Chap3 primary memory
- 2. contents Introduction Memory hierarchy Random Access Memory
- 3. introduction Computers are used to perform various tasks in science, engineering , business, education, entertainment and many other fields. They work at high speed and can handle huge amount of data with great accuracy, can perform operation in sequence without any human interventions. Computer requires memory to process data, hold intermediate results and to store the output. Computer memory refers to the electronic holding place for instructions and data where the processor can reach quickly. Two categories : primary and secondary memory.
- 4. Primary memory is used to process data and instructions Secondary memory is used to store output The major limitations with the primary memory is it is volatile in nature i.e. when power is turned off the data is lost forever. To store data permanently a computer requires some non-volatile storage medium like a hard disk, this kind of memory is called secondary storage
- 5. Memory representation All quantities are measured in units. Length is measured in meters, time in seconds, weight in kgs. Digital computers work in two states, ON(1) OFF(0) O represents a false value, +5 volts represents a true state i.e. 1. Each of these values is called binary digit or bit. Group of eight bits is called a byte.
- 6. Bit: smallest unit of data on a machine and a single bit can only hold one of the value 0 or 1. Byte: A unit of eight bits is known as byte Kilobyte: in decimal system, kilo stands for 1000 but in binary system kilo refers to 1024. Kilobytes is 1024 bytes. Megabytes: 1024 kilobytes Gigabytes: 1024 MB Terabytes : 1024GB
- 7. Memory hierarchy The memory in a computer system is of three fundamental types. Internal processor memory: Is placed within the CPU or it is attached to a special fast bus. Includes cache memory and special registers Used to compensate the speed gap between the primary memory and the processor. Primary memory RAM , ROM come under this categories Also known as main memory. BIOS
- 8. Secondary memory Also called as auxiliary memory. Provides a backup storage for instructions and data Eg: magnetic disk and tapes Least expensive and have large physical storage. Data stored in these, are permanently stored forever. They can be removed if the user wants to or it is destroyed. They can be used as overflow memory, when the primary memory is full The data and instructions have to be shifted from secondary to main memory, later it is shifted to the processor.
- 9. Memory hierarchy When data comes from permanent storage , they first go into RAM. CPU has to access the hard disk constantly to retrieve every piece of required data , it would operate slowly. When the data is kept in primary storage, it can accessed quickly.
- 10. Memory hierachy
- 11. Random access memory It allows computer to store data for immediate manipulation, and keep track of what is currently being processed. Its place where the computer keep operating system, application and data in current use, so that the processor can access it quickly. Data in RAM in it, only till computer is ON. When the computer goes down, the data is lost which present in RAM. When the computer is started again , the operating system and files are loaded into RAM.
- 12. Interaction of memory and storage with processor Whenever the user enters a command from keyboard , the CPU interprets the command and instructs the hard disk to “load” the command or program into the main memory Once the data is loaded into the main memory, the CPU is able to access it faster than secondary storage.
- 14. Types of RAM Two types of RAM Static RAM (SRAM) Dynamic RAM (DRAM) SRAM The word static indicates that memory retains the content as long as power is supplied When power goes down, the data stored in it is lost forever. Does not need to be refreshed periodically Very fast but much more expensive than DRAM Used as cache memory due to its high speed.
- 15. DRAM Named so because it is unstable in nature The data continue to move in and out of the memory as long as power is available It has to be continuously refreshed in order to maintain the data in it. Done by placing the data in refresh circuit that rewrites the data several hundred times per second. Used in most of the system, as its inexpensive and small in size.
- 16. Difference between SRAM and DRAM BASED On SRAM DRAM Life of data Data is present as long as power is there, power is turned off, data is lost It is continuously refreshed for every 15 microseconds, even when there is continuous power supply Circuitry Dense Simple, no. of cells present in DRAM is less than SRAM Profit More beneficial, its fast and has low latency, no need of refreshing Slower and have longer latency then SRAM Cost Expensive, requires more power to operate, produces lots of heat Cheaper, simple, small and space efficient Access time 25 nanoseconds 60 nanoseconds