Chapter1.2.Fall2009

Chapter1.2.Fall2009 - Java Software Solutions Chapter 1.2...

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1/73 Java Software Solutions Chapter 1.2 Computer Systems (continued)
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2/73 Input/Output Devices For our purposes, the primary I/O devices will be the keyboard and the monitor . Output on a monitor is achieved by breaking the screen picture into small pieces called picture elements ( pixels ). The pixels are represented in the computer as numbers (as is everything else) and each pixel has a specific ‘screen address.’ A typical monitor can display a number of ‘screen resolutions’ such as 640x480, 800x720, 1280 x 1024 and others… These numbers represent the number of pixels used to display the screen information.
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3/73 Peripherals Almost all devices in a computer other than the CPU and main memory are called peripherals . Controllers are devices that coordinate the activities of specific peripherals. A unit with several disks attached will have a disk controller for timing, contention, data access, … Input/Output (I/O) devices and secondary memory devices are considered peripherals.
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4/73 Main Memory and Secondary Memory Main memory is made up of a series of consecutive memory locations called addresses . Associated with each memory location is a unique address . On many computers, each memory location consists of 8 bits, or one byte of information. The storage capacity of a device such as main memory is the total number of bytes it can hold. A kilobyte (KB) is 1,024 bytes or 2 10 bytes. Larger units are a megabyte (MB), a gigabyte (GB), and a terabyte (TB). It is usually easier to think about these capacities by rounding them off.
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5/73 Characteristics of Main Memory When data is stored in a main memory location, it overwrites (thus destroying) any information that was previously stored there. Main memory is usually volatile . The information stored in it will be lost if its electrical power is turned off. (meaning of volatile in this context) You should frequently save your work onto a secondary memory device (such as a disk) in case the power goes off.
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6/73 Secondary Memory Devices A disk is a direct access (also referred to as random access ) device since the read/write head can move very quickly to the information needed. In the prerequisite to this course, you have seen pictures of disks and how they operate. Disk is both a direct access device and can be accessed sequentially just like a tape unit. A tape must be sequentially processed / rewound / or fast- forwarded to get to the desired information. A tape is thus a considered a sequential access device. While tapes used to be the medium of choice when disks were expensive and access was controlled to those types of applications really needing disks, tapes now are almost exclusively used for backup storage .
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