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The subtract (SUB) instruction

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• The SUB machine instruction
  • Subtraction:

    The subtract instruction is similar to the add instruction

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  • Subtract in a data register:

    SUB.B Src, Dn * Subtract the 8 bits values: Dn = Dn - Src SUB.W Src, Dn * Subtract the 16 bits values: Dn = Dn - Src SUB.L Src, Dn * Subtract the 32 bits values: Dn = Dn - Src

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  • Note:

    The subtract instruction is very similar to the add instruction You can use the above examples to learn about the subtract instruction The examples will work if you replace: ADD ---> SUB

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  • Subtract in an address register:

    SUBA.W Src, An * Subtract the 16 bits values: An = An - Src SUBA.L Src, An * Subtract the 32 bits values: An = An - Src

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• Reminder: When to use data and address registers
  • Data registers:

    When you perform calculation: always use data registers

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  • Address registers:

    When you access an array element or a field in a linked list:     use address registers That's because the indirect addressing mode used to access an array element or a field in a list: m(An) requires an address register An !!!

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    Example:

    short B[10]; (We assumed array B have been defined) ===> Get the value of B[3] into register D0 In assembler code: /* ========================================================= We want to access B[3]: ==> put base address in address register ========================================================= */ movea.l #B,A0 * A0 = base address of array B * An address is 32 bits, so we use .l !!! /* ========================================================= We put the values B[3] in data register D0 ========================================================= */ move.w 6(A0),D0 * Get short (16 bits) variable B[3] into register D0 * We use an address register A0 because the * indirect addressing mode 6(A0) NEEDS to use * an address register * We put the value into a data register D0 * because we want use the value in B[3] in computations !!!