30-10-2012, 04:06 PM
STRUCTURED ORGANISATION
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CPU
• A CPU is on a chip called a microprocessor
• It continuously follows the fetch-decode-execute cycle:
• The speed of a CPU is controlled by the system clock
• The system clock generates an electronic pulse at regular intervals
• The pulses coordinate the activities of the CPU
• The speed is usually measured in gigahertz (GHz)
Registers
Registers are fast stand-alone storage locations that hold data temporarily. Multiple registers are needed to facilitate the operation of the CPU. Some of these registers are shown in Figure 5.2.
• Data registers
• Instruction register
• Program counter
THE CONTROL UNIT
The third part of any CPU is the control unit. The control unit controls the operation of each subsystem. Controlling is achieved through signals sent from the control unit to other subsystems.
MEMORY
MAIN MEMORY
• Main memory is the second major subsystem in a computer. It consists of a collection of storage locations, each with a unique identifier, called an address.
• Data is transferred to and from memory in groups of bits called words. A word can be a group of 8 bits, 16 bits, 32 bits or 64 bits (and growing). If the word is 8 bits, it is referred to as a byte. The term “byte” is so common in computer science that sometimes a 16-bit word is referred to as a 2-byte word, or a 32-bit word is referred to as a 4-byte word.
Address space AND memory address
To access a word in memory requires an identifier. Although programmers use a name to identify a word (or a collection of words), at the hardware level each word is identified by an address. The total number of uniquely identifiable locations in memory is called the address space. For example, a memory with 64 kilobytes and a word size of 1 byte has an address space that ranges from 0 to 65,535.
Cache memory
1. A small block of high-speed SRAM memory
1. Stores most frequently and most recently used data and instructions
2. Microprocessor looks for what it needs in cache first
1. Transferred from cache much faster than from memory
2. If not in cache, control unit retrieves from memory
1. The more cache “hits” the faster the system performance
Cache memory is faster than main memory, but slower than the CPU and its registers. Cache memory, which is normally small in size, is placed between the CPU and main memory
INPUT/OUTPUT SUBSYSTEM
The third major subsystem in a computer is the collection of devices referred to as the input/output (I/O) subsystem. This subsystem allows a computer to communicate with the outside world and to store programs and data even when the power is off.
Input/output devices can be divided into two broad categories:
• non-storage
• storage devices.
• Storage devices (SECONDARY STORAGE) although classified as I/O devices, can store large amounts of information to be retrieved at a later time. They are cheaper than main memory, and their contents are nonvolatile—that is, not erased when the power is turned off. They are sometimes referred to as auxiliary storage devices. We can categorize them as either magnetic or optical.
SUBSYSTEM INTERCONNECTION
The previous sections outlined the characteristics of the three subsystems (CPU, main memory, and I/O) in a stand-alone computer. In this section, we explore how these three subsystems are interconnected. The interconnection plays an important role because information needs to be exchanged between the three subsystems.