Introduction to computer: executes instructions

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Introduction to computer executes instructions
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Overview

• Topics discussed in this webnote
• Structure and operation of the CPU
• Program flow
• Types of instructions that a computer can execute
  

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Overview

• Topics discussed in this webnote
• Structure and operation of the CPU
• Program flow
• Types of instructions that a computer can execute
  

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Structure of a CPU
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Components of a CPU

• General purpose registers
• Arithmetic and Logic Unit (ALU)

• Register is a synonym for memory in computer
  science
• A general purpose register is a memory cell
• Each general purpose register has a unique name
• It is used to store (and recall) intermediate
  result of complex computations

• Is a complex electrical circuit that can perform
  Mathematical (, -, , /) and logical operations
  (lt, gt, , and, or)
• The output (result) of the computation (obtained
  by the ALU) is often stored in a general purpose
  register

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Structure of a CPU (cont.)

• Instruction register (IR)
• Program counter (PC)

• Contains the current instruction being executed
  by the CPU
• The CPU will perform the operation indicated by
  the instruction code contained in the instruction
  register

• The program counter is a register (memory cell)
  !
• This register contains the address (location in
  memory) of the next instruction after the CPU
  finishes executing the current instruction in the
  instruction register
• The value in the program counter will be
  increased after the CPU finishes executing one
  instruction

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Structure of a CPU (cont.)

• Processor status register (PSR)

• This register contains the various information
  about the CPU
• Among the information contained in the PSR is
• When the CPU compares 2 numbers a and b, the
  outcome of the comparison is stored in the PSR
  The outcome of a compare operation will allow the
  CPU to determine the following fact between a and
  b

• The result of a comparison operation

• equal
• not equal
• less than
• less than or equal
• greater than
• greater than or equal

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Operation of a CPU

• The CPU performs the following sequence of
  operations repeatedly

• After executing an instruction, the CPU obtains
  ("fetches") the (next) instruction at the address
  (location) given in the program counter
• The fetched instruction is stored in the
  instruction register in the CPU and the program
  counter is increased to point to the next
  instruction in the memory
• The fetched instruction (stored in the
  instruction register) is the executed (the CPU
  will do the operation indicated by the
  instruction code in the instruction register)

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Operation of a CPU (cont.)

• Example

• Supposed the CPU has just finished executing the
  instruction (instruction code 0) in the
  instruction register
• (Instruction code 0 in the instruction register
  means "add" and it has just been executed)

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Operation of a CPU (cont.)

• The following figures illustrates the CPU's
  execution cycle (that goes on indefinitely)
• The CPU will cause the memory to retrieve
  (recall) the value stored at memory address
  (location) 1

1. The CPU first sends a request to retrieve
(recall) the data stored at memory location given
by the program counter (PC) (in the figure, the
value of the PC 1)
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Operation of a CPU (cont.)

• 2. In response, the RAM memory sends the value
  stored in memory location 1 ( the number 4) to
  the CPU which stores it in the instruction
  register (IR)
• This number (4) will be interpreted as a code
  for a computer instruction.
• The old instruction code (0) will be
  over-written by the new instruction code

• In response, the RAM memory sends the value
  stored in memory location 1 ( the number 4) to
  the CPU which stores it in the instruction
  register (IR)

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Operation of a CPU (cont.)

• 3. The program counter is then increased
• Notice that the CPU is now ready to fetch the
  next instruction when the current instruction
  (stored in the instruction register) is
  processed.

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Operation of a CPU (cont.)

• 4. The CPU will now execute the instruction in
  the program counter (instruction code 4)
• When the CPU is finished, it will repeat these
  steps again (and again, until you turn the
  computer off)...

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Pointers pointing to a location in memory

• Pointer

• A pointer is "something" that points to
  somewhere Examples

3 pointers to different directions
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Pointers pointing to a location in memory (cont.)

• In Computer Science, a pointer always points to a
  location in memory
• Examples

Pointer to memory location 1
Pointer to memory location 4
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Pointers pointing to a location in memory (cont.)

• The computer does not have "pointing finger" that
  point to some location in memory
• The pointer is represented in the computer by the
  value of the address (location) of the memory
  where the pointer is pointing at

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Pointers pointing to a location in memory (cont.)

• Examples

Pointer to memory location 1
Pointer to memory location 4

• Storing the value 4 in some register/memory cell
  will remember the memory location 4
• The value 4 in the register/memory cell is
  effectively pointing to the memory location 4

• Storing the value 1 in some register/memory cell
  will remember the memory location 1
• The value 1 in the register/memory cell is
  effectively pointing to the memory location 1

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Example of a pointer the program counter (PC)

• The program counter (PC) is in fact a pointer
• Example

When the program counter (PC) contains the value
1 the program counter (PC) is in fact
pointing to the memory location 1
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Overview

• Logical (functional) view of a computer
• Program flow
• Types of instructions that a computer can
  execute

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Program flow

• Definition program flow
• Default program flow

• Program flow the sequence of instructions from
  the program executed by the CPU      

• After executing a non-branching (or non-jumping)
  instruction at memory location n, then next
  instruction that is executed is the instruction
  at memory location n1

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Program flow (cont.)

• Example

• All of the instructions in the figure are
  non-branching
• Therefore, the sequence of instructions executed
  by the CPU starting at memory location 0 is
• 1. 0 ("add")
• 2. 4 ("compare")
• 3. 1 ("subtract")
• 4. And so on (in sequence)

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Branching changing the program flow

• Branch instruction

• A branch instruction can alter (change) the
  default program flow
• When the CPU executes a branch x instruction,
  the next instruction that will be executed by the
  CPU is the instruction at memory location x

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Branching changing the program flow (cont.)

• Example

• Notice there is a branch to location 4
  instruction in the example.
• Therefore, the sequence of instructions executed
  by the CPU starting at memory location 0 is
• 1. 0 ("add")
• 2. 64 ("branch to location 4")
• 3. 56 ("negate" --- this is the instruction at
  memory location 4)
• 4. 0 ("add")
• 5. And so on (in sequence again)

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Overview

• Logical (functional) view of a computer
• Program flow
• Types of instructions that a computer can
  execute

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The types of instructions that a computer can
execute

• The computer is a computing (reckoning) machine
• All existing computers (actually the CPU) execute
  the following 3 types of instructions

Arithmetic and logic operations The result
of an arithmetic and logic operation is often
stored in a general purpose register

• -
• /
• AND           
• OR
• NOT

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The types of instructions that a computer can
execute (cont.)

• Memory transfer operations

• Transfer the content from some specific memory
  location to a specific register (memory cell) in
  the CPU.
• and vice versa (transfer the content from some
  specific register (memory cell) in the CPU to
  some specific memory location).

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The types of instructions that a computer can
execute (cont.)

• Branch operations

• A branch instruction will cause the CPU to
  branch (jump) to the specified location in memory
  
• After the jump has occurred, the CPU will
  continue to execute the instructions in sequence,
  until another branch/jump instruction is
  encountered
• There are 2 kinds of branch operations

• A unconditional branch instruction will always
  cause the CPU to jump to the target location
• A conditional branch instruction will only cause
  the CPU to jump to the target location when the
  specified condition is met