Read-only memory

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A read-only memory or read-only memory ( English read-only memory , ROM ) is a data storage on which in normal operation only read access can not write, and the non-volatile memory is. That means: It keeps its data even when there is no power. It is used in embedded systems with unchangeable software, but now mostly flash memory is installed instead , the content of which can be subsequently corrected / changed. On PCs, for example, the “hard-wired” BIOS was originally located in a ROM that could only be changed by exchanging or externally reprogramming.

Fixed-value core memory (core rope memory), very likely from a Nixdorf System 820 , storage capacity max. 256 wires per 16 lines, results in 4096 handwired command words of 18 bits

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 .

Overview

Originally, semiconductor ROMs were also "hard-wired" during manufacture. Since this wiring with a " mask is exposed onto direct" (a kind of film negative) on the chip, it is called here by a mask-programmed ROM or shortly mask ROM . Since this process is only economical in large-scale production, a - constantly growing - family of additional memory modules was developed which can also be filled with information after production, the so-called PROMs ( Programmable ROM ).

The writing of data in a ROM is called programming of the component and cannot be compared with the write access in a read-write memory ( random access memory , hard disk ). A distinction must be made between modules with reversible and irreversible programming. However, as ROM allows RAM a random access to the data.

functionality

A ROM chip has a number n address pins to which the querying logic (e.g. a processor ) applies the address from which it wants to read in the ROM chip. 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 adjacent address is meant for another (ROM) chip that is connected to the same address lines.

Example: A 64 kB ROM can store 1 byte = 8 bits at 2 ¹⁶ = 65536 addresses, i.e. addresses 0..65535. It therefore has 16 address input lines and 8 data output lines.

Computing ROM

Instead of a program, a ROM chip can only contain data. In a special form of this, a ROM chip can also "calculate" a hardware function directly: If its address pins are not viewed as an address but as an "input value", an output value can be burned in for each input value, which the ROM chip can later returns with this input. A ROM chip can thus map any Boolean function (“logic function”) that has a maximum of n input bits and m output bits.

variants

There are now quite a large number of different types of ROM:

• Mask ROM - only programmable at the time of production, the cheapest version for mass production
• Programmable Read-Only Memory (PROM), one-time programmable
• Erasable Programmable Read-Only Memory (EPROM), erasable with UV light
• Electrically Erasable Programmable Read-Only Memory (EEPROM), electrically erasable

Flash memory is often used today as a substitute for the ROM - also available as FLASH EEPROM with optional addressing.

Web links

• ( Page no longer retrievable , search in web archives: formerly threaded read-only memory, still in use in a museum )
• Computer History Museum: http://www.computerhistory.org/semiconductor/timeline/1965-ROM.html