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Important property of computer (= machine) instructions

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• Very important property of computer (machine) instructions of a computer
  • Very important fact:

    The CPU can only perform operations on 2 values of the same data type with each other In other words: The CPU can add 2 byte values     or       The CPU can add 2 short (2 bytes) values,     or       The CPU can add 2 int (2 bytes) values,     or       The CPU can add 2 float (4 bytes) values,     or       The CPU can add 2 double (8 bytes) values And so on !!!! The CPU can subtract 2 byte values     or       The CPU can subtract 2 short (2 bytes) values,     or       The CPU can subtract 2 int (2 bytes) values,     or       The CPU can subtract 2 float (4 bytes) values,     or       The CPU can subtract 2 double (8 bytes) values And so on !!!! Etc. etc....

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  • Therefore:

    When the CPU needs to perform an operation on values of different data types: The CPU must (use a machine instruction to) convert the different types to a common (= same) data type Then perform the desired operation on 2 values of the same type !!!

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• Order of presentation on material on operations using different data types
  • Presentation:

    We will first learn how the computer operate on data/values of the same (data) type We will study arithemtic operations using the same data type Next, we study how the computer operate on data/values of the different (data) types: How the computer store values of a different type How the computer performs computations on values of a different type (You will see that the computer must perform conversions first !!!)

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• Teaching this material in M68000...
  • Unfortunate "distraction" in M68000:

    The M68000 can perform the add operation in a data register or in an address register Example: add.l #3, D0 // Add 3 to the value in register D0 adda.l #3, A0 // Add 3 to the value in register A0 The behavior of the operation is slightly different when you use an address registers

    It's unfortunate because ideally:

    the behaviour of each operation should be the same (so you don't need to remember additional rules when you write a M68000 program