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Initializing and copying variables - simple high level programming statements in assembler: b = 4 and b = a

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• Initializing variables with some value
  The assignment statement:

  b = 4 (or any other value)

  is executed by the computer as follows:

  You first move the value 4 (or any other value) into a (unused) register using a mov (if the value is small) or the movw and movt instructions (this will work with any value) See: click here and click here Then you move (= store) the value in the register to the (memory) variable b (= the memory location(s) given by the memory address marked by the label b) See: click here

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  • Important note:

    Make sure that you use the correct store instruction (ldrb, ldrh or ldr) to store the variable b: Use strb when b is a byte typed variable Use strh when b is a short typed variable Use str when b is a int typed variable

    You will practice this in project 4

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• Copying variables
  The assignment statement:

  b = a

  is executed by the computer as follows:

  You first move (= load) the value from the (memory) variable a on the RHS (right hand side) into a (unused) register You must sign-extend the value to int to achieve maximum precision See: click here Then you move (= store) the value in the register to the (memory) variable b (= the memory location(s) given by the memory address marked by the label b) See: click here

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  • Important note:

    Make sure that you use the correct load instruction (ldrsb, ldrsh or ldr) to load the variable a: Use ldrsb when a is a byte typed variable Use ldrsh when a is a short typed variable Use ldr when a is a int typed variable Make sure that you use the correct store instruction (ldrb, ldrh or ldr) to store the variable b: Use strb when b is a byte typed variable Use strh when b is a short typed variable Use str when b is a int typed variable

    You will practice this in project 4