CS255 Syllabus

Intro to M68000 Assembler Programming....

Assembler Instruction Format:

[Label] Opcode Operand(s) [Comment] Example: Loop: move.b d0,d1 The rest of line is comment

Effect of processing an assembler instruction:

The assembler will replace the mnemonic Opcode Operand(s) with:
The machine instruction code will be stored at the next memory location in the assembly processs
If a label is
included on the assembler code line, then:

the symbolic name label is set equal to the address of the instruction in memory

NOTE:

Comment line:

If column 1 contains the character "*" (star), then the entire line is a comment line

Example:

    123456789 <----------------- column number

    * This whole line is a comment
     * This line will cause a problem - because * did not start on column 1

The effect of labeling assembling instructions:

It is crucial to know that when you assign a label to an assembler instruction, the symbolic name of the label will be equated to the memory location at which the assembler instruction is stored.
(Because we will be labeling instructions like a madman when we write programs in assembler...)

Here is an assembler program with 4 labels (L1, L2, L3 and L4) attached to instructions along with 3 labels (A, B, and C) attached to ds directives: click here

Assemble it and look at the assembler listing file a.lst

You should see the following:

1 000000 * Demonstrate the effect of DS directive 2 000000 * Assemble with: as255 instr 3 000000 * Look at the output file a.lst 4 001000 ORG $1000 5 001000 1200 move.b d0, d1 6 001002 1200 L1: move.b d0, d1 7 001004 1200 move.b d0, d1 8 001006 A: ds.b 10 9 001010 1200 move.b d0, d1 10 001012 1200 L2: move.b d0, d1 11 001014 1200 move.b d0, d1 12 001016 B: ds.w 10 13 00102A 1200 move.b d0, d1 14 00102C 1200 L3: move.b d0, d1 15 00102E 1200 move.b d0, d1 16 001030 C: ds.l 10 17 001058 1200 move.b d0, d1 18 00105A 1200 L4: move.b d0, d1 19 00105C 1200 move.b d0, d1 20 00105E end SYMBOL TABLE ************ A 001006 B 001016 C 001030 L1 001002 L2 001012 L3 00102C L4 00105A

Notice that:

The symbol L1 is equal to the address value (number) 001002₍₁₆₎
In fact:
This will be important when we use labels to access variable and in the branch instruction:

Overview M68000 assembler instructions

M68000 instructions can be subdivided into 5 broad categories:
- Data movement instructions (data copy is a better name)
- arithmetic operations (add, sub, mult, div)
- logic operations (and, or, not, shift, rotate)
- branch and jump instructions (include subroutine call & return)
- system control instructions - will not be covered here

M68000 instructions can have:

0, 1, or 2 operands

In binary operations (instructions with 2 operands), the second operand doubles as destination

 ADD D0, D1       - operands are D0 and D1 (D0 + D1)
		  - The second operand doubles as destination
		    i.e., the result of the operation is stored 
		    in D1

Categories of M68000 instructions
- Data movement (MOVE)
- Arithmetic (ADD, SUB, MULS, DIVS)
- Logic (AND, OR, NOT)
- Branching (BRA, Bcc, BSR, JSR, RTS)
- Controlling the CPU (not covered)
Mapping of High Level Language constructs in M68000 Assembler
- Program in High Level Language is first translated into assembler and then into computer code !
  So there is always a way to express any construct found in any High Level Language in assembler language
- Variable definitions
  - DS and DC directives
- Constant definitions
  - EQU directive
- Statements
  - Assignment statement
    - MOVE
    - ADD, SUB, MULS, DIVS (to evaluate the expression)
    - AND, OR, NOT
  - Conditional statements (if, if-else)
    - BRA
    - Bcc
  - Loop statements (while, for, do-while)
    - BRA
    - Bcc
- Subroutine call
  - BSR, JSR
  - RTS
Handout two sheets of papers describing M68000 instructions....
- M68000 instruction nmenomic codes and their meaning: click here
- M68000 instruction nmenomic codes and the allowed addressing modes: click here

How to read allowed effective addresses:


     Instruction     | Size | #  | Dn | An | ...
   ------------------+------+----+----+----+-----------
      .....          |      |    |    |    |
     move <ea>,<ea>  | BWL  | *X | *  | *X |



     Size = BWL         instruction can use sizes B, W and L
			So you may use:

				move.b .. , ..
				move.w .. , ..
				move.l .. , ..

     The other columns list the "addressing modes"

         #    represents a constant as operand
	 Dn   represents a data register as operand
	 An   represents an address register as operand
	 ...

     <ea> is the "effective address"
     The marking in the columns give you information about
     whether certain type of operands may be used for <ea>

       *  means allowed, unless restricted by other comments
       X  means not allowed in destination

     So:

       *  under Dn means: data registers can be used both in source
			   and in destination 

       *X under # means:  constants can be used in source 
			  but not in destination 
			  (make sense, you can't change a value
			  of a constant...)

       *X under An means: address registers can be used in source 
			  but not in destination 
			  (recall that we must use movea for address
			  registers !)

		          i.e.:	movea ..., An

I found an online M68000 instruction sheet that is not so descriptive, but at least it's in electronic form that you can do a "search" on: click here