Lecture 11
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The document discusses different types of computer memory. It describes main memory (RAM), secondary memory (hard disks, floppy disks, etc.), and internal processor memory. It provides details on RAM types (SRAM and DRAM), ROM types (PROM, EPROM, EEPROM), memory units (bit, byte, word), memory access times, volatile vs. non-volatile memory, and sequential vs. direct access storage for secondary memory devices. The document serves as a comprehensive overview of computer memory components and characteristics.
Lecture 11
- 3. Memory 11-Sep-17 Memory may be defined as the devices which is used to store data and information. Memory components of a computer system are divided into three groups- Main Memory or Primary Memory Secondary Memory or Auxiliary Memory or Back-up Memory Internal Processor Memory
- 4. Main Memory or Primary memory 11-Sep-17 Locations in main memory can be directly accessed by the CPU instruction set. Used for program and data storage during computer operation. Example: RAM
- 5. Secondary Memory 11-Sep-17 Much larger in size but much slower than memory. Used for storing system program and large data files which are continually not required by CPU Example: Hard Disk
- 6. Internal Processor Memory 11-Sep-17 Usually comprise of small set of high speed registers used as working registers for temporary storage of instructions and data Example: Program counter (PC), Accumulator (Acc)
- 7. Why Need Memory 11-Sep-17 To store program and data during execution To store program for repetitive use To store data for future or periodical use. To store results for execution.
- 8. Main Memory 11-Sep-17 The smallest unit of information that can be stored by a digital computer is called Bit (binary digit). A collection of four bits is called Nibble. A combination of eight bits is called a Byte. Unit of bits that a computer can process at a time is called Word. The fixed number of bits that a word can store is called the word length which varies from computer to computer. 32 bit computer: Set of bits processed by microprocessor as a unit is 32.
- 9. Continue 11-Sep-17 A 1KB computer storage has exactly 1024 (210) storage locations typically presented as 0000 to 1023. A computer with 64K storage has 64536 storage locations in memory. 1 MB is roughly 1 Million (Exactly, 220) bytes of storage. Roughly 1GB has 1 Billion bytes of storage. Roughly 1TB has 1 Trillion bytes of storage. Two kinds of main memory are commonly used in modern computers. Semiconductor Memory Magnetic Memory
- 10. Memory Access 11-Sep-17 Memory Access Instructions or data are read from or written into the memory one word at a time. Reading from the memory or writing data into memory is usually called Memory Access. Access Time The performance of a memory device is primarily determined by the rate at which information i.e. one word from the memory is read/written known as Access Time.
- 11. Types of Memory 11-Sep-17 Volatile memory If stored information is destroyed when power goes off. Example: Most semiconductor memory (RAM) Non-volatile memory Stored information is preserved when power goes off or power failure. Most Magnetic Memory (disk)
- 12. Types of Memory 11-Sep-17 Serial access memory Storage locations can be accessed in only in certain pre-defined sequence. Example: Magnetic tap Random access memory (RAM) If memory locations can be accessed in any order and access time is independent of the location being accessed. Categorized as – Static RAM (SRAM) Dynamic RAM (DRAM)
- 13. Static RAM (SRAM) 11-Sep-17 Made up of flip flop Stores bit as voltage High -> 1, Low -> 0 Need no refreshing Require greater power supply Four times larger in area than DRAM Contain less data Costlier
- 14. Dynamic RAM (DRAM) 11-Sep-17 Made up of capacitor Stores Bit as charge; presence of charge -> 1, else -> 0 High density and low power consumptions Required refreshing logic to charge the capacitor periodically Smaller area Less costly
- 15. Read Only Memory (ROM) 11-Sep-17 There are some random access memories in which data are permanently recorded during fabrication itself. The memory only allows information stored into it to be read and would not permit any writing or modification by users. All microcomputers have at least one ROM unit that contains a small program called BOOTSTRAP LOADER. This program is automatically copied into primary storage when a computer is powered up and being to execute. This program is responsible for reading in a copy of the operating system from a secondary storage device.
- 16. Programmable ROM (PROM) 11-Sep-17 Special types of ROMs PROM enables the user to enter the contents using special equipment called PROM programmer. But once programmed, it cannot be altered further
- 17. Erasable Programmable ROM (EPROM) 11-Sep-17 Contents of EPROM can be altered as many times as user likes. Ultraviolet ray is used for erasing Erasing process is slow Full erase of the ROM
- 18. Electrically Erasable PROM (EEPROM) 11-Sep-17 Contents of EPROM can be altered as many times as user likes. Electrical pulse is used for erasing Erasing process is fast Full or partly erase of the ROM
- 20. Secondary memory 11-Sep-17 Characteristics: Non-volatile storage Mass storage Cost efficiency Lack of direct processing capability Common secondary memory: Magnetic Tape Magnetic disk Floppy disk Optical disk
- 21. Primary vs. Secondary memory 11-Sep-17
- 22. Secondary storage 11-Sep-17 There are two basic type of storage: Sequential Access Storage To read one particular record in the file all records preceding it must be read E.g. magnetic tape Direct Access Storage Media where an individual record can be located and read immediately without reading any other record. Directly retrieved by selecting and using any of the locations on storage media. Each storage position must- Have unique address Can be individually accessed in approximately the same length of time