presentation on memory hierarchy

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In the Computer System Design, Memory Hierarchy is an enhancement to organize the memory such that it can minimize the access time. The Memory Hierarchy was developed based on a program behavior known as locality of references. The figure below clearly demonstrates the different levels of the memory hierarchy.

Why Memory Hierarchy is Required in the System?

Memory Hierarchy is one of the most required things in Computer Memory as it helps in optimizing the memory available in the computer. There are multiple levels present in the memory, each one having a different size, different cost, etc. Some types of memory like cache, and main memory are faster as compared to other types of memory but they are having a little less size and are also costly whereas some memory has a little higher storage value, but they are a little slower. Accessing of data is not similar in all types of memory, some have faster access whereas some have slower access.

Types of Memory Hierarchy 

This Memory Hierarchy Design is divided into 2 main types:

• External Memory or Secondary Memory: Comprising of Magnetic Disk, Optical Disk, and Magnetic Tape i.e. peripheral storage devices which are accessible by the processor via an I/O Module.
• Internal Memory or Primary Memory: Comprising of Main Memory, Cache Memory & CPU registers . This is directly accessible by the processor.

Memory Hierarchy Design

1. registers.

Registers are small, high-speed memory units located in the CPU. They are used to store the most frequently used data and instructions. Registers have the fastest access time and the smallest storage capacity, typically ranging from 16 to 64 bits.

2. Cache Memory

Cache memory is a small, fast memory unit located close to the CPU. It stores frequently used data and instructions that have been recently accessed from the main memory. Cache memory is designed to minimize the time it takes to access data by providing the CPU with quick access to frequently used data.

3. Main Memory

Main memory , also known as RAM (Random Access Memory), is the primary memory of a computer system. It has a larger storage capacity than cache memory, but it is slower. Main memory is used to store data and instructions that are currently in use by the CPU.

Types of Main Memory

• Static RAM: Static RAM stores the binary information in flip flops and information remains valid until power is supplied. It has a faster access time and is used in implementing cache memory.
• Dynamic RAM: It stores the binary information as a charge on the capacitor. It requires refreshing circuitry to maintain the charge on the capacitors after a few milliseconds. It contains more memory cells per unit area as compared to SRAM.

4. Secondary Storage

Secondary storage, such as hard disk drives (HDD) and solid-state drives (SSD) , is a non-volatile memory unit that has a larger storage capacity than main memory. It is used to store data and instructions that are not currently in use by the CPU. Secondary storage has the slowest access time and is typically the least expensive type of memory in the memory hierarchy.

5. Magnetic Disk

Magnetic Disks are simply circular plates that are fabricated with either a metal or a plastic or a magnetized material. The Magnetic disks work at a high speed inside the computer and these are frequently used.

6. Magnetic Tape

Magnetic Tape is simply a magnetic recording device that is covered with a plastic film. It is generally used for the backup of data. In the case of a magnetic tape, the access time for a computer is a little slower and therefore, it requires some amount of time for accessing the strip.

Characteristics of Memory Hierarchy

• Capacity: It is the global volume of information the memory can store. As we move from top to bottom in the Hierarchy, the capacity increases.
• Access Time: It is the time interval between the read/write request and the availability of the data. As we move from top to bottom in the Hierarchy, the access time increases.
• Performance: Earlier when the computer system was designed without a Memory Hierarchy design, the speed gap increased between the CPU registers and Main Memory due to a large difference in access time. This results in lower performance of the system and thus, enhancement was required. This enhancement was made in the form of Memory Hierarchy Design because of which the performance of the system increases. One of the most significant ways to increase system performance is minimizing how far down the memory hierarchy one has to go to manipulate data.
• Cost Per Bit: As we move from bottom to top in the Hierarchy, the cost per bit increases i.e. Internal Memory is costlier than External Memory.

Advantages of Memory Hierarchy

• It helps in removing some destruction, and managing the memory in a better way.
• It helps in spreading the data all over the computer system.
• It saves the consumer’s price and time.

System-Supported Memory Standards       

According to the memory Hierarchy, the system-supported memory standards are defined below:

1. What do you mean by Memory Hierarchy?

Memory Hierarchy can be simply illustrated as the organization of the memory for saving the access time. Because of the nicely written codes or program, memory hierarchy works well. It takes less time in accessing at the current level.

2. Explain the types of Memory Hierarchy.

Here are the some types of the Memory Hierarchy: External Memory or Secondary Memory Internal Memory or Primary Memory

3. In DBMS, How memory Hierarchy is listed?

In Database Management System, Memory Hierarchy can be illustrated as follows: CPU Registers Cache Memory Main Memory or Primary Memory

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Memory Hierarchy

Memory hierarchy computer organization and assembly languages yung-yu chuang 2006/01/05 with s by cmu15-213 – powerpoint ppt presentation.

• Computer Organization and Assembly Languages
• Yung-Yu Chuang
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• Chapter 6 from Computer System A Programmers Perspective
• We assume memory is a linear array which holds both instruction and data, and CPU can access memory in a constant time.
• The gap widens between DRAM, disk, and CPU speeds.
• Some fundamental and enduring properties of hardware and software
• Fast storage technologies cost more per byte, have less capacity, and require more power (heat!).
• The gap between CPU and main memory speed is widening.
• Well-written programs tend to exhibit good locality.
• They suggest an approach for organizing memory and storage systems known as a memory hierarchy.
• Most programs tend to access the storage at any particular level more frequently than the storage at the lower level.
• Locality tend to access the same set of data items over and over again or tend to access sets of nearby data items.
• A programmer needs to understand this because the memory hierarchy has a big impact on performance.
• You can optimize your program so that its data is more frequently stored in the higher level of the hierarchy.
• For example, the difference of running time for matrix multiplication could up to a factor of 6 even if the same amount of arithmetic instructions are performed.
• Principle of Locality programs tend to reuse data and instructions near those they have used recently, or that were recently referenced themselves.
• Temporal locality recently referenced items are likely to be referenced in the near future.
• Spatial locality items with nearby addresses tend to be referenced close together in time.
• In general, programs with good locality run faster then programs with poor locality
• Locality is the reason why cache and virtual memory are designed in architecture and operating system. Another example is web browser caches recently visited webpages.
• Reference array elements in succession (stride-1 reference pattern)
• Reference sum each iteration
• Instructions
• Reference instructions in sequence
• Cycle through loop repeatedly
• Being able to look at code and get a qualitative sense of its locality is important. Does this function have good locality?
• Does this function have good locality?
• typedef struct
• Cache a smaller, faster storage device that acts as a staging area for a subset of the data in a larger, slower device.
• Fundamental idea of a memory hierarchy
• For each k, the faster, smaller device at level k serves as a cache for the larger, slower device at level k1.
• Why do memory hierarchies work?
• Programs tend to access the data at level k more often than they access the data at level k1.
• Thus, the storage at level k1 can be slower, and thus larger and cheaper per bit.
• Program needs object d, which is stored in some block b.
• Program finds b in the cache at level k. E.g., block 14.
• b is not at level k, so level k cache must fetch it from level k1. E.g., block 12.
• If level k cache is full, then some current block must be replaced (evicted). Which one is the victim?
• Placement policy where can the new block go? E.g., b mod 4
• Replacement policy which block should be evicted? E.g., LRU
• Cold (compulsory) miss occurs because the cache is empty.
• Capacity miss occurs when the active cache blocks (working set) is larger than the cache.
• Conflict miss
• Most caches limit blocks at level k1 to a small subset of the block positions at level k, e.g. block i at level k1 must be placed in block (i mod 4) at level k.
• Conflict misses occur when the level k cache is large enough, but multiple data objects all map to the same level k block, e.g. Referencing blocks 0, 8, 0, 8, 0, 8, ... would miss every time.
• Cache memories are small, fast SRAM-based memories managed automatically in hardware.
• CPU looks first for data in L1, then in L2, then in main memory.
• Typical system structure
• Locate the set based on ltset indexgt
• Locate the line in the set based on lttaggt
• Check that the line is valid
• Locate the data in the line based onltblock offsetgt
• Simplest kind of cache, easy to build(only 1 tag compare required per access)
• Characterized by exactly one line per set.
• Set selection
• Use the set index bits to determine the set of interest.
• Line matching and word selection
• Line matching Find a valid line in the selected set with a matching tag
• Word selection Then extract the word
• float dotprod(float x8, y8)
• float sum0.0
• for (int i0 ilt8 i)
• float dotprod(float x12, y8)
• Characterized by more than one line per set
• identical to direct-mapped cache
• must compare the tag in each valid line in the selected set.
• Word selection is the same as in a direct mapped cache
• High-order bit indexing
• adjacent memory lines would map to same cache entry
• poor use of spatial locality
• Multiple copies of data exist
• Main Memory
• What to do when we write?
• Write-through
• Write-back (need a dirty bit)
• What to do on a replacement?
• Depends on whether it is write through or write back
• Options separate data and instruction caches, or a unified cache
• Repeated references to variables are good(temporal locality)
• Stride-1 reference are good (spatial locality)
• Examples cold cache, 4-byte words, 4-word cache blocks
• Read throughput number of bytes read from memory per second (MB/s)
• Memory mountain
• Measured read throughput as a function of spatial and temporal locality.
• Compact way to characterize memory system performance.
• Slice through the memory mountain (stride1)
• illuminates read throughputs of different caches and memory
• Slice through memory mountain (size256KB)
• shows cache block size.
• Major cache effects to consider
• Total cache size
• Exploit temporal locality and keep the working set small (e.g., use blocking)
• Exploit spatial locality
• Description
• Multiply N x N matrices
• O(N3) total operations
• N reads per source element
• N values summed per destination
• but may be able to hold in register
• Line size 32B (big enough for four 64-bit words)
• Matrix dimension (N) is very large
• Approximate 1/N as 0.0
• Cache is not even big enough to hold multiple rows
• Analysis method
• Look at access pattern of inner loop
• 2 loads, 0 stores
• misses/iter 1.25
• 2 loads, 1 store
• misses/iter 0.5
• misses/iter 2.0
• Miss rates are helpful but not perfect predictors.
• Code scheduling matters, too.
• Example Blocked matrix multiplication
• Here, block does not mean cache block.
• Instead, it mean a sub-block within the matrix.
• Example N 8 sub-block size 4
• Innermost loop pair multiplies a 1 X bsize sliver of A by a bsize X bsize block of B and accumulates into 1 X bsize sliver of C
• Loop over i steps through n row slivers of A C, using same B
• Blocking (bijk and bikj) improves performance by a factor of two over unblocked versions (ijk and jik)
• relatively insensitive to array size.
• Programmer can optimize for cache performance
• How data structures are organized
• How data are accessed
• Nested loop structure
• Blocking is a general technique
• All systems favor cache friendly code
• Getting absolute optimum performance is very platform specific
• Cache sizes, line sizes, associativities, etc.
• Can get most of the advantage with generic code
• Keep working set reasonably small (temporal locality)
• Use small strides (spatial locality)

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Memory Hierarchy

Oct 04, 2014

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Memory Hierarchy. Er . Gurpreet Singh Assistant Professor Department of Information Technology, MIMIT Malout. Objective. Study about the various types of memories. Memory Hierarchy.

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Memory Hierarchy Er. Gurpreet Singh Assistant Professor Department of Information Technology, MIMIT Malout

Objective • Study about the various types of memories

Memory Hierarchy • The memory unit is an essential component in any digital computer since it is needed for storing programs and data • Not all accumulated information is needed by the CPU at the same time • Therefore, it is more economical to use low-cost storage devices to serve as a backup for storing the information that is not currently used by CPU

Memory Hierarchy • Computer Memory Hierarchy is a pyramid structure that is commonly used to illustrate the significant differences among memory types. • The memory unit that directly communicate with CPU is called the main memory • Devices that provide backup storage are called auxiliary memory • The memory hierarchy system consists of all storage devices employed in a computer system from the slow by high-capacity auxiliary memory to a relatively faster main memory, to an even smaller and faster cache memory

MEMORY HIERARCHY Memory Hierarchy is to obtain the highest possible access speed while minimizing the total cost of the memory system Auxiliary memory Magnetic I/O Main tapes memory processor Magnetic disks Cache CPU memory Register Cache Main Memory Magnetic Disk Magnetic Tape

Memory Hierarchy • The main memory occupies a central position by being able to communicate directly with the CPU and with auxiliary memory devices through an I/O processor • A special very-high-speed memory called cache is used to increase the speed of processing by making current programs and data available to the CPU at a rapid rate

Memory Hierarchy • CPU logic is usually faster than main memory access time, with the result that processing speed is limited primarily by the speed of main memory • The cache is used for storing segments of programs currently being executed in the CPU and temporary data frequently needed in the present calculations • The typical access time ratio between cache and main memory is about 1to7 • Auxiliary memory access time is usually 1000 times that of main memory

Main Memory • Most of the main memory in a general purpose computer is made up of RAM integrated circuits chips, but a portion of the memory may be constructed with ROM chips • RAM– Random Access memory • Integated RAM are available in two possible operating modes, Static and Dynamic • ROM– Read Only memory

Main Memory • A RAM chip is better suited for communication with the CPU if it has one or more control inputs that select the chip when needed • The Block diagram of a RAM chip is shown next slide, the capacity of the memory is 128 words of 8 bits (one byte) per word

SRAM vs DRAM Summary Tran. Access per bit time Persist? Sensitive? Cost Applications SRAM 6 1X Yes No 100x cache memories DRAM 1 10X No Yes 1X Main memories, frame buffers

RAM • Read/write memory, that initially doesn’t contain any data • The computing system that it is used in usually stores data at various locations to retrieve it latter from these locations • Its data pins are bidirectional (data can flow into or out of the chip via these pins), as opposite to those of ROM that are output only • It loses its data once the power is removed, so it is a volatile memory • It has a directional select signal R/W’; When R/W’=1, the chip outputs data to the rest of the circuit; when R/W’ = 0 it inputs data from the rest of the circuit

Random-Access Memory Types • Static RAM (SRAM) • Each cell stores bit with a six-transistor (Diode) circuit. • Retains value indefinitely, as long as it is kept powered. • Relatively insensitive to disturbances such as electrical noise. • Faster and more expensive than DRAM. • Dynamic RAM (DRAM) • Each cell stores bit with a capacitor and transistor. • Value must be refreshed every 10-100 ms. • Sensitive to disturbances. • Slower and cheaper than SRAM.

Random-Access Memory • Key features • RAM is packaged as a chip. • Basic storage unit is a cell (one bit per cell). • Multiple RAM chips form a memory. • Static RAM (SRAM) • Each cell stores bit with a six-transistor circuit. • Retains value indefinitely, as long as it is kept powered. • Relatively insensitive to disturbances such as electrical noise. • Faster and more expensive than DRAM. • Dynamic RAM (DRAM) • Each cell stores bit with a capacitor and transistor. • Value must be refreshed every 10-100 ms. • Sensitive to disturbances. • Slower and cheaper than SRAM.

ROM • ROM is used for storing programs that are PERMENTLY resident in the computer and for tables of constants that do not change in value once the production of the computer is completed • The ROM portion of main memory is needed for storing an initial program called bootstrap loader, witch is to start the computer software operating when power is turned off

ROM • Data is programmed into the chip using an external ROM programmer • The programmed chip is used as a component into the circuit • The circuit doesn’t change the content of the ROM • Can be used as lookup tables to implement various functions • Used by PCs to store the instructions that form their Basic Input/Output System (BIOS) • When power is removed from a ROM chip, the information is not lost, so it is a nonvolatile type of memory • It has a OE (Output Enable) specific control pin. Both OE and CE must be enabled in order for the ROM to output data; otherwise its data output is tri-stated.

ROM Types • Masked ROM – programmed with its data when the chip is fabricated • PROM – programmable ROM, by the user using a standard PROM programmer, by burning some special type of fuses. Once programmed will not be possible to program it again • EPROM – erasable ROM; the chip can be erased and chip reprogrammed; programming process consists in charging some internal capacitors; the UV light (method of erase) makes those capacitors to leak their charge, thus resetting the chip • EEPROM – Electrically Erasable PROM; it is possible to modify individual locations of the memory, leaving others unchanged; one common use of the EEPROM is in BIOS of personal computers.

Nonvolatile Memories • DRAM and SRAM are volatile memories • Lose information if powered off. • Nonvolatile memories retain value even if powered off. • Generic name is read-only memory (ROM). • Misleading because some ROMs can be read and modified. • Types of ROMs • Programmable ROM (PROM) • Eraseable programmable ROM (EPROM) • Electrically eraseable PROM (EEPROM) • Flash memory • Firmware • Program stored in a ROM • Boot time code, BIOS (basic input/ouput system) • graphics cards, disk controllers.

Auxiliary Memory • The main memory construction is costly. Therefore, it has to be limited in size. The main memory is used to store only those instructions and data which are to be used immediately. However, a computer has to store a large amount of information. The bulk of information is stored in the auxiliary memory. This is also called backing storageor secondary storage. They include hard disk, floppy disks, CD-ROM, USB flash drives, etc. • When the electricity supply to the computer is off, all data stored in the primary storage is destroyed. On the other hand, this is not true for secondary storage. The data stored in secondary storage can be stored for the desired time.

SOME T I P S FOR AUXILIARY MEMORY

Disk Geometry • Disks consist of platters, each with two surfaces. • Each surface consists of concentric rings called tracks. • Each track consists of sectors separated by gaps. tracks surface track k gaps spindle sectors

Disk Geometry (Muliple-Platter View) • Aligned tracks form a cylinder. cylinder k surface 0 platter 0 surface 1 surface 2 platter 1 surface 3 surface 4 platter 2 surface 5 spindle

Disk storage • Disks are used to store data, applications software and operating systems software. Whereas the primary form of storage in the early days of computing was magnetic tape, this has been replaced by predominantly disk based medium today. The reasons for this trend has been • decreasing cost per bit • reliability • reduced access times • higher transfer rates (more data per second) • reduced size and power requirements • increased capacity • One trend during the past few years is a move to optical storage medium. Many software companies offer both operating systems software and application software on optical medium (CDROM or DVDROM)

Disk storage technology • Disk storage systems work on magnetic principles. • In magnetism, there are two opposing polarities called poles, the north and the South pole. Opposite polarity attracts, whilst like polarity repels. • In computers, data is represented in binary format. • Binary data has two states, a 1 or a 0. It just so happens that magnetism also has two states, north and south, so in effect, magnetism is a good way of storing data also • A rotating disk is coated with very fine ferrous oxide particles, each of which act and behave like little magnets • All that is required now is a mechanism of converting the digital data of 0's and 1's into magnetic states of north and south poles. • In a storage disk drive, the mechanism which performs the function of converting the digital 0's and 1's into magnetic states which can magnetize the surface areas of the disk is called the write head. A similar head, called the read head, is used to detect the magnetic states on the surface of the disk and convert them back into digital states

Disk Capacity • Capacity: maximum number of bits that can be stored. • Vendors express capacity in units of gigabytes (GB), where 1 GB = 10^9. • Capacity is determined by these technology factors: • Recording density (bits/in): number of bits that can be squeezed into a 1 inch segment of a track. • Track density (tracks/in): number of tracks that can be squeezed into a 1 inch radial segment. • Areal density (bits/in2): product of recording and track density. • Modern disks partition tracks into disjoint subsets called recording zones • Each track in a zone has the same number of sectors, determined by the circumference of innermost track. • Each zone has a different number of sectors/track

Computing Disk Capacity • Capacity = (# bytes/sector) x (avg. # sectors/track) x • (# tracks/surface) x (# surfaces/platter) x • (# platters/disk) • Example: • 512 bytes/sector • 300 sectors/track (on average) • 20,000 tracks/surface • 2 surfaces/platter • 5 platters/disk • Capacity = 512 x 300 x 20000 x 2 x 5 • = 30,720,000,000 • = 30.72 GB

The read/write head is attached to the end of the arm and flies over the disk surface on a thin cushion of air. By moving radially, the arm can position the read/write head over any track. Disk Operation (Single-Platter View) The disk surface spins at a fixed rotational rate spindle spindle spindle spindle spindle

Disk Operation (Multi-Platter View) read/write heads move in unison from cylinder to cylinder arm spindle

Disk Access Time • Average time to access some target sector approximated by : • Taccess = Tavg seek + Tavg rotation + Tavg transfer • Seek time (Tavg seek) • Time to position heads over cylinder containing target sector. • Typical Tavg seek = 9 ms • Rotational latency (Tavg rotation) • Time waiting for first bit of target sector to pass under r/w head. • Tavg rotation = 1/2 x 1/RPMs x 60 sec/1 min • Transfer time (Tavg transfer) • Time to read the bits in the target sector. • Tavg transfer = 1/RPM x 1/(avg # sectors/track) x 60 secs/1 min.

Disk Access Time Example • Given: • Rotational rate = 7,200 RPM • Average seek time = 9 ms. • Avg # sectors/track = 400. • Derived: • Tavg rotation = 1/2 x (60 secs/7200 RPM) x 1000 ms/sec = 4 ms. • Tavg transfer = 60/7200 RPM x 1/400 secs/track x 1000 ms/sec = 0.02 ms • Taccess = 9 ms + 4 ms + 0.02 ms • Important points: • Access time dominated by seek time and rotational latency. • First bit in a sector is the most expensive, the rest are free. • SRAM access time is about 4 ns/doubleword, DRAM about 60 ns • Disk is about 40,000 times slower than SRAM, • 2,500 times slower then DRAM.

Optical Disks • The data is accessed from the underside of the CD-ROM. According to the initial specification devised by Philips and Sony, data is stored in a single track which is embedded into a polycarbonate material • The track starts at the inner of the disk, and ends at the outer radius of the disk. The track length is thus one long tightly wound spiral, the equivalent of over 3 miles long • The track is comprised of indentations or bumps which are created on a master disc. This master disc is then used to create the actual CDROM's which are shipped to customers. This technique is similar to the technique which was used to create audio records. • The laser beam is shone onto the surface of the disk. Data is stored as a sequence of surface variations called lands (flat surface) and pits (bumps or holes). The light is scattered by the pits and reflected by the lands. These two variations encode the binary 0's and 1's. The laser beam is moved to follow the spiral track of the data stored on the disk, detected the pits and lands as it follows the spiral track. • A light sensitive diode picks up the reflected laser light from the surface of the disk, and converts the light to digital data.

Optical Disk Technology • The pit and lands vary in length. The speed of rotation of the CD is adjusted so that the speed of the pits and lands passing above the laser is always the same speed (slower when it is in the inner and faster on the outer). This is called Constant Linear Velocity. • The amount of time that occurs between a pit and a land is measured and converted into digital data. Note that the information is stored permanently as pits and lands on the CD-ROM. It cannot be changed once the CD-ROM is mastered, this is why its called CD-ROM. • Single speed CD-ROM has a transfer speed of 150KB/s

DVD (Digital Versatile Disk) • This new standard offers higher data storage and faster data transfers than existing CD-ROM. Differences between DVD and CD-ROM • standard DVD holds 4.7GB per layer, dual layer single sided DVD holds 8.5GB on a single side • error correction is more robust than CD-ROM • every DVD is a bonded disc, composed of two 0.6mm substrates joined together • smaller pits are used and tracks are closer together than CD-ROM • DVD uses MPEG2 compression for high quality full screen pictures • a single layer DVD can hold a two hour 13 minute movie, with full digital sound in three languages • dual layer single sided DVD can hold a movie greater than 4 hours long • DVD-ROM drives have a much faster transfer rate than CD-ROM drives • DVD-ROM drives will read and play existing CD-ROM's and CDA disks • DVD is ideal for companies that wish to deliver enhanced training that includes high quality video. It has both the storage capacity and transfer speeds to support this type of application. In addition, movie companies are producing full length movie pictures on DVD, as MPEG-2 compression provides full screen high quality definition with multiple language track capability.

Tape Storage Systems • Magnetic Reel and Cartridge Tape • Digital Audio Tape (DAT) • Digital Data Storage (DDS) • Digital Linear Tape (DLT)

Cache memory • If the active portions of the program and data are placed in a fast small memory, the average memory access time can be reduced, • Thus reducing the total execution time of the program • Such a fast small memory is referred to as cache memory • The cache is the fastest component in the memory hierarchy and approaches the speed of CPU component

Cache memory • When CPU needs to access memory, the cache is examined • If the word is found in the cache, it is read from the fast memory • If the word addressed by the CPU is not found in the cache, the main memory is accessed to read the word

Cache memory • The performance of cache memory is frequently measured in terms of a quantity called hit ratio • When the CPU refers to memory and finds the word in cache, it is said to produce a hit • Otherwise, it is a miss • Hit ratio = hit / (hit+miss)

Cache memory • The basic characteristic of cache memory is its fast access time, • Therefore, very little or no time must be wasted when searching the words in the cache • The transformation of data from main memory to cache memory is referred to as a mapping process, there are three types of mapping: • Associative mapping • Direct mapping • Set-associative mapping

Cache memory • To help understand the mapping procedure, we have the following example:

Memory Address Map • Memory Address Map is a pictorial representation of assigned address space for each chip in the system • To demonstrate an example, assume that a computer system needs 512 bytes of RAM and 512 bytes of ROM • The RAM have 128 byte and need seven address lines, where the ROM have 512 bytes and need 9 address lines

Memory Address Map

Memory Address Map • The hexadecimal address assigns a range of hexadecimal equivalent address for each chip • Line 8 and 9 represent four distinct binary combination to specify which RAM we chose • When line 10 is 0, CPU selects a RAM. And when it’s 1, it selects the ROM

Associative mapping • The fastest and most flexible cache organization uses an associative memory • The associative memory stores both the address and data of the memory word • This permits any location in cache to store an word from main memory • The address value of 15 bits is shown as a five-digit octal number and its corresponding 12-bitword is shown as a four-digit octal number

Associative mapping

Associative mapping • A CPU address of 15 bits is places in the argument register and the associative memory searched for a matching address • If the address is found, the corresponding 12-bits data is read and sent to the CPU • If not, the main memory is accessed for the word • If the cache is full, an address-data pair must be displaced to make room for a pair that is needed and not presently in the cache

Direct Mapping • Associative memory is expensive compared to RAM • In general case, there are 2^k words in cache memory and 2^n words in main memory (in our case, k=9, n=15) • The n bit memory address is divided into two fields: k-bits for the index and n-k bits for the tag field

Direct Mapping

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