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The data type used in an assembler instruction

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• Important fact in assembler programming
  • Important fact:

    Every machine (= assembler) instruction uses one (1) specific type of operand(s)

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  • In other words; an assembler instruction will never say:

    Add these 2 values          Subtract these 2 values

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  • An assembler instruction will always say:

    Add these 2 byte values          or          Add these 2 short values        or          Add these 2 int values        or          Add these 2 float values        or          Add these 2 double values            or          And so on Subtract these 2 byte values          or          Subtract these 2 short values        or          Subtract these 2 int values        or          Subtract these 2 float values        or          Subtract these 2 double values            or          And so on

    I.e.:

    Each machine (= assembler) operation will always include a specific data type as part of the operation !!!

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• Denoting the operand type in an M68000 assembler instruction
  • Fact:

    Different computers has different way to denote the data type used by a machine instruction (Just like diffeent counties has different syntax for their languages...)

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  • The M68000 way to denote the type of the operand in an instruction/operation:

    Operation.Qualifier The operation nmemonic specifies the operation and the data type        A qualifier specifies the precision of the data type

    Example:

    The nmenomic for the integer addition operation in m68000 is: add operand1, operand2 // add two 2's complement coded integer operands Operations on different precision of int typed operands are specified as follows: add.b operand1, operand2 means: add 2 byte integer operands add.w operand1, operand2 means: add 2 word integer operands add.l operand1, operand2 means: add 2 long word integer operands

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  • Note:

    The M68000 was a older processor invented around 1980: At that time, the technology was not advanced enough to provide on-chip floating point operations So: The M68000 CPU does not have floating point operations..... The later (more advanced) processor M68020 does have floating point operations: fadd operand1, operand2 means: Add the floating point operands operand1 and operand2 And: fadd.l op1, op2 means: add 2 single precision (32 bits) float values fadd.x op1, op2 means: add 2 double precision (64 bits) float values

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• Example that illustrate the fact that each assembler instruction assumes one specific data type
  • Recall:

    Unsigned integer (whole number) uses the binary number system as encoding (see: click here) Signed integer (whole number) uses the 2's complement number system as encoding (see: click here)

    Example:

    This 16 bits binary number pattern: 1111111111111110 represents the value 65534(10) if it is an unsigned integer represents the value -2(10) if it is an signed integer

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  • There are 2 different multiplication instructions for integer values in M68000:

    MULU - multiply unsigned integers Meaning: perform a multiplication operation assuming that the representation used is for unsigned numbers (= binary number system) Example: 1111111111111110 = 65534(10) (unsigned) 0000000000000010 = 2(10) (unsigned) ---------------------------------------------- 00000000000000011111111111111100 = 131068(10) (unsigned) MULS - multiply signed integers Meaning: perform a multiplication operation assuming that the representation used is for signed numbers (= 2's complement number system) Example: 1111111111111110 = -2(10) (2's complement) 0000000000000010 = 2(10) (2's complement) ---------------------------------------------- 11111111111111111111111111111100 = -4(10) (2's complement)

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile:   as255 data-type To run: use          m68000

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• Default operand data type for integer (= whole number) data types
  • Default operand data type for integer (= whole number) data types:

    If an integer assembler instruction does not specify .B, .W or .L, then: The default data type is assumed to be: .W !!!

    Example:

    add op1, op2 is equal to: add.w op1, op2

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• Addressing mode reminded
  • Important reminder:

    To specify where to find an operand: Use the addressing modes that you have learned previously !!!! Example: add.b d0, d2: use the byte operands in d0 and d2 in the byte integer addition operation add.w d0, d2: use the short operands in d0 and d2 in the short integer addition operation add.w d0, d2: use the int (4 bytes) operands in d0 and d2 in the int integer addition operation add.b 0, D2: use the operands in memory address 0 and register D2 in the byte integer addition operation and so on...