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【正文】 ses ? Address = a “ name” to uniquely identify one cell in the puter’ s main memory ? The names for cells in a puter are consecutive numbers, usually starting at zero ? Cells have an order: “ previous cell” and “ next cell” have reasonable meanings ? Random Access Memory = memory where any cell can be accessed independently Figure Memory cells arranged by address Measuring memory capacity: Not quite like the metric system ? “ Kilo” normally means 1,000。 data remains when puter is off ? Usually much bigger than main memory ? Usually rotating disks ? Hard disk, floppy disk, CDROM ? Much slower than main memory ? Data access must wait for seek time (head positioning) ? Data access must wait for rotational latency Figure A disk storage system Figure CD storage format Figure A magic tape storage mechanism Files ? File = the unit of data stored on a mass storage system. ? Logical record and Field = natural groups of data within a file ? Physical record = a block of data conforming to the physical characteristics of the storage device. ? Buffer = main memory area sometimes set aside for assembling logical records or fields of a file Figure Logical records versus physical records on a disk Figure The message “ Hello.” in ASCII Representing text ? Each printable character (letter, punctuation, etc.) is assigned a unique bit pattern. ? ASCII = 7bit values for most symbols used in written English text ? Unicode = 16bit values for most symbols used in most world languages today ? ISO proposed standard = 32bit values Representing numeric values ? Binary notation – uses bits to represent a number in base two ? Limitations of puter representations of numeric values ? Overflow – happens when a number is too big to be represented ? Truncation – happens when a number is between two representable numbers Figure The sound wave represented by the sequence 0, , , , , , , , 0 Figure The base ten and binary systems Figure Decoding the binary representation 100101 Figure
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