Data and instructions are stored in memory to be executed. This memory is a temporary storage unit for data, instructions, and information. It is also called primary storage. It consists of one or more chips on the motherboard. The memory stores the following three items:
1. Operating system and other system software to control the use of computer system
2. Application programs to perform specific tasks
3. Data to be processed by application programs
The instructions and data are stored in memory as bytes during execution. Each byte in the memory has its unique address that identifies its location. There are two types of memory in a computer that are as follows:
- Volatile Memory - Volatile memory loses its contents when the computer is turned off.
- Non-volatile Memory — It does not lose its contents when the computer is turned off.
RAM stands for Random Access Memory. It is also called main memory or Direct Access Memory. Random access means that each individual byte in the entire memory is directly accessible. A program must be loaded into RAM before execution.
RAM is temporary memory. When the power is turned off, the information in this memory is lost. Thus it is called volatile memory.
RAM plays a very important role in the processing speed of a computer. A bigger RAM size provides larger amount of space for processing. 50 the processing speed is Md. CPU can read data from RAM and write data to RAM. That is why RAM is also known as reading/write memory. It is used to store data and instruction while it is being executed.
There are two types of RAM:
SRAM stands for Static Random Access Memory. The memory cells are made from digital gates. Each cell can store its value without any need to refresh the data as long as the power is available.
SRAM is more expensive. It does not need to be power-refreshed. CPU does not wait to am data. That is why it is faster. It utilizes more power than DRAM. It is normally used to build a very fast memory known as cache memory.
DRAM stands for Dynamic Random Access Memory. It is a type of memory that is used in most computers. It is the least expensive kind of RAM.
DRAM requires an electric current to maintain its electrical state. The electrical charge of DRAM decreases with the time that may result in loss of data. DRAM is recharged or refreshed again and again to maintain its data. The processor cannot access the data of DRAM when it is being refreshed. That is why it is slow.
Differences between SRAM and DRAM
- It is faster than DRAM.
- It is more expensive.
- It does not need to be power-refreshed.
- .It utilizes less power.
- It is slower than SRAM.
- It is less expensive.
- It has to be refreshed after each read operation.
- It utilizes more power
RAM is mounted on a small circuit board called the memory module. The memory modules mounted on the motherboard. Three types of memory modules are as follows:
- SIMM — SIMM stands for a single inline memory module. In this module, the pins on opposite sides of the circuit board connect together to form a single set of contacts.
- DIMM — DIMM stands for a double inline memory module. In this module, the pins on opposite sides of the circuit board do not connect and form two sets of contacts.
- RIMM — RIMM stands for Rumbas inline memory module. It houses SDRAM chips.
Read-Only Memory (ROM)
ROM stands for read-only memory. The instructions in ROM prepare the computer for use. These instructions can only be read but cannot be changed or deleted. It is not possible to write new information or instructions into the ROM.
ROM-stores dam and instructions permanently. When the power is switched off, the instructions stored in ROM are not lost. Therefore ROM is called non —volatile memory.
The information in ROM is stored by the manufacturer. When the computer is switched on, the instructions in the ROM are automatically loaded into the memory of the computer. Different types of ROM are as follows.
PROM stands for Programmable Read-Only Memory. This form of ROM is initially blank. The user or manufacturer can write data on it using special devices. The user can write data and instructions on it only once If there 15 any error in writing the instructions the error cannot be removed from PROM. The chip becomes unusable. It is used to store the programs or data developed by the user.
EPROM stands for Erasable Programmable Read-Only Memory User can erase instructions or data stored in EPROM chip by exposing the chip to ultraviolet light and write a new program.
EEPROM stands for Electronically Erasable Programmable Read-Only Memory. In this memory, the user can erase and write instructions with the help of electrical pulses. If there is an error in writing the instructions, the user can erase the contents electrically.
Difference between RAM & ROM
- RAM is a temporary memory.
- RAM enables data to be both read and written to memory and data can be changed or deleted.
- Instructions in RAM are continuously changing as different programs are executed and new data are processed.
- When the power is turned off, all the programs and data are erased from RAM. So RAM is a volatile memory.
- The instructions are written into the RAM at the time of execution.
- ROM is permanent memory.
- The instructions written in ROM can only be read but cannot be changed or deleted.
- It is not possible to write new information or new instructions into the ROM, so ROM is a non-volatile memory.
- When the power is turned off, the instructions in ROM are not lost, therefore ROM is a non-volatile memory
- The instructions are written into ROM at RAM at manufacturing time.
Difference between PROM and EPROM
- FROM is a programmable memory.
- The user can write instructions on PROM only once.
- The instructions written by the user cannot be erased from PROM.
- If there is an error while writing on PROM, it becomes unusable.
- It provided less usability as instructions are written only once.
- EPROM is an electronically programmable memory.
- The user can write instructions on EPROM many times.
- The instructions written by the user can be erased from EPROM.
- If there is an error while writing on EPROM, it can still be used again.
- It provides more usability as instructions written many times.
A cache ( pronounced” cash”) is a small and very fast memory. It is designed to speed up the transfer of data and instructions. It is faster than RAM. The data and instructions that are most recently or most frequently used by CPU are stored in the cache.
The data and instructions are retrieved from RAM when the CPU uses them for the first time. A copy of that data or instructions is stored in the cache. The next time the CPU needs that data or instructions, it first looks in the cache. If the required data is found there, it is retrieved from cache memory instead of the main memory. It speeds up the working of CPU.
Types of Cache Memory
Different types of cache memory are as follows:
Level 1(L1) Cache: It is also called primary or internal cache. It is built directly into the processor chip. It has a small capacity from 8KB to 128 KB.
Level 2 (L2) Cache: It is slower than the L1 cache. Its size is large i.e. from 64KB to 4Mb. New processors contain advanced transfer cache on processor chip that is a type of L2 cache. The common size of the cache is 256 KB.
Level 3 (L3) cache: This cache is separate from the processor chip on the motherboard. It exists on a computer that well-advanced transfer cache.
Flash memory is also called flash ROM or flash RAM. It is a type of nonvolatile memory. It can be erased electronically and reprogrammed. Many computers are using flash BIOS. Flash BIOS can be updated easily if required.
Flash memory chip stones data on handheld computers like cellular phones, digital cameras, etc. Another type of flash memory is Flash memory cards that stores flash memory on a removable device instead of chip.
CMOS stands for complementary metal-guide semiconductor. It stores configuration information of the computer. The information includes the type of disk drivers, keyboard and monitor, current date & time, and other startup information required during the booting process.
CMOS chip uses battery power to retain information even when the computer is turned off. The information in CMOS can be changed.
Memory Access Times
The amount of time required by the processor to read data, instructions, and information from memory is called access time. Access time is defined by different types of terminologies that are as follows:
- Millisecond: It is one-thousandth of a second. It is denoted by ms.
- Microsecond: It is one-millionth of a second. It is denoted by μs.
- Nanosecond: It is one-billionth of a second. It is denoted by ns.
- Picosecond: It is one-trillionth of a second. It is denoted by ps.
- Megahertz: It can be converted into nanoseconds by dividing it into 1 billion ns. It is denoted by MHz.