Are you curious about the several kinds of ROM (Read-Only Memory) that exist? If you didn’t know they did, now you do. What’s more, the percentage of their individual use has changed over the years to accommodate technological advancement. However, nearly all types of ROM are still in use today, even if only for repairing old electronics or making replicas. Moreover, they can change in appearance, e.g., users familiar with huge electronics from the past may not recognize their downsized version on modern electronics, at least at first. To help you understand their purpose and recognize the way they look, we’ll go over all ROM types.
Overview of ROM (Read-Only Memory)
First, you must understand the base term. ROM is the primary non-volatile memory that retains its data. To clarify, “non-volatile” means that, unlike RAM (Random Access Memory), it does not lose stored data when powered off. Therefore, it is used for storing crucial information that does not need to change. Some notable applications of ROM are calculators, peripheral devices, the chip that stores the program needed for booting a computer (“bootstrap”), firmware for hardware, or embedded systems with pre-determined programming.
The name also suggests that we can only read from the memory, not write to it. Hence, we use ROM for the permanent storage of information. More precisely, ROM cannot be changed electronically after it leaves the factory. Since we anticipated users’ wonder about updating firmware or the apparent ability to write data (especially after reading about EEPROM), we need to make one thing clear. The only way to “write” any data is to obtain special apparatus and change the contents electronically. That is the definition of “programming” the memory. To put things into simple terms, ROM chips cannot be reprogrammed in the target device, only using dedicated equipment.
1. MROM (Masked/Mask Read-Only Memory)
MROM, also known as classic or mask-programmed, is the oldest among types of ROM and is thus rarely in use outside of making replicas or fixing decades-old computers. Though inexpensive today, this form is rudimentary and is made up of integrated circuits. The chip is programmed at the factory and cannot be reprogrammed in any way. Also, it utilizes a particular input-output pathway to send a current and fuses organized in columns and rows to determine the pathway’s location. Thus, rewiring is practically impossible. The only way MROM could be useful nowadays is if a particular ROM chip template is available and in high demand. That makes copies cheap to produce.
2. PROM (Programmable Read-Only Memory)
Programmable ROM is the type of memory that is a blank slate, more specifically, a ROM chip that was manufactured but not programmed. Users can purchase these blank ROM chips and utilize the special equipment, “programmer”, to customize the functionality and decide the type of data they want to program. Unlike the type above, PROM lets the current flow through all available pathways without a defined input or output.
Thus, the programmer tool generates the desired pathway by deciding which fuses will remain active. Fuses that are unnecessary are burned out by high voltage and become unusable from that day on. One common worry is that because fuses are so sensitive, static electricity has the chance to ruin PROM before it is programmed. Therefore, they are considered more damage-prone compared to other ROMs.
3. EPROM (Erasable Programmable Read-Only Memory)
EPROMs are among the types of ROM that permit users to program and reprogram the contents of the chip many times. They may or may not be programmed at the factory, depending on whether there’s a need for data to be present from the get-go. EPROM uses a specific quartz window combined with a purpose-made EPROM programmer (different from regular ones). The programmer can generate ultraviolet light (UV-C) at a specific frequency (254 nm wavelength). This will burn out any charges inside the EPROM chip and remove any existing pathways, returning the chip to a blank state. Thereafter, the chip essentially becomes PROM and can be programmed as such.
Here’s an example. If the chip stores data in bits, you can program any type or number of bits at once. However, a benefit of EPROMs is that users can reprogram the chip without erasing the contents, so long the bit stays 0 or changes from 1 to 0. With that in mind, if the bit needs to change from 0 to 1, the chip must be erased. Further, the chip cannot be wiped clean infinitely. Luckily, thanks to advances in technology, the current EPROMS can be erased over 1000 times.
Also, to fit as many distinct needs, EPROM is sold with or without the quartz window. Though those sold without one are cheaper, they’re titled OTP EPROM (One-Time Programmable EPROM). It’s pivotal to note that the window needs to be covered when unused. That’s because sunlight contains UV-C light components and may erase the chip. Finally, there are two prominent types of EPROM. The one with a “C” in the name is a CMOS (Complimentary Metal Oxide Semiconductor) EPROM. In contrast, the chip with “N” in the name is an NMOS (N-channel Metal Oxide Semiconductor) EPROM.
4. EEPROM (Electrically Erasable Programmable Read-Only Memory)
EEPROM is, as you can guess, an advanced type of EPROM that brings several advantages. For instance, we can reprogram or erase it over 10,000 times. Also, instead of using UV light, which erases the entire memory, EEPROM can electrically erase individual bytes, and even permit their overwriting. While the programming time is many times slower (typically 4 to 10 milliseconds per byte), the precision is impeccable. Moreover, some manufacturers add functions of programming blocks to save time when needed. Instead of individual bytes, functions can simultaneously program 64, 128, or 256 bytes as a single block. Unsurprisingly, these benefits raise the price, often considerably.
Since the late 2000s, manufacturers have been working on an even more powerful solution. Flash EEPROMs can be wiped and reprogrammed much faster, and have a drastically higher lifespan, even exceeding a million cycles. Due to increased utilization of the chip, their capacity has increased, and NAND flash EEPROM chips with 32 GB were available back in 2007. Those in 2022 range between 128 and 176 layers used for storage. Manufacturers such as Samsung claim they can produce between 200 and 256 layers for the 8th generation of V-NAND chips and speculate their future 3D NAND can go up to 1000 layers.