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134 lines
4.1 KiB
Markdown
134 lines
4.1 KiB
Markdown
---
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title: "01-bits-and-bytes"
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aliases:
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tags:
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- cosc204
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- lecture
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sr-due: 2022-07-21
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sr-interval: 7
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sr-ease: 250
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---
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[memory](notes/memory.md)
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[unicode](notes/unicode.md)
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[ASCII](notes/ASCII.md)
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[digital-data](notes/digital-data.md)
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# What is data
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- A fact — a piece of information
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- corresponds to discreete facts about phenomena from which we gain information abou the world
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- The concept of a *value* is fundamental to data
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- e.g., 25, $356.00, April, "this is a sentence", colours etc
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- Vaues are abstract, they are interpretations of data
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- There are many way of storing the same data
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- e.g., 12, twelve, XII, 1100, · ··, ·----··---
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# How computers represent data
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- In *Binary*
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- Stored in one of two states, true/false, 1/0, on/off, voltage/no voltage
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- Each instance of a state is called a *bit*. (binary digit)
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- *Values* are represented as a sequence of bits.
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- e.g., 1000001
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- The computer doesn't "know" what any given sequence means, **you** know.
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- could be 65, A, or anything **You** want it to mean
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# Computer memory
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- SImilar to the switch board in your home
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- Each switch has a number
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- they are all always there
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- you can switch the state by flipping the switch
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- Each switch has:
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- A address/location (swtich number)
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- A value (on/off)
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- computer languages allow us to name some of the locations, its easuer than remembering its number (variable)
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# Bits, Nibbles, Bytes
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- The smallest unit of storage is a buit (0 or 1)
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- (for convenience) bit are grouped into larger units.
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- a nibble is 4 bits
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- a byte is 8 bits
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- For convenience bytes are given addresses, not nibbles or bits. (they are too small to work with most of the time)
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# A Word of memory
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- The word is the number of bits the cpu uses internally, varies between manufacturers and CPUs.
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- Now its usually 64 bits
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- [amount of bits for different devices](https://i.imgur.com/nHrz1zX.png)
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# Characters
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- A written symbol.
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- In english are represented as a single byte, (other languages use 2 bytes or more)
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- e.g., [different types of characters](https://i.imgur.com/DBLVhw8.png)
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- characters are joined together to make human readable numbers and words
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- `char ch`
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- ch is a variable name (identifier)
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- used to label a location in the computer's memory where a byte is stored
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- when the code is compile, the name is assigned an address, in memory. The meaning of that data depends on how a human interprets it. it might be small integer, or a character, or a color etc.
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## ASCII Character Code
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1. The computer uses ch as a integer index into a pre-existing table
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2. the computer screen is made up of a thousand little dots called pixels. theyre in a rectangular grid like a table.a
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- [ascii code example](https://i.imgur.com/9uvKRVo.png)
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- There are several tables that describe what to draw
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- fonts describe how to draw them
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- ASCII (american standard code for information) describes what should be drawn for Roman (english like) alphabets
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- e.g.,
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- A 1000001 (65)
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- B 1100001 (97)
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- 9 0111001 (57)
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- There are only a few letter numbers and punctuation marks. The remaining ASCII code are non-printing and have other meaning (line feed, for feed, tab etcc)
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- ASCII characters are stored using 7-bits
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- so there are 128 (2^7) possible characters
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- stored as a byte with the 8th bit set to zero
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- For sorting purposes characters are compared on their numeric value (called the *collating sequence*)
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- 'A' is before 'Z' but 'a' is after 'Z'!
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## Unicode
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- Other non roman languages
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- greek, arabic, chinese, hebrew, japanese, thai etc.
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- atrology symbols
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- emoji etc
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- Unicode
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- developed by the Unicode Consortium
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- coordinated with ISO/IEC 10646
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- a 21-bit code with 144,697 characters from 159 scripts
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- unicode maps from character numbers (code points) into glyphs (graphical representations)
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- Some(many) are reserved
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# Homework
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- How are character strings (e.g. “hello world”) stored in a computer?
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- Is this different between different programming languages (for example; C and Java)? |