[CrashCourse CS] #5 How Computers Calculate - the ALU

Arithmetic and Logic Unit - the ALU

• Mathmatical brain of a computer.
• Intel 74181 - the most famous ALU, 1970.
  • The first complete ALU that fit entirely inside of a single chip.

An ALU has two units in one - arithmetic unit and a logic unit.

Arithmetic unit

• Responsible for handling all numerical operations in a computer (add, subtract...).
• It can be built by using logic gates (and, or, not, xor)

Half Adder

• Takes two inputs (A, B) to add them together (A, B, and the output is single bit).
• 4 cases
  • 0 + 0 = 0
  • 1 + 0 = 1
  • 0 + 1 = 1
  • 1 + 1 = 1 0
• This matches the boolean logic of an XOR gate, so we can use it as a one bit adder.
• For the 4th case, we need an extra output wire for the carry bit.
• Carry bit is only true when the inputs are 1 and 1 (only time when the result is bigger than 1 bit can store) -> use AND gate.

Full Adder

• Takes three bits as inputs (A, B, C).
• Total of 4 output cases (00, 01, 10, 11)

• Building an 8-bit adder (8-bit Ripple Carry Adder)

• If there is a carry into the 9th bit, it means the sum of the two numbers is too large to fir into 8-bit: "overflow"
• Overflow occurs when the result of an addition is too large to be represented by the number of bits you are using. this can usually cause errors and unexpected behavior.
• In order to avoid overflow, you need to add more gates, but operation would take more time.
• Thus, modern computers use the faster carry-look-ahead adder.

Multiplication/Division

• Simple ALU's dont have circuits for multiply/divide.
  • Perfrom series of additions instead.
  • Thermostat, tv remote, microwave.
• Laptop, smartphones - have dedicated circuits for multiplication.

Logic Unit

• Performs logical operations(AND, OR, NOT...)
• Performs simple numerical tests, like chcking if the number is 0.
  

Arithmetic & Logic Unit

• Symbol for ALU - V (another level of abstraction).
  

• Operation codes : tells what operation to perform.
• Flags(wires)
  • overflow : true if the number has overflowed.
  • zero : true if the number is zero.
  • negative : true if the number is negative.
    
    

영어 표현

• pièce de résistance - the most impressive thing in a series or collection. (가장 중요한 것)
• crème de la crème - best of person or thing of kind. (일류 중의 일류)
• rite of passage - 통과의례
• admittedly - 인정하건데
• feat - impressive and difficult achievement
• mint - make coins or medals
  
  

느낀점

• 작은 스위치들을 조합하여 기계 부품을 만들어간다는게 신비롭다. 다음편이 기대된다.