The Difference Between RAM, ROM, and Flash

Jun 19, 2023

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The Difference Between RAM, ROM, and Flash

Semiconductor memory is an essential component of modern computing systems. Two of the most common types of semiconductor memory are ROM and RAM. ROM stands for read-only memory, while RAM stands for random access memory. ROM is capable of retaining data even when the system is powered off, while RAM cannot. Computer memory is a typical example of RAM.

However, many people may not be clear about the difference between flash memory, ROM, and RAM. In this article, we will explore these types of memory in detail, providing a comprehensive guide to basic memories. Whether you're a tech enthusiast or just looking to learn more about computer hardware, this article is for you.

What are types of RAM?

1. Static RAM (SRAM)

SRAM is currently the fastest storage device for reading and writing data. However, due to its high cost, it is typically reserved for demanding applications, such as serving as the first and second level buffer for CPUs.

2. ynamic RAM (DRAM)

Computer memory is a crucial component in the performance of a computer. Dynamic Random Access Memory (DRAM) is a type of memory that stores data for a short time and operates slower than Static Random Access Memory (SRAM), but is still faster than Read-Only Memory (ROM). Moreover, DRAM is much cheaper than SRAM, making it the most common type of memory used in computers.

There are many different types of DRAM, such as FPRAM/FastPage, EDORAM, SDRAM, DDR, RAM, RDRAM, SGRAM, and WRAM. DDR RAM, or Double-Date-Rate RAM, is an improved version of SDRAM that can read and write data twice in one clock cycle, effectively doubling the speed of data transmission.

DDR RAM is the most widely used memory in computers today due to its cost advantage and superior performance compared to other types of memory, such as Rambus DRAM. High-end graphics cards also use high-speed DDR RAM to increase bandwidth, which enhances the pixel rendering capability of 3D accelerator cards.

What are types of ROM?

PROM (Programmable Read-Only Memory) is an early type of memory that can only be programmed once and cannot be modified. However, due to its limitations, it is no longer widely used.

EPROM (Erasable Programmable Read-Only Memory) is a type of memory that can be erased and reprogrammed using ultraviolet radiation. It is a universal memory that can be used for various applications.

EEPROM (Electrically Erasable Programmable Read-Only Memory) is a type of memory that can be electronically erased and reprogrammed. It is more expensive and has a longer writing time than other types of memory. EEPROM is commonly used in mobile phones, where the last dialed numbers are temporarily stored in SRAM before being written to the call log in EEPROM.

Computer memory is used to store data and programs that are currently in use. The most common type of memory used in computers is dynamic random-access memory (DRAM). The "dynamic" in DRAM refers to the fact that data written to the memory will be lost after a period of time. To maintain the continuity of the data, an extra circuit is needed to refresh the memory.

When data is written to a DRAM cell, it is stored as a charge on a capacitor. Over time, the charge on the capacitor will leak away, causing the data to be lost. To prevent this, the refresh operation regularly checks the capacitor to see if the charge is above or below a certain threshold. If the charge is above the threshold, it is considered to represent a 1 and the capacitor is fully charged. If the charge is below the threshold, it is considered to represent a 0 and the capacitor is discharged to maintain the continuity of the data.

What is Flash Memory?

Flash memory is a type of memory that combines the advantages of both ROM and RAM. It is electrically erasable and programmable, like EEPROM, and can read data quickly without power loss, like NVRAM. Flash memory is commonly used in USB drives and MP3 players. In the past, embedded systems used ROM (EPROM) as their storage device, but in recent years, FLASH has completely replaced ROM (EPROM) to store Bootloader, operating systems, program code, or directly as a hard disk (U disk).

There are two types of flash memory: NOR Flash and NAND Flash. NOR Flash is suitable for storing small amounts of code, and its fast reading speed allows users to run the code loaded in NOR FLASH directly, which reduces the capacity of the SRAM and saves cost. NAND Flash is cheaper and is suitable for storing data that requires high storage density. However, users cannot run program code on NAND directly, so many developers using NAND have to use a small NOR Flash to startup and run code.

Intel, AMD, Fujitsu, and Toshiba are the main manufacturers of FLASH on the market, while Samsung and Toshiba are the main manufacturers of NAND Flash.

NOR and NAND are the two main non-volatile flash memory technologies in the market. NOR flash technology was developed by Intel in 1988, while Toshiba published its NAND flash architecture in 1989. NOR flash is characterized by simple application, no special interface circuit, high transmission efficiency, and in-chip execution (XIP). NOR flash is generally used to store a small amount of code, especially in code storage media. NAND flash, on the other hand, provides extremely high cell density, high storage density, and fast write and erase speed. However, its application is more difficult due to the management of flash and the need for a special system interface.

Performance Comparison

Flash memory is a type of non-volatile memory that can be erased and reprogrammed on a memory cell block. However, the write operation of any flash device can only be performed on an empty or erased cell. This means that in most cases, the erasure must be performed before writing.

NAND devices perform the erasure operation easily, while NOR requires that all bits in the target block be written as 1 before erasing. NOR devices are erased as a block of 64~128KB, which takes around 5 seconds to perform a write/erase operation. In contrast, NAND devices are erased as a block of 8~32KB, and the same operation only takes a maximum of 4 milliseconds.

The performance gap between NOR and NAND is further widened by the difference in block size during erasure. For a given set of write operations, especially when updating small files, more erasure operations must be performed in NOR-based cells. Therefore, when choosing a storage solution, the designer must weigh the following factors:

- NOR has slightly faster read speeds than NAND, but slower writing speeds.

- NAND has a much faster erasure speed of 4ms compared to NOR.

- The NAND erasure unit is smaller, and the corresponding erasure circuit is less complex.

Interface Difference

NOR flash memory is a type of non-volatile storage that uses a SRAM interface and has enough address pins to easily store and program each byte. This makes it ideal for applications that require fast read and random access.

In contrast, NAND flash memory uses complex I/O ports to access data in a serial manner. The data access method can vary from product to product or manufacturer to manufacturer. NAND flash memory uses 8 pins to transmit and control address and data information.

NAND flash memory reads and writes data in 512-byte blocks, which is similar to hard disk management. This makes NAND memory suitable for high-density storage and applications that require large block sizes. In fact, NAND-based memory can even replace hard drives or other block devices.

Capacity and Cost

NAND flash has almost half the cell size of NOR devices, making it a more cost-effective option for higher capacity storage. The simpler manufacturing process of NAND flash allows for more storage within a given die size, resulting in a lower price.

In the 1~16MB flash market, NOR flash is the dominant technology, while NAND flash is mainly used in 8~128MB products. NOR flash is primarily used for code storage, while NAND flash is better suited for data storage. NAND flash is commonly used in CompactFlash, Secure Digital, PC Cards, and MMC memory cards.

Reliability and Durability

When using flash media, reliability is a key factor that needs to be considered. Flash is a very suitable storage solution for systems that need to extend MTBF. The reliability of NOR and NAND can be compared in terms of bit switching, lifetime (durability), and bad block processing.

1. Lifetime

NAND flash memory has a maximum number of erasures per block of one million times, while NOR has 100,000 times. NAND memory also has the advantage of 10:1 erasure speed and each block has fewer deletions at a given time, making it more durable than NOR.

2. Bit Switching

All flash devices are susceptible to bit switching, where a bit may reverse or its reversion is reported. This can cause system failure if it occurs on a critical file. Although bit inversion is more common in NAND flash memory, it can be solved by using an error detection/correction (EDC/ECC) algorithm. It is recommended to use NAND flash memory and EDC/ECC algorithm together to ensure reliability.

3. Bad Block Processing

Bad blocks in NAND devices are randomly distributed. Detecting and marking bad blocks is essential to ensure the reliability of the device. If this treatment cannot be carried out by a reliable method, the failure rate will be high. Although previous efforts have been made to eliminate bad blocks, they found that the yield was too low and the cost was too high.

Simple Operation

NOR-based flash memory is simple and straightforward to use, as it can be connected like other memory and can run code directly on it.

On the other hand, NAND flash memory is more complex due to the need for an I/O interface. Access methods for various NAND devices can vary depending on the manufacturer. When using NAND devices, it is important to first write to the driver before doing anything else. Additionally, designers must be careful not to write to bad blocks when writing information to NAND devices, which requires the use of virtual mapping.

Software Support

When discussing software support for flash memory devices, it is important to distinguish between basic read/write/erase operations and higher-level software for disk emulation and flash management algorithms, including performance optimization.

While running code on a NOR device does not require much software support, using a NAND device typically requires a Memory Technology Driver (MTD) for writing and erasing operations. However, NOR devices require less MTD because many vendors offer advanced software for NOR devices, such as M-System's TrueFFS driver, which is used by manufacturers such as Wind River System, Microsoft, QNX Software System, Symbian, and Intel. This software can simulate DiskOnChip products and manage NAND flash memory, including error correction, bad block handling, and loss balancing.

Intel and Micro used to be the main manufacturers of NOR flash and it was the mainstream flash product, but its market share has been squeezed by NAND flash. One advantage of NOR flash is that it can run programs directly from the flash, but the process is complex and expensive. NAND flash, on the other hand, is mainly supplied by Samsung and Toshiba and is widely used in USB drives, memory cards, and MP3 players. It has a larger storage capacity and is cheaper than NOR flash, but it cannot address running programs directly and is only used to store data. Additionally, NAND flash is prone to bad blocks, so it is necessary to detect them using related algorithms.

NAND flash is used to store data and programs in laptops, but it must be started with NOR flash. Mainstream processors for laptops, other than Samsung, do not support NAND flash startup directly. Therefore, the machine must be started with a small piece of NOR flash, and the OS and other software must be loaded into SDRAM through NAND flash.

Conclusion

DRAM uses the charge on the gate capacitor of the MOS transistor to store information. However, the charge on the gate capacitor will leak over time, causing the stored information to be lost. To prevent this, DRAM requires regular refresh operations to replenish the charge on these gate capacitors. This type of memory is called dynamic RAM. Despite this limitation, DRAM is highly integrated and can perform a lot of work.

SRAM, on the other hand, uses registers to store information. Unlike DRAM, SRAM does not require refresh operations to maintain the stored information. However, the data will be lost once the power is down. This type of memory is called static RAM and is mainly used in systems that require large capacity and do not need to recover data through refresh operations.

Flash ROM uses the capacitor on the floating gate to store the charge and information. Because the floating gate does not leak, the information can still be saved even after the power is off. Additionally, Flash ROM has a simple mechanism and can be highly integrated, allowing for large storage capacity. However, before writing new information, Flash ROM must first be erased by electricity. Unlike EEPROM, which can be erased in bytes, Flash ROM can only be erased in sectors. Flash ROM is mainly used for BIOS, U disk, and MP3 devices that require large capacity and data storage when power is down.