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M68000 Immediate Addressing Mode

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• Operand specified with the immediate addressing mode is a constant operand

• Recall the context that the address mode is used within the MOVE instruction:

  MOVE <EA>,<EA> ^ ^ | | | +--- Source operand 2 and Destination +-------- Source operand 1

• Syntax to specify the immediate addressing mode:

  #N (N = constant number)

• Semantics (meaning):
  • The operand is the constant number N

• Examples:

  (1) MOVE.B #0, D0 move byte (8 bits) constant 0 into D0 (2) MOVE.W #0, D0 move word (16 bits) constant 0 into D0 (3) MOVE.L #100, D0 move long word constant 100 into D0 NOTE: you cannot use immediate mode in second operand (destination), that should be obvious... You cannot change a constant

• Constants in other number base systems:
  • $-prefix indicates a hexadecimal constant, e.g.: $FA1F
  • @-prefix indicates a octal constant, e.g.: @70167
  • %-prefix indicates a binary constant, e.g.: %11011011

• Examples:

       MOVE.B #%10101010, D0  move byte binary number 10101010 into D0
       MOVE.W #$FFFF, D0      move word constant FFFF (hex) into D0
   

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• Advanced examples:

  (1) MAX: EQU 100 ... ... MOVE.L #MAX,D0 move 100 into reg. D0

  • You must remember that every symbolic name will be replaced by its corresponding number.
  • MAX is defined to be equal to 100
  • So, the MOVE instruction will become (after replacing MAX by 100):

    MOVE.L #MAX,D0 --> MOVE.L #100,D0

  • Result: move 100 into reg. D0

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  (2) MOVE.L #A,D0 move the address of the variable A in D0 ... ... A: DS.L 10

  • You must remember that:

    A: DS.L 10

    equates the symbolic name A to the address of the memory location where the (array) variable is defined.

  • So the symbolic name A will be replaced by this address

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

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• The magic of symbolic names revealed:

  006000 org $6000 000007 MAX: EQU 7 006000 203C move.l #-1, d0 FFFF FFFF FFFFFFFF = -1 (in 32 bits) 006006 303C move.w #1, d0 0001 0001 = 1 (in 16 bits) 00600A 103C move.b #MAX, d0 0007 MAX is replaced by 7 ! 00600E 207C move.l #A, a0 0000 601A A is replaced by 0000601A ! (addr is 32 bits) 006014 227C move.l #B, a1 0000 6042 B is replaced by 00006042 ! 00601A A: ds.l 10 <---- array of 10 int at A 006042 B: ds.l 10 <---- array of 10 int at B 00606A end

  • You can see that the symbolic name A is equated to the hexadecimal number 00601A

    The "move.l #A, a0" replaces A by 0000601A

  • You can also see that the symbolic name B is equated to the hexadecimal number 006042

    The "move.l #B, a1" replaces B by 00006042

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  • So:

    YOU write Assembler knows that: Final assembler instr: ------------------ ----------------------- ------------------------ move.b #MAX,d0 MAX == 7 (dec) move.b #7,d0 move.l #A,d0 A == 601A(hex) move.l #$601A,d0 move.l #B,d0 A == 6042(hex) move.l #$6042,d0