In computing, memory refers to the physical devices used to store programs (sequences of instructions) or data on a temporary or permanent basis for use in a computer or other digital electronic device
Types of Memory
- External Memory
- Internal Memory
External memory typically refers to storage in an external hard drive or on the Internet. The main “memory” in the computer is the computer work-space, not its storage facility.
External memory which is sometimes called backing store or secondary memory, allows the permanent storage of large quantities of data. Some method of magnetic recording on magnetic disks or tapes is most commonly used
The capacity of external memory is high, usually measured in hundreds of megabytes or even in gigabytes
The most common form of external memory is a hard disc which is permanently installed in the computer and will typically have a capacity of hundreds of megabytes
Types of External Memory’s:
- Magnetic Tapes
- Hard disk
- Magnetic Disk
- Optical Drives (CD-R/W, CD-ROM
Internal memory typically refers to main memory (RAM), but may also refer to ROM and flash memory. In either case, internal memory generally refers to chips rather than disks or tapes.
- In a computer, all of the storage spaces that are accessible by a processor without the use of the computer input-output Internal memory usually includes several types of storage, such as main storage, cache memory, and special registers, all of which can be directly accessed by the processor.
- Primary storage (or main memory or internal memory), often referred to simply as memory, is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required. Any data actively operated on is also stored there in uniform manner.
- It’s also called ( Primary /Main/Temporary/Semiconductor) Memory type.
Types of Internal Memory
RAM (Random Access Memory)
Random access memory, or RAM, is memory storage on a computer that holds data while the computer is running so that it can be accessed quickly by the processor. RAM holds the operating system, application programs and data that is currently being used.
RAM data is much faster to read than data stored on the hard disk. RAM is stored in microchips and contains much less data than the hard disk. RAM can never run out of memory, but the processor must overwrite old data if the RAM is filled, which results in slower computer function. Any file stored in RAM can be accessed directly if the user knows the row and column where the data is stored.
- Random access memory is used to store temporary but necessary information on a computer for quick access by open programs or applications.
- RAM, is a volatile yet fast type of memory used in computers. RAM is more expensive to incorporate.
- RAM allows reading and writing (electrically) of data at the byte level
- RAM is the Volatile memory.
Types of RAM
- Static RAM
- Dynamic RAM
Static RAM stores a bit of information in a flip-flop. Static RAM is usually used for applications that do not require large capacity RAM memory.
Static(RAM) is a memory technology based on flip-flops. SRAM has an access time of 2 – 10 nanoseconds. All of main memory can be viewed as fabricated from SRAM, although such a memory would be unrealistically expensive.
Dynamic RAM data store one bit of information as a payload. Dynamic RAM using a substrate capacitance gate MOS transistors as memory cells shut. To keep dynamic RAM stored data remains intact, the data should be refreshed again by reading and re-write the data into memory. Dynamic RAM is used for applications that require large RAM capacity, for example in a personal computer (PC)
- EDO (Extended Data-output) and SD (Synchronous Dynamic Random Access Memory) are type of Dynamic RAM.
- Dynamic RAM (DRAM) is a memory technology based on capacitors
- Dynamic RAM is cheaper than static RAM and can be packed more densely on a computer chip
- DRAM has an access time in the order of 60 – 100 nanoseconds, slower than SRAM.
ROM (Read only Memory)
Sometimes can be erased for reprogramming, but might have odd requirements such as UV light or erasure only at the block level
Sometimes require special device to program, i.e., processor can only read, not write.
- Data are written into a ROM when it is manufactured.
- ROM is mask programmed by the manufacturer in the factory with the contents ordered by the customers.
- The contents are fixed by metal masks used during chip fabrication.
- Once programmed, the contents cannot be erased.
- Even a single bit wrongly programmed the ROM chip is useless
- Used to store control programs such as micro program.
- Character generation, code conversion
- Permanent storage – nonvolatile
- Library subroutines
- Systems programs (BIOS)
- Function tables
- Embedded system code
Types of ROM
- PROM (Programmable Read Only Memory)
- EPROM (Erasable Programmable Read Only Memory )
- EEPROM (Electrically Erasable Programable Read Only Memory)
If the content is determined by the vendor ROM, PROM sold empty and can then be filled with a program by the user. Having completed the program, fill PROM cannot be removed.
PROM (Programmable Read Only Memory)
- PROM is a field programmable device.
- The customer buy a blank PROM and store desired data using PROM programmer(burner).
- Programmability achieved by inserting a fuse at point P.
- Before programmed, the memory contains all 0s
- The user can insert 1 by burning out the fuse in the particular cell using high current pulse.
- The PROM chip can be programmed only once and its contents cannot be erased.
- PROM are flexible, faster and less expensive because they can be programmed directly by the user.
EPROM (Erasable Programmable Read Only Memory )
- A rewritable chip that holds its contents without power. Previous data can be erased and new data can be inserted
- EPROM chips are written on an external programming device before being placed on the circuit board. Capable of retaining stored information for a long time.
- Eraser contd., requires breakup the charges trapped in the transistors of memory cell.[this is done by break the chip to ultraviolet light].
- This reason EPROM packaged with transparent window.
- Disadvantages: Entire EPROM is erased as a whole and selective erasing is not possible.
- Should be removed from the chip for reprogramming.
- Unlike the PROM, EPROM contents can be deleted after being programmed. Elimination is done by using ultraviolet light.
EEPROM (Electrically Erasable Programable Read Only Memory)
EEPROM can store data permanently, but its contents can still be erased electrically through the program. One type EEPROM is Flash Memory. Flash Memory commonly used in digital cameras, video game consoles, and the BIOS chip.
- It can be both programmed and erased electrically ( flashed back to Zero).
- They do not need to removed when the chip content erasure.
- Also, erase selected content in the chip.
- Erasing and programming dynamically without removing the EEPROM from the circuit.
- Different voltages are required for erasing, reading and writing the data.
Cache is generally divided into several types, such as L1 cache, L2 cache and L3 cache. Cache built into the CPU itself is referred to as Level 1 (L1) cache. Cache is in a separate chip next to the CPU is called Level 2 (L2) cache. Some CPUs have both, L1 and L2 cache built-in and assign a separate chip as cache Level 3 (L3) cache. Cache built in CPU faster than separate cache. However, a separate cache is still about twice as fast from Random Access Memory (RAM). Cache is more expensive than RAM, but the motherboard with built-in cache very well in order to maximize system performance.
Cache serves as a temporary storage for data or instructions needed by the processor. In addition, the cache function to speed up data access on the computer because the cache stores data/information has been accessed by a buffer, to ease the work processor.
Another benefit of cache memory is that the CPU does not have to use the bus system motherboard for data transfer. Each time the data must pass through the system bus, the data transfer rate slow the ability motherboard. CPU can process data much faster by avoiding obstacles created by the system bus.