to the data.
How it works
A ROM chip has a number n of address pins to which the querying logic (e.g. a processor) applies the address from which in the ROM chip it wishes to read. In addition, the ROM chip has m data output pins on which it then outputs the value that is burned into it at the requested address. Many ROM chips also have a single pin called "chip select" which indicates whether the ROM chip is meant, or the address present is meant for another (ROM) chip connected to the same address lines.
Example: A 64-kB-ROM can store 1 byte = 8 bits at 216 = 65536 addresses, i.e. address 0..65535. It therefore has 16 address input lines and 8 data output lines.
