CSCI130

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Lecture 6

• CSCI130
• Instructor Dr. Imad Rahal

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Circuit for Equivalence

• We need to compare the data contents of two
  registers
• Data is in binary
• compare them bit by bit
• Start right to left
• Take two inputs
• If both 0s or 1s, output 1
• Otherwise, output a zero
• ABAB
• (Sum of products method)
• ? (AB) AB (simpler)
• Draw circuit

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Circuit for Addition

• The rules of addition
• 00 ?
• 01 ?
• 10 ?
• 11 ?
• Half-adder
• Takes two inputs and produces one output
• Ignores carry
• Just the negation of equivalence
• AB AB
• Draw circuit

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• Need to include carry
• Full-adder
• Two inputs and one carry from addition (i.e. 3
  inputs)
• Produces two outputs (sum and carry)
• Drive expressions
• Sum ABC ABC ABC ABC
• Carry out ABC ABC ABC ABC
• Draw circuit diagram (ABC in center)

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Processor Design and Machine Language (4.1-4.5)
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Von Neumann Machine

• Seen how hardwired operations are accomplished
• Comparison and addition
• Build up other operations/tasks using those
  basic/hardwired operations ones
• Programmed
• Initially we had to physically connect the memory
  registers containing the input to the required
  circuits in order
• Rewire for different programs
• (1945) John von Neumann (Mathematician) showed
  how to encode and store programs and their data
  in memory
• led to the von Neumann design
• Store programs and data together in memory
• Any program now can stored and processed without
  the need for hardware change

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Programs Algorithms

• Characteristics of an algorithm
• List of steps to complete a task
• Each step is PRECISELY defined and is suitable
  for the machine used
• Add 5 to variable X
• Increase the value of X
• Eat a sandwich!
• Finite number of steps
• The process terminates in a finite amount of time
• No infinite loops

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Programs Algorithms

• Program A formal representation of a method for
  performing some task
• Written in a code/programming language understood
  by a computer
• Detailed and very well-organized (computers just
  do what they are told)
• Follows an algorithm method for fulfilling the
  task
• Plan to do something VS the actual performance

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CPU

• Connects to main memory (MM) thru a bus
• Bus bundle of wires
• Data between CPU and memory
• Width of bus determines speeds
• e.g. 32-bit bus (Higher ? faster)
• Where is data stored when copied to CPU?

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CPU

• Divided into CU and ALU
• CU (Control Unit)
• Orchestra organizer
• Oversees transfer of data between MM and CPU
• Accesses info in ROM (e.g. during reboot)
• Has a bunch of registers
• ALU (Arithmetic Logic Unit)
• Circuits to do computations
• Built for each basic operation (,-, x, /, ,gt,lt,
  NOT)
• Includes a special register accumulator AC to
  hold results

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Fetch-Execute Cycle

• Programs are up of instructions
• Fetch-Decode-Execute
• CU supervises the fetch-execute cycle
• Fetches one instruction from memory
• Decodes instruction
• Supervises its execution
• Saves results
• Fetches another until we STOP

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Fetch-Execute Cycle

• To supervise the fetch-execute cycle, the CU has
  two registers
• PC (program counter)
• Contains the address of the next instruction in
  memory to fetch
• Initially points to first instruction of a
  program
• After fetching an instruction, it is incremented
  by one unless there is a jump to some other
  instruction
• IR (instruction register)
• Holds the current instruction
• Also has a timing clock
• Clock chip that generates a steady stream of
  electric pulses
• At every pulse, the CPU proceeds by one step
• Fetch, send data to ALU, etc
• Higher frequency clocks result in faster
  computers
• 16-3000 MHz (3.0 GHz)

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The Machine Language

• Instruction set depending on CPU
• Hardwired
• Binary code for each instruction (Opcode)
• Different CPUs might have different operations
  for which circuits exit in the ALU
• Read about RISC and CISC in book
• Limit VS increase circuits
• Programs can only use these opcodes
• The set of all opcodes (i.e. instructions)
  together is known as the machine language

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A Simple 8-bit Computer

• Use a simplified version of real computers to
  understand machine language programs
• 32 8-bit main memory registers
• 5 bits (25 registers) to represent an address
• 00000 to 11111
• PC holds memory addresses ? 5-bit PC
• For simplicity, assume only positive and negative
  integers (2s complement notation)
• 8-bit AC

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A Simple 8-bit Computer

• Two 1-bit flags
• zflag 1 if AC zero, 0 otherwise
• pflag 1 is AC is positive, 0 otherwise
• ALU supports the following basic operations
• Addition, subtraction, if AC0, if ACgt0
• 8 instructions (next slide)
• 3 bits (8 instructions)
• 000 to 111
• 8 bits per memory location ? rest 5 bits of the
  instruction contain the address of the input data
  to the instruction
• ooo ddddd
• 8-bit IR

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A Simple 8-bit Computer

• Memory holds 8-bit data or instructions
• 8 instructions (next slide)
• 3 bits (8 instructions)
• 000 to 111
• 8 bits per memory location ? rest 5 bits of the
  instruction contain the address of the input data
  to the instruction
• ooo ddddd
• 8-bit IR