Code: The Hidden Language of Hardware and Software

Best Friends
- Friends can use flashlights to communicate (i.e. morse code)
- In book, code means system for transferring info among people and machines
- All facets of communication (audio, visual, etc) corresponds to some kind of code
Codes and Combinations
- Easier to send than receive because harder to map dots and dashes than letters
- Could group based on combo length and organization
- Each length grouping, doubles in size
- number of codes = 2 ^ (number of dots and dashes)
- Morse code is binary (only has dots or dashes)
- Analyzing binary codes is combinatorial analysis
Braille and Binary Codes
- Braille uses 2*3 dots that are raised or not
- Results in 2^6 or 64 combinations
- after letters, includes various codes for common words, combinations of letters or special indicators such as caps
- Escape codes let you change routine interpretation
Anatomy of a Flashlight
- Flash light consists of battery, wires, bulb
- electricity happens when electrons dislodged from atoms
- circuit takes from negative side to positive side
- Resistance in bulb section causes it to glow and emit light
Seeing Around Corners
- If you wanted to connect wires so you and friend could have a lights, need common connection
- Could use earth as one side of wire
- Lets you communicate beyond line of sight
- Trade offs with distance, thickness of wire and voltage required
Telegraphs and Relays
- Morse used electromagnetism to control metal lever that would make sounds representing dashes or dots
- Relay system could amplify signal and resend in between
Our Ten Digits
- Our base 10 counting system arbitrary
- Likely arose because 10 fingers
- Current number system from arab world, positional system and invention of zero resulted in huge benefits
Alternatives to Ten
- Can create different base variations e.g. base 8
- In base 8, each position represents 8’s
- by position: 8^0, 8^1, 8^2, 8^3, etc.
- Can create base 2 or binary
- gets long very quickly
- 1, 10, 11, 100, 101, 111 corresponds to 1,2,3,4,5
- first position is how many 1’s, next how many 2’s, next how many 4’s, next how many 16’s
- John Tukey realized binary digits would be very important with rise of computers so created word bit to shorten
Bit by Bit by Bit
- Bit is zero or one
- simplest number system and basic building block of information
- adding a digit doubles possibles
- If you know how many codes you need can use base two log
- log2 200 = 7.64 or 8 bits
- Bits can represent anything, but most fundamentally are numbers
Logic and Switches
- Aristotle wrote extensively about logic in the Organon
- George Boole invented mathematical way to represent logic
- Made algebra more abstract and divorced it from concept of numbers
- Operands refer to classes or sets
- + or U is a union or an OR
- x or n is an intersection or an AND
- Circuit with parallel switches (OR) and sequential switches (AND) can use Boolean algebra to determine if something satisfies conditions
Gates (Not Bill)
- Logic gates decide whether electricity flows or not
- switches are input devices and lightbulb is output device
- Relays can be configured to create AND or OR circuits
- An inverter changes the flow to the opposite
- can create NOR (inverted AND) or NAND (inverted OR)
A Binary Adding Machine
- Adding binary digits result in a sum and a carry out
- The carry out is described by an XOR operator
- sum is described with an AND operator
- combining these shown with a half adder
- This only works for the first digit, but not other digits because must take in a carry in
- This is simplified to a full adder
- Computers function similarly
But What about Subtraction
- Minuend – Subtrahend = Difference
- Can do subtraction and avoid borrowing
- e.g. 32 – 28 = x
  - 32 – 28 + 99 + 1 – 100 = x
  - 32 + (99 – 28) + 1 – 100 = x
  - 32 + 71 + 1 – 100 = x
  - 104 – 100 = 4
- Can do the same with binary. 111111 minus anything will just result in inverse
- Don’t need to use signs (+/-) to represent negatives
  - -128 to 127 represented by 1000000 to 01111111
  - adding and subtracting easier, but need to watch out for overflow
- When using binary and digits, need to know if signed (-128 to 127) or unsigned (0 to 255)
Feedback and Flip-flops
- Oscillators change between states by itself e.g. an inverter that feeds its output to itself
- One cycle is called a period
- 1/period is frequency of hertz
- Can set up to have multiple stable states
- “Flip-flop” circuits retain information
- Can configure to count in binary
Bytes and Hex
- 8 bits is known as a byte
- base 16 number system is hexadecimal
- Each byte can be represented by two hexadecimal digits
- A-F used to represent after 9
An Assemblage of Memory
- Telegraph relays when assembled into logic gates and then flip-flops can store information
- Can use three select inputs to represent 8 values
- This config of latches known as read/write memory or RAM (Random Access Memory)
- Random access because each latch accessed by changing address inputs (select)
- number of values in RAM array = 2^(Number of Address Inputs)
- 1024×8 RAM, means can store 2^10 or 1028 1-byte values
- 1024 bytes is kilobyte because 2^10 is roughly 10^3
- 1024 kilobytes is a megabyte and etc..
- Can use switches to select or write over values in any of the addresses
- volatile memory because will disappear if lose electricity
Automation
- A latch that accumulates a running total of numbers is an accumulator
- It often holds one number and then that number plus or minus a number
- Can make an automatic adder more versatile by introducing instructions such as load, add, store
- To do this need some kind of numeric code that indicates what the automated adder should do
- Easiest way is with separate RAM array that is accessed at the same time
- These two arrays are labeled Data and Code
- The new automatic adder needs to be able to write sums into the original RAM array
- Each code RAM corresponds to value in the Data RAM
- Numeric codes often called instruction codes, operation codes, or opcodes
- using low-order bytes, high order-bytes, and a carry latch, can add values with more than 8bits
- if we make each instruction 3 bytes, with a code, and two bytes for addresses, can have one RAM array
- The conditional jump (controlled repetition or looping) is what separates computers from calculators
- This is a digital computer and has four parts: a processor, memory, at least one input device, and at least one output device
  - Memory: 64-KB RAM array
  - Input/Output: switches and lightbulbs
  - Processor: everything else
- Processor is also called a central processing Unit or CPU
- Processor has several components
  - Accumulator: latch that holds a number
  - Arithmetic Logic Unit or ALU: 8-bit inverter and 8-bit adder. adds or subtracts in this example
  - Program Counter: 16-Bit counter
  - operation codes that processor responds to AKA machine codes/ machine language
- If you were writing, assembly language first then machine language
From Abaci to Chips
- Herman Hollerith helped automate the census by creating hole punch system
- Created and sold punch-card equipment with Tabulating Machine Company
- In 1915, President Thomas Watson changed name to International Business Machines or IBM
- While we covered primarily digital, most inventions were analog
- Programs used to be written on paper tape with punched holes
- code and data in memory is more recent concept
- Vacuums could replace relays, but problems with that too
- Alan Turing helped design Colossus to break German Enigma machine
- J. Presper Ekert and John Mauchly designed ENIAC using 18,000 vacuum tubes in 1945
- John Von Neumann helped design successor or the EDVAC
- Computer should have as much memory as possible and used to store both code and data
- instructions sequential in memory and addressed with a program counter that allowed conditional jumps (stored-program concept)
- von Neumann architecture but also von Neumann bottleneck, significant time spent fetching instructions
- John Bardeen and Walter Brattain in 1947 at Bell Labs wired a new amplifier out of germanium (a semiconductor)
- This was the first transistor
- resistance of semiconductors can be manipulated
- semiconductors can be doped or combined with impurities
- N-type semiconductors (negative) and P-type semiconductors (positive)
- Semiconductors can be made into amplifiers by sandwiching P-type with two N-types (NPN transistor)
- These three pieces known as collector, base, and emitter
- turning voltage off on base, turns off transistor
- In 1956, Shockley started Shockley Semiconductor Laboratories in Palo Alto
- Vacuums and transistors originally developed for amplification, but could be used for switches in logic gates
- Kilby and Noyce are both credited for invention of integrated circuit (single piece of silicon with transistors, resistors, and other electrical components)
- In 1965, Gordon Moore, noticed that technology was improving in such a way that number of transistors that could fit on a single chip would double every year
- In 1970, IBM put whole processors on a single chip (4004)
- Three measures for comparing microprocessors: 1) bits, 4004 was 4, 2) clock speed, and 3) addressable memory
Two Classic Microprocessors
- The 8080 and 6800 two most historically significant chips
- These were single-chip microprocessors
- Took inputs and outputs and required an oscillator to make it go
- microprocessor requires energy and synchronized clock inputs
- The 8080 had A0 – A15 for address 2^16 or 65,536 bytes of memory
- 8-bit microprocessor means can read or write 8-bits at a time
- RETURN
ASCII and a Cast of Characters
- using code to represent text is known as coded character set
- ASCII or American Standard Code for Information Interchange developed to help computers communicate with text (7bit)
- Control characters can be used to format text
- Unicode was developed as an alternative taking into account other languages (16 bit)
Get on the Bus
- processor most important, but storage important because flash doesn’t retain information without power
- boards communicate by means of a bus (signals provided to every board on computer: address, data output, data input, control)
- RETURN
The Operating System
- when computer is first turned on, won’t be productive without software
- need initialization code for when microprocessor is reset
- once turned on, computer starts operating at a given address after instructions are written there
- inputting machine code very tedious and difficult (0’s and 1’s)
- keyboard designed to interrupt microprocessor when pressed
- Can save code in ROM (read-only memory)
- can store data in file system
- file system is almost always part of a larger collection of software known as an operating system
- UNIX popular because can be adapted to run on a variety of computers
Fixed Point, Floating Point
- Computers deal with numbers as discrete values
- Number of discrete values you can represent directly related to number of bits available
- fractions can be represented with binary-coded decimal (BCD)
- common to store two BCD digits in 1 byte, don’t use two’s complement so need a sign bit
- This is fixed-point format, bc decimal is always fixed at a particular number place
- works good if you know the numbers aren’t going to get too large
- Alternative is floating-point, good for storing small and large, based on scientific notation
Languages High and Low
- bytes of machine code associate with short mnemonics such as MOV, ADD, CALL, HLT
- An assembler can take assembly-language program and output an executable file written in machine code
- Fairly simple because assembly to machine is 1-to-1 conversion
- Working in assembly is tedious and isn’t portable since works differently with different chips
- High-level programming refers to anything other than assembly
- Languages need a syntax and a compiler
  - converts the statements to machine code
- They are easier to learn, write, concise, portable
- High level language can’t use features specific to certain processors
- FORTRAN is the oldest high-level language and is still in use today
- Interpreters read source code and executes it directly
The Graphical Revolution
- One way in which we can put the extra power and speed of computers is by improving the user interface
- Computers originally weren’t interactive
- Step from character to graphic displays started in 1950’s, but very slowly
- increased interactivity
- GUI started at PARC
- Graphical OS require much more space
- Almost never written in assembly language
- OO helped programmers think about objects on screens in a way the user perceived them
- OO doesn’t let you do anything more, but provides a superior structure to problems
- Raster graphics (bitmap graphics) encodes an image as a rectangular array of bits that correspond to the pixels of an output device
- Graphics Interchange Format (GIF) is very popular bitmap file format since uses lossless compression
- Digitized sound made a big splash in 1983
- Introduction of sound, music, and video into personal computer was known as multimedia
- Internet allows computers to communicate through protocols