CIM101 : Introduction to computer Lecture 3 Memory

Devices that uses memory See

Memory Computer memory refers to devices that are used to store data or programs (sequences of instructions) on a temporary or permanent basis for use in an electronic digital computer. refer to the semiconductor technology that is used to store information in electronic devices. Current primary computer memory makes use of integrated circuits consisting of silicon-based transistors. There are two main types of memory: volatile and non- volatile. 10/25/2015 3

Types of Computer Memory 10/25/20154

Volatile memory Volatile memory is computer memory that requires power to maintain the stored information. Current semiconductor volatile memory technology is usually either static RAM (see SRAM) or dynamic RAM (see DRAM). 10/25/20155

Non-volatile memory Non-volatile memory is computer memory that can retain the stored information even when not powered. Examples :read-only memory ( ROM), flash memory, most types of magnetic computer storage devices (e.g. hard disks, floppy discs and magnetic tape), optical discs, and early computer storage methods such as paper tape and punch cards. 10/25/20156

Classification based on access time 1. Internal processor memory (called registers). 2. Main memory (primary memory). 1. Uses semiconductor technology. 2. Capacity more than processor memory. 3. Secondary memory. 1.Capacity more than main memory. 10/25/20157

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Properties of memory devices Access time: ◦ Rate at which data can be read from or written to memory. ◦ Depends on physical characteristics. ◦ Low access time means high cost. Punch card CD-ROM Magnetic Disks RAM COSTAccess Time 10/25/20159

Cost ◦ The purchase price. Access mode ◦ Random access(access time independent of loc) ◦ Serial access (access time dependent of loc) Alterability ◦ Read-only (ROM). Can not be erased. ◦ Read-write (RAM). Cycle time & data transfer rate ◦ Time needed to perform any read or write task. ◦ Maximum amount of data that can be transferred every second. (bits/sec) 10/25/201510

Physical characteristics ◦ Technology used to store data. ◦ The popular technologies are electronic, magnetic, mechanical and optical. Capacity ◦ The number of 0’s and 1’s the memory can hold. ◦ Bit(Binary Digit). ◦ Byte (= 8 bits). ◦ Kilobyte (= 1024 bytes or 2^10 byte). KB. ◦ Megabyte (= 1024 KBs). MB. 10/25/201511

Memory Hierarchies Microprocessor Register Built in cache External Cache Main Memory Secondary Memory Cost per bit Access time Closeness with respect to processor 10/25/ Capacity

Read Only Memory ROM ◦ Used to store permanent data/program. ◦ nonvolatile. ◦ Contains the BIOS(Basic Input Output System ). PROM( Programmable ROM) ◦ Initially nothing stored. ◦ Data written by special programming device. 10/25/201513

Read Only Memory EPROM( Erasable Programmable ROM) ◦ Can be erased and re-programmed. ◦ Ultraviolet light is used for alteration. ◦ Need special programming device EEPROM(Electrically EPROM) ◦ Same as EPROM ◦ Uses electrical pulses for alteration. ◦ Can be re-programmed by keyboards commands 10/25/201514

RAM Read/write. Volatile. Holds data and program that will be used by the processor. Access time (microseconds to nanosecs). MOS and bipolar are used for RAM. A single RAM chip can be 1 MB. Higher capacity RAMs are built by connecting more than 1 RAM chip 10/25/201515

RAM 10/25/201516

Classification Static ◦ Does not require refreshing. ◦ High speed, costly, less complexity, used for cache. Dynamic ◦ Needs periodical refreshing. ◦ Slower, low cost, main memory, high complexity. 10/25/201517

Cache Memory Used for better performance. Placed between CPU and RAM. Its faster than RAM but slower than CPU. High speed memory that holds recently/frequently used data. CPU Cache Main Memory 10/25/201518