COMPUTER ORGANISATION AND ARCHITECTURE MEMORY HIERARCY. 1

Lecture Objectives Understanding Computer Memory Memory hierarchy Memory X-tics Types of memory

Overview Historically, the limiting factor in a computer’s performance has been memory access time. Memory speed has been slow compared to the speed of the processor A process could be bottlenecked by the memory system’s inability to “keep up” with the processor Our goal in this section is to study the development of an effective memory organization that supports the processing power of the CPU General memory organization and performance “Internal” memory components and their use “External” memory components and their use

Memory Hierarchy Major design objective of any memory system To provide adequate storage capacity at -An acceptable level of performance -At a reasonable cost

Computer Org & Arch - BScIT 1 @ UCU Memory Hierarchy Cost/ bit Access rate Processor Registers Cache Memory Main Memory Flash/ USB memory Hard Drives Tape Backup Computer Org & Arch - BScIT 1 @ UCU 5

Going Down the Hierarchy Decreasing cost per bit Increasing capacity. (The number of bytes (characters) a storage medium can hold). Slower access time ( Time it takes to perform a read or write operation. That is the time from the instant that an address is presented to memory to the instant that data has been stored )

Internal : Processor registers, Cache Internal : Processor registers, Cache. Directly accessible by the processor. ii) Main : System Ram. iii) Online Mass Storage: Secondary Storage. For example Hard Disk iv) Offline Bulk Storage:Offline Storage. ( Backup Tapes )

Hierarchy List Registers(A, special high-speed storage area within the CPU. All data must be represented in a register before it can be processed. For example, if two numbers are to be multiplied, both numbers must be in registers, and the result is also placed in a register.. Cache. used by the cpu to reduce the average time to access memory. It stores copies of the data from the most frequently used main memory locations. Main memory Flash / USB ( Non volatile computer storage that can be electrically erased and reprogrammed.) Hard Disk Tape Backup 8

Characteristics of the memory hierarchy Consists of distinct “levels” of memory components. Each level characterized by its size, access time, and cost per bit As one goes down the hierarchy, the following occur: Decreasing cost per bit Increasing capacity Increasing access time( Time it takes to perform a read or write operation. That is the time from the instant that an address is presented to memory to the instant that data has been stored )

Cache Small amount of fast memory that contains a copy of portions of main memory. The memory cache is high-speed memory available inside the CPU in order to speed up access to data and instructions stored in RAM memory. Sits between normal main memory and CPU May be located on CPU chip or module 10

Cache Memory Cache memory is intended to give memory speed approaching that of the fastest memory available. (CPU registers) Compared to the size of main memory, cache is relatively small. Very expensive compared to main memory Cache contains copies of sections of main memory

Cache Operation When the processor attempts to read a word from memory, The microprocessor first checks the cache memory and if it finds the data there (from a previous reading of data), it does not have to do the more time- consuming reading of data from larger memory.

Memory Classification According to Volatility: RAM ROM According to Technology: Static RAM Dynamic RAM

RAM/ROM Memory is either Read/Write (Volatile) or Read only (Non-Volatile) ROM – Read Only Memory Mostly used for installation into toys or appliances Programmed once although there are exceptions PROM/EPROM

Main Memory There are has two kinds of memory: Random-access memory (RAM) and Read-only memory (ROM).

Random Access Memory A temporary holding place for data and instructions. Consists of one or more chips on the motherboard. Sometimes called primary storage RAM is not a permanent storage place for information. It is active only when computer is on. If computer is switched off, the information is deleted from RAM.

ROM . Read Only Memory ROM is the memory where information is only read. When the computer is turned a off, information is not deleted from ROM. Writing information to ROM is made by vendor. ROM – Read Only Memory Mostly used for installation into toys or appliances Programmed once although there are exceptions PROM/EPROM

FEATURES OF RAM. The distinguishing feature is that RAM is volatile. RAM must be provided with a constant power supply. If power is interrupted, then the data is lost. The other distinguishing feature of RAM is that it is possible both to read data from the memory and to write new data into the memory easily and rapidly. Both the reading and writing are accomplished through the use of electrical signals.

Dynamic RAM The term dynamic refers to this tendency of the stored charge to leak away even with power continuously applied. Needs refreshing even when powered The presence and absence of charge on a capacitor is interpreted as a binary 1 or 0. Because capacitors have a natural tendency to discharge, DRAM requires periodic charge refreshing to maintain data storage. 19

Characteristics of DRAM Simpler construction Less expensive Need to refresh circuits Slower. Used for Main memory

Static RAM and its characteristics Bits stored as on/off switches (to be looked at when we get to logic gates) No charges to leak No refreshing needed when powered More complex construction More expensive Faster Used for Cache. 21

Read Only Memory (ROM) As the name suggests Read-only memory contains a permanent pattern of data that cannot be changed. Data is “wired in” during fabrication at a chip manufacturer’s plant . It is nonvolatile - no power source is required to maintain data It is possible to read from ROM but it is not possible to write new data into it. ASSIGNMENT What do you think are some of the advantages and drawbacks (disadvantages/shortcomings) associated with ROM? 22

Types of ROM Read “mostly” Erasable Programmable (EPROM) Erased by UV Electrically Erasable (EEPROM) Takes much longer to write than read Flash memory Erase whole memory electrically

Types of ROM Programmable ROM (PROM) Data can be programmed/written once by the user employing a PROM programmer. Writing process is performed electrically and may be performed by a supplier. Used to store programs permanently. Normally in video game controllers, mobile phones and the programming is done after the device is constructed. 24

Read-Mostly memory Three common forms of read-mostly memory. Useful for applications in which read operations are far more frequent than write operations but for which nonvolatile storage is required. Three common forms of read-mostly memory. Erasable Programmable ROM (EPROM) Read and written electrically as with PROM However, before a write operation. All storage cells must be erased at the same initial stage by exposure to ultraviolet radiation Erased by intense ultraviolet light, that is shone through a window designed like a memory chip. Erasure process can take 20 minutes. More expensive than PROM but has an advantage of multiple update capability

Electrically Erasable Programmable ROM (EEPROM) Can be written into at any time without erasing prior contents, only the data addressed is updated Takes much longer to write than read Nonvolatile and flexible. EEPROM is more expensive than EPROM and is less dense, supporting fewer bits per chip

Flash memory This is a non volatile computer storage that can be electrically erased and reprogrammed. This technology is used in memory cards and USB flash drives It is a special type of EEPROM( electrically Erasable Programmable Read only Memory) that is erased and programmed. It is Non-Volatile , no power is needed to maintain information. Memory erased electrically in a few seconds

Goal of the memory hierarchy Try to match the processor speed with the rate of information transfer from the lowest element in the hierarchy.