Bit与Byte的区别(转载)

在工作中遇到一些概念模糊的地方, 需要记住了
bit意为“位”或“比特”，是计算机运算的基础； 
byte意为“字节”，是计算机文件大小的基本计算单位； 

说到usb2.0标准接口传输速率。许多人都将“480mbps”误解为480兆/秒。其实，这是错误的，事实上“480mbps”应为“480兆比特/秒”或“480兆位/秒”，它等于“60兆字节/秒”，大家看到差距了吧。 

这要从bit和byte说起：bit和byte同译为"比特"，都是数据量度单位，bit=“比特”或“位”。 
byte=字节即1byte=8bits,两者换算是1：8的关系。 
mbps=mega bits per second(兆位/秒)是速率单位，所以正确的说法应该是说usb2.0的传输速度是480兆位/秒,即480mbps。 
mb=mega bytes(兆比、兆字节)是量单位，1mb/s（兆字节/秒）=8mbps（兆位/秒）。 

我们所说的硬盘容量是40gb、80gb、100gb，这里的b指是的byte也就是“字节”。 
1 kb = 1024 bytes =2^10 bytes 
1 mb = 1024 kb = 2^20 bytes 
1 gb = 1024 mb = 2^30 bytes 

比如以前所谓的56kb的modem换算过来56kbps除以8也就是7kbyte，所以真正从网上下载文件存在硬盘上的速度也就是每秒7kbyte。 
也就是说与传输速度有关的b一般指的是bit。 
与容量有关的b一般指的是byte。 

最后再说一点: usb2.0 480mbps=60mb/s的传输速率还只是理论值，它还要受到系统环境的制约（cpu、硬盘和内存等），其实际读、取写入硬盘的速度约在11～16mb/s。但这也比usb1.1的12mbps(1.5m/s)快了近10倍。 

 

 

Bits and Bytes

< CS101

 

At the smallest scale in the computer, information is stored as bits and bytes. In this section, we'll look at how that works.

Bit

• Bit, like an atom, the smallest unit of storage
• A bit stores just a 0 or 1
• "In the computer it's all 0's and 1's" ... bits
• Anything with two separate states can store 1 bit
• &nbps;-Nick's tennis racket example
• Chip uses areas of electric charge as 0/1 states
• Hard drive uses spots North/South magnetism 0/1 states
• A bit is too small to be much use
• Group 8 bits into a byte

Everything in a computer is 0's and 1's ... what does that mean? The bit stores just a 0 or 1 .. it's the smallest building block of storage.

Byte

• One byte = grouping of 8 bits
• e.g. 0 1 0 1 1 0 1 0
• One byte can store one letter, e.g. 'A' or 'x'

How much exactly can one byte hold?

• How many distinct patterns can be made with 1, 2, or 3 bits?
• (You don't need to memorize all the details, but you should be familiar with the derivation.)

Number of bits	Distinct Patterns
1	0 1
2	00 01 10 11
3	000 001 010 011  100 101 110 111

• 3 Bits
• Consider just the leftmost bit
• It can only be 0 or 1
• Lefmost bit is 0, then append 2-bit patterns
• Leftmost bit is 1, then append 2-bit patterns again
• Result ... 3-bits has twice as many patterns as 2-bits

Number of bits	Distinct Patterns
1	0 1
2	00 01 10 11
3	000 001 010 011  100 101 110 111

• In general: add 1 bit, double the number of patterns
• 1 bit - 2 patterns
• 2 bits - 4
• 3 bits - 8
• 4 bits - 16
• 5 bits - 32
• 6 bits - 64
• 7 bits - 128
• 8 bits - 256
• Mathematically: n bits yields 2ⁿ patterns (2 to the nth power)

One Byte - 256 Patterns

• Need to know:
• 1 byte is group of 8 bits
• 8 bits can make 256 different patterns
• How to use the 256 patterns?
• How to store a number in a byte?
• Start with 0, go up, one pattern per number, until run out of patterns
• 0, 1, 2, 3, 4, 5, ... 254, 255
• One byte holds a number 0..255
• i.e. with 256 distinct patterns, we can store a number in the range 0..255
• Code: pixel.setRed(n) took a number 0..255. Why?
• Red/green/blue image values each stored in one byte

Bytes

• "Byte" - unit of information storage
• A document, an image, a movie .. how many bytes?
• 1 byte is enough to hold 1 typed letter, e.g. 'b' or 'X'
• Later we'll look at storage in: RAM, hard drives, flash drives
• All measured in bytes, despite being very different hardware
• Kilobyte, KB, about 1 thousand bytes
• Megabyte, MB, about 1 million bytes
• Gigabyte, GB, about 1 billion bytes
• Terabyte, TB, about 1 trillion bytes (rare)

The space that data takes up in the computer is measured in by the "byte". One byte is big enough to hold a single typed letter, like 'a'. Here we'll look at storing data in RAM memory and in persistent storage like a hard drive. All of that storage space will be measured in bytes. We'll look at byte arithmetic in more detail later.

Bytes and Letters - ASCII Code

• ASCII is an encoding representing each typed letter by number
• Each number is stored in one byte of space in the computer (0..255)
• A is 65
• B is 66
• a is 96
• space is 32

32 space 33 ! 34 " 35 # 36 $ 37 % 38 & 39 ' 40 ( 41 ) 42 * 43 + 44 , 45 - 46 . 47 / 48 0 49 1 50 2 51 3 52 4 53 5 54 6 55 7 56 8 57 9 58 : 59 ; 60 < 61 = 62 > 63 ? 64 @

65 A 66 B 67 C 68 D 69 E 70 F 71 G 72 H 73 I 74 J 75 K 76 L 77 M 78 N 79 O 80 P 81 Q 82 R 83 S 84 T 85 U 86 V 87 W 88 X 89 Y 90 Z 91 [ 92 \ 93 ] 94 ^ 95 _ 96 `

97 a 98 b 99 c 100 d 101 e 102 f 103 g 104 h 105 i 106 j 107 k 108 l 109 m 110 n 111 o 112 p 113 q 114 r 115 s 116 t 117 u 118 v 119 w 120 x 121 y 122 z 123 { 124 | 125 } 126 ~

Typing, Bytes, and You

• An example of bytes in your daily life
• When you type letters on your phone or computer
• Each letter is stored as a number in a byte, as below
• When you send, say, a text message, the numbers are sent