There are two main classes of primary storage, RAM and ROM.
RAM stands for Random Access Memory, while ROM stands for Read Only Memory.
Generally, the difference between these two technologies is that RAM is volatile while ROM is non-volatile.
Volatile memory requires a constant power supply to retain data.
This means that the data is lost when the computer is turned off.
Non-volatile memory does not need a continuous power supply, making it suitable for storing long-term data.
Note:The boundaries of RAM and ROM can be challenging to discern.
For example, NVRAM is non-volatile, and modern types of ROM can be reprogrammed.
Hard drives also dont neatly fit into either category, secondary storage.
In early computers such as the Commodore 64, all storage was provided with only RAM and ROM.
Instead, custom programs could be run by inserting a memory cartridge containing more ROM.
Contents
Core Memory Mask ROM and PROM
The key part of ROM is that it is read-only.
This is very distinctly clear in early forms of storage.
With the advent of the integrated circuit, mask ROM became the preferred choice.
Mask ROM devices were custom-designed electronic circuits.
They were designed by creating a circuit diagram mask and then produced to match that mask.
They could not be reprogrammed as the data was physically encoded in the circuit.
Mask ROM has four core problems.
Thirdly, R&D work is impractical due to the long turnaround times and high costs.
Finally, suppose a fault is found after the product launch.
This allowed companies to order giant batches of essentially blank PROM chips and then program them as needed.
Like PROM, blank chips could be purchased in bulk and programmed as needed.
Critically EPROM could also be erased.
The process for doing so was a little complex.
However, it requires the chip to be exposed to intense ultraviolet light.
EPROM chips were housed in cases with a transparent quartz window to facilitate UV exposure.
A branded sticker was generally placed on top to prevent accidental disclosure and erasure.
The sticker was typically foil-backed to ensure that UV light could not reach the chip.
EEPROM, or Electronically Erasable Programmable Read Only Memory, developed in 1972, generally solved the fourth issue.
It could be erased electronically.
Flash Memory is a form of EEPROM invented in the early 1980s.
Reprogramming can be limited to a portion of the chip rather than the whole thing.
The erasure and reprogramming process is much faster, earning the name Flash.
In Modern Computers
In modern devices, RAM is still universal.
ROM maintained popularity for a long time, precisely mask ROM because it was cheap to produce.
In the contemporary world, though, it is critical to be able to push firmware updates to devices.
This has generally led to replacing ROM with flash memory almost completely.
It performs the same functionality but can be reprogrammed as needed.
Typically, reprogrammable ROM can be configured with write protection to become read-only.
This isnt a fail-proof solution, though.
This name comes from the shortening of the term ROM images.
Conclusion
ROM stands for Read Only Memory.
It originally referred to primary memory that could only ever be programmed during manufacture.
