Types of Memory
Memory can be divided into two types, volatile memory and non-volatile memory. Volatile,. memory temporarily stores the data. It loses data as soon as the system supply is turned it off. Non-volatile memory stores data permanently. It does not lose the data even if the system supply is turned off. Further, memory is classified into physical, flash and cache memory.
Physical memory is the total amount of memory installed in the computer. For example, if the computer has two 1 GB memory modules installed, it has a total of 2GB of physical memory.
The Different types of physical memory are:
RAM – Stands for Random Access Memory. It is a semiconductor-based memory, where the CPU or the other hardware devices can read and write data. It temporality stores the data and it is a volatile memory. Once the system turns off, it loses the data As a result. RAM is used as a temporary data storage area.
ROM – Stands for Read Only Memory. It stores the data permanently and it is a non-volatile memory. H does not lose the data even after the system turns oft As a result ROM is a permanent data storage area the different types of ROM are.
PROM – Stands fur the Programmable Read-Only Memory It stores programs permanently and is a non-volatile memory. Programming the ROM is sometimes ii referred to as burning and it requires a special machine called a device programmer or ROM burner Each binary 1 bit can be thought of as a fuse. when a PROM is programmed. a higher voltage blows or bums the fuses at the desired locations thus turning any given 1 into a 0. Due to this, PROM chips we often referred to as One Time Programmable (OTP) chips.
EPROM – Stands for the Erasable Programmable Read-Only Memory. Ultra-violet (UV) rays can remove the programs from this memory. It can be easily recognized by the clear quartz crystal window set on the top of chip. An EPROM eraser is a device that contains a UV light source that erases the chip by causing a chemical reaction, which essentially melts the fuses back together.
EEPROM/Flash ROM– Stands for the Electrically Erasable Programmable Read-Only Memory. Electrical signal removes the programs from this memory EEPROki can be erased by an electric field, rather than exposed to UV. Also, the data can be erased bit by bit allowing only selected portions of the code to be replaced But in case of Flash ROM, data is rewritten in blocks of 512 bytes instead of bit by bit, sc rewriting is faster Flash memory can erase or reprogram blocks of bytes where as EEPROM erases individual bytes.
This is also called hybrid memory as it reads and writes data similar to the RAM but, maintains data similar to the ROM It is a mixture of RAM and ROM.
Flash memory is the high-density device. It is a non-volatile memory. It is fast in reading and writing data. This is because it writes data in chunks or blocks. Flash memory is an electrically re-programmable device. The contents from the flash memory are erased in blocks and not in bytes. The block size can range from 256 bytes to 16 KB. Flash memory can replace hard disk in portable computers.
It is primarily used in memory cards and USB flash drives for general storage and transfer of data between computers and other digital products. Some of the latest memory cards include Multimedia card (MMC), Secure Digital (SD), CompactFlash, Memory Stick, xD-F fire Card and SmartMedia.
Cache memory is a small and fast memory which is placed between the CPU and main memory. It is accessed at a very high speed than the system memory. As a result, the programs which access the same data or instructions over and over run faster. The CPU does not have to traverse to the main memory to get the data. It will first access the cache to find the data. Figure 2.6 shows the structure of cache memory.
CPU does a check on the cache to find the data, if the data is not found then it proceeds to the system memory. This results in faster access to the data in the cache. But, it will be, lagging if the data is not in the cache and on the system memory. This can be countered: with modern hardware and better cache design. The time taken to traverse the cache it very less as compared to the whole RAM as it is small and fast memory.
Cache levels refer to the electronic pathways and connecting circuits via which cache (memory is connected to the CPU. They also indicate the physical closeness of cache memory to CPU. Most of the modern computers use two or three cache levels so the. processor does not have to wait for longer time for information from the memory.
Different types of cache levels are:
►L1L1 cache is referred to the cache which is built in the processor. This is the rest cache in the computer. This cache is also known as primary cache or internal The most common size of this type of cache memory is 8 KB to 64 KB.
► L2– New CPUs like from Pentium pro onwards usually incorporate the L2 cache directly on the processor chip. Earlier. L2 cache was located outside the processor i.e, commonly present on the motherboard. The most common size of this type of cache memory is 64 KB to 8 MB. L2 cache consists of two components.
► Data Store: This is the actual area where the cache data is stored.
► Tag RAM: This is a small area used to keep a map of the addresses of the system memory which are cached in the data store.