Transcript
Processor and Memory Modern computer architecture is accredited to John Von Neumann. This was first noted in his paper criticising the methods used in computing at that time. Von Neumann suggested that programs for the computer could be represented in digital form in the computer's memory along with the data.
Processor and Memory - System Buses A PC passes data between the CPU, internal memory, and Input/Output controller (I/O) via a data highway called a bus.
Several types of buses transmit power, data, address and control signals. Collectively these are known as system buses. System buses are classified by their data bandwidth (8 bit, 16 bit, 32 bit...). A higher data width increases the speed transfer of data.
Processor and Memory – CPU The Von Neumann architecture is based on a single computing element, the Central Processing Unit (CPU). Processor chips vary slightly in look depending on which company has made them. They are all made up from the same elements except some work faster and more efficiently than others. The above examples are Intel® processor chips, a popular brand.
CPU – Architecture The CPU architecture executes instructions one after another. It consists of a single control unit, an arithmetic logic unit (ALU), and registers. A control unit (CU) handles all processor control signals. It directs all input and output flow, fetches code for instructions from microprograms and directs other units and models by
providing control and timing signals. A CU component is considered the processor brain because it issues orders to just about everything and ensures correct instruction execution. An arithmetic logic unit (ALU) is a major component of the central processing unit of a computer system. It does all processes related to arithmetic and logic operations that need to be done on instruction words. In some microprocessor architectures, the ALU is divided into the arithmetic unit (AU) and the logic unit (LU). An ALU can be designed by engineers to calculate any operation. As the operations become more complex, the ALU also becomes more expensive, takes up more space in the CPU and dissipates more heat. That is why engineers make the ALU powerful enough to ensure that the CPU is also powerful and fast, but not so complex as to become prohibitive in terms of cost and other disadvantages. An arithmetic logic unit is also known as an integer unit (IU). A processor register is a local storage space on a processor that holds data that is being processed by CPU. Processor registers generally occupy the top-most position in the memory hierarchy, providing high-speed storage space and fast access to data. A register may include the address of the memory location instead of the real data itself.
CPU - Program Execution The CPU is responsible for acting as the 'brain' of the computer. The function of the CPU is to execute programs stored in the main memory by fetching instructions, examining them, and executing them one after the other.
CPU - Control Unit Tasks carried out by a CPU are listed below: -
decoding the instructions within a computer sequencing the reading and writing of data within the CPU and externally on the data bus controlling the sequence in which instructions are executed controlling the operations performed by the ALU
CPU – ALU The arithmetic logical unit (ALU) is responsible for performing arithmetic and logical operations and comparisons of data.
CPU – Registers The CPU also contains a small high speed memory which is used to store temporary results and control information.
This memory consists of a number of registers, each performing a specific function. Accumulators - serve the purpose of holding data used in calculations. Address Registers - are used for storing the memory location of data or instructions to be used by a program. Stack Pointer - this register is used during sub-routine nesting and stack based arithmetic. Status Register - this register provides a service to the CPU by maintaining the status of the last operation carried out by the ALU. Instruction Pointer - sometimes referred to as the program counter, the pointer is responsible for retaining the memory address of the next instruction to be executed.
Fetch and Execute The following pages illustrate the operations performed within a computer during a fetch and execute cycle. The control bus performs a read operation:
The control unit in the CPU prompts memory to put the instruction onto the data bus enabling the CPU to read the instruction onto its instruction decoder which is part of the control unit:
The next step involves the CPU decoding the instruction.
This instruction is then executed. For example, if the instruction is for the Control Unit to load the contents of the memory location 112 into the accumulator:
Performing a write operation The control unit sets the address bus to location 112 and puts the value of the accumulator onto the data bus. Finally the control bus performs a memory write operation:
Memory This section discusses the physical arrangement of memory, along with the various types of memory found in a computer system. The memory of a computer is constructed from microchips. Memory can be thought of as a set of pigeon holes or cells with each one having a unique address. Each pigeon hole can store 8 bits of data. Older memory chips have 30 pins connecting the memory to the computer. It is more usual to have 72 or 128 pins now. A computer contains two types of memory, Random Access Memory (RAM) and Read Only Memory (ROM). A module of RAM showing 30 pins.
A ROM chip
Random Access Memory Random Access Memory (RAM) is responsible for storing the instructions and data that the computer is using at that present moment in time.
It is described as volatile memory as the contents of RAM chips can be lost when the computer is turned off or when new data is being written to RAM while other data is being processed.
RAM – types RAM chips are produced on pieces of silicon in a manner similar to that of microprocessors.
Semiconductor RAM memory can be divided into two major groups, Static Ram and Dynamic Ram.
Static RAM Static memory is more expensive to produce than Dynamic memory, but because of its lower power consumption it is often used in small to medium sized systems. Static memory retains data within a cell until the data is overwritten or lost as a result of power being shut down.
Dynamic RAM Dynamic Memory is often referred to as volatile memory. Data is stored within the capacitance of a transistor. The capacitor is unable to prevent the charge from slowly discharging. This would result in the loss of data. A solution to this problem is the introduction of additional circuitry which performs a 'memory refresh' by periodically restoring the charge. Dynamic memory is cheaper than Static memory and is used in larger memory systems.
Read Only Memory ROM is responsible for storing permanent data and instructions. Developments in the storage capacity of ROM chips have enabled their use in the storage of systems software. Previously systems software was stored on floppy disks. Other applications of ROM can be found where a microprocessor is dedicated to a particular task. In these instances ROM has provided a cost effective way of storing software to be used by the microprocessor. In this example ROM is used to store the various programs that may be run under a microprocessor dedicated to controlling a washing machine.
ROM – Types ROM can be divided into several types: Mask programmed, PROM, EPROM, FLASH EPROM and EEPROM.
A selection of ROM chips The majority of ROM chips are mask-programmed (the programs are set during manufacturing). We will focus on two frequently encountered types of ROM chip - PROM (Programmable Read Only Memory) and EPROM (Erasable Programmable Read Only Memory).
PROMs PROMs are chips manufactured with no predefined program.
A PROM chip
PROMs have a low access time which has led to their use as a logic element rather than a program memory storage medium. They are used to store data that may then be referenced as a look-up table.
EPROMs EPROM - this is a special ROM chip that allows for the contents of the chip to be erased and rewritten. The programs written to an EPROM are written for applications where updating is not a regular occurrence.
An EPROM chip
The program can be erased by exposure of the normally covered central window to ultraviolet light.
Storage Devices In many cases the information that has been processed is stored in machine-readable format so that it may be accessed at a later time by a computer. This data is stored in binary form in 'bits'. This practice requires the use of storage devices. Storage Devices: -
Hard Disk CD/DVD disk Blue Ray disk
Storage Devices - Hard Disk The hard disk is a direct-access storage medium with a rigid magnetic disk. The data is stored as magnetised spots arranged in concentric circles (tracks) on the disk. Each track is divided into sectors. The number of tracks and sectors on a disk is known as its 'format'. High data rates demand that the disk rotates at a high speed (about 3,600 rpm). As the disk rotates read/write heads move to the correct track. The disk is sealed and lubricated and the head hovers on a cushion of air just above the disk to avoid damage. These are therefore called floating heads. The storage capacity of a hard disk can be Gigabytes (Gb), i.e. thousands of Megabytes (1000Mb), of information.
Storage Devices - Optical Disk (CD/DVD/BLUE RAY) An optical disk is impressed with a series of spiral pits in a flat surface. A master disk is burnt by high-intensity laser beams in bit-patterns from which subsequent copies are formed which can be read optically by laser. The optical disk is a random access storage medium; information can be easily read from any point on the disk. A Compact disc read-only memory (CD-ROM) is a storage device that can be read but not written to. CD-ROM was a common convention for delivery of audio and other data through the years before small solid-state flash drives and other devices began to take over.
As magnetic tape had replaced vinyl, the compact disc replaced magnetic tape as a durable, easy way to store information. In many ways, the CD-ROM was the last physical data storage method, coinciding with the use of floppy disks for computers. By contrast, today's data storage and data transmissions are mostly 'completely digital’ in the sense that tiny pieces of hardware can handle the information that would have been put in dozens of individual compact discs or floppy disks. As compact discs became a common data format for both music and other kinds of data, writable CDs allow users to download data from their computers to be used in other devices, for instance, in replicating songs and playlists for use in stereo systems with compact disc capability. As compact discs became useful for storing and delivering software in addition to music, companies worked on specific technical protocols for different kinds of digital data written onto CD-ROM products. These continue to help manage video, individual files and different kinds of data that may be on a compact disc. CD–Read Writable (CD-RW) refers to an optical CD that may be written and rewritten multiple times. CD-RW allows for data erasing during each rewritable session. However, data cannot be changed during CD-RW sessions. Some CD-RW discs have a multisession feature, in which additional data may be written at a later time if extra space is available. A CD-RW can hold data for several years if the disc is protected from direct sunlight. Most CDRW discs hold approximately 74 minutes and 640 MB of data, but some hold 80 minutes and 700 MB of data. Experts claim that a CD-RW's rewriting cycle may occur up to 1000 times. The CD-RW term is also known as CD-Rewritable (CD-RW). Introduced in 1997, CD-RW followed the CD-Magneto Optical (CD-MO) format, which introduced the multisession writing standards via a magneto-optical CD recording layer. Although never commercially available, CD-MO was established in Part 1 of the Trusted Computer System Evaluation Criteria (Orange Book) of the Rainbow Series, which was originally released by the U.S. government department of defense (DoD) in 1990. Most CD-RW discs have a multisession format feature capable of adding data during different sessions. Additionally, individual data files and directories may be deleted or updated as needed. This feature links one or more previous recorded (burned) sessions without consuming additional space, and subsequent recording sessions are linked to previous sessions. A CD-RW without the multisession format feature looks at the first session only and overwrites all of the disc data. Thus, most audio CD players c A digital versatile disc (DVD) is an optical disc storage medium similar to a compact disc, but with enhanced data storage capacities as well as with higher quality of video and audio formats.
Codeveloped by Sony, Panasonic, Philips and Toshiba in 1995, the DVD is widely used for video formats, audio formats as well software and computer files. Digital versatile discs are also known as digital video discs.annot read written multisession data. A digital versatile disc has a large capacity, starting at 4.7 GB. They are written at a speed of 1820x and have a video compression ratio of 40:1 with the help of MPEG-2 compression. The materials and manufacturing techniques used in the case of a digital versatile disc are same as that of CDs. The layers in the DVD are made by polycarbonate plastic. Digital versatile discs can be categorized in different ways based on their applications. If they are used for reading only and cannot be written, then they are classified as DVD-ROM. If the DVDs can be used to record any type of data, then they are called DVD-R. If the disc can be read, written and then erased and rewritten, it is called DVD-RW. There are many advantages associated with the DVD format. Compared to a CD, the audio quality is superior thanks to DTS or Dolby Digital technology. The picture quality is also superior to CD and the DVD player is capable of taking one to a specific moment in the video or audio, unlike a CD player. Based on the user needs, the formats of the digital versatile disc can be altered. Again, digital versatile discs are capable of storing more data compared to CD. This is due to smaller size of the pits and bumps and high density of the tracks in the DVDs. A large amount of space is wasted in codes, to avoid errors in information in the case of CDs. Digital versatile discs are backward compatible as well. One disadvantage of DVDs is the higher access time, which is mainly because of the higher amount of data and greater density. A Blu-ray disk (BD) is a high-capacity optical disk medium developed for recording, rewriting and playing back high definition video. It can store large amounts of data and was designed to supersede the DVD. Blu-ray was jointly developed by a group of personal computer and consumer electronics companies called the Blu-ray Disc Association. Blu-ray disks support higher resolutions and more advanced video and audio formats compared to DVDs. Blu-ray technology gets its name from the blue-violet laser that is used to read Blu-ray disks. Compared to a DVD’s red laser, a blue laser permits more information to be stored at a greater density. For example, while a DVD can store 15 GB per layer, a Blu-ray disk can store 25 GB per layer, and dual-layer disks can hold up to 50 GB. Compared to a DVD, Blu-ray also provides much higher resolution; while a DVD with standard definition can provide definition of 720x480 pixels, Blu-ray high definition has 1920X1080 pixel resolution.
