1. Assembly Language Programming
Introduction and Addressing Modes

2. Intro to Assembly Language: Goals
Introduction to Assembly language
Basic instructions: MOVE, ADD, EXT, etc.
Operand size (B, W, L)
Register Transfer Notation (RTN)
Addressing Modes
Register direct, immediate, absolute long, register indirect, indexed basic, autoincrement, autodecrement
When to use the various modes.
Assigned Reading
HVZ Chapter 2.4, 2.5, 2.6, 3.8, 3.9, 3.10, 3.11
Reference: HVZ Appendix C

3. Introduction: Basic Instructions
Instructions begin with a mnemonic which represents the operation to be performed.
MOVE, ADD, SUB, CLR, etc.
The mnemonic is followed by a character representing the length of the data to be operated on.
Byte (B), Word (W), and Long Word (L)
In most instructions, the instruction is followed by one or more operands.
How an operand is interpreted depends on the information provided and the addressing mode used for that operand.

4. Introduction: A Few Example Instructions
ADD/SUB/MUL/DIV/AND/OR source, dest
Performs operation on dest and source and stores result in dest.
NEG location
Computes 2’s complement of value in location, then stores it into location.
NOT location
Computes complement of value in location, then stores it into location.
CLR location
Sets value of byte/word/long at location to 0.
MOVE source, dest
Moves value of source to dest location.

5. Assembly: Operand size
Because the is capable of performing operations on bytes, words and long words:
In assembly language, a size indicator is appended to the instruction mnemonic (W=word, B=byte, L=long word):
MOVE.W, MOVE.B, MOVE.L, ADD.W, ADD.B, and ADD.L are examples of this.
If the size indicator is omitted, a WORD operation is assumed.

6. Assembly: Common Instructions - Examples
MOVE
MOVE.L D1, D2
D2 = D1
Contents of long word in D1 is copied into D2.
Destroys contents of D2!
Contents of D1 do not change!
ADD/SUB/MUL/DIV
ADD.W D1, D2
D2 = D1 + D2
Sum of words in D1 and D2 is placed into D2.

7. Assembly: Common Instructions - EXT
EXT.W extends a byte to a word
Bit 7 is copied into bits 8-15.
EXT.L extends a word into a long word
Bit 15 is copied into bits 16-31

8. Assembly: Common Instructions - EXT
Sign extension does not change the value of positive or negative 2’s complement numbers!
= 3
EXT.L : = 3
= 3
EXT.L : = 3

9. Register Transfer Notation: Introduction
Symbolic and precise way to describe the effect of an instruction.
Example:
MOVE D3,D2
The contents of register D3 are copied to register D2.
A common notation to designate this is: D2  [D3]
Brackets around D3 indicate “contents of”
Left arrow indicates “receives”

10. Register Transfer Notation: Instructions
DEST  Result
SUB D5, D7
can be described by:
D7  [D7] - [D5]
ADD D2, $001004
Here, $ is a memory address. The same notation still suffices:
$  [$001004] + [D2]

11. Addressing Modes: Register Direct
MOVE D2, D3
Register Direct: directly accesses the contents of the indicated register.
RTN:
D3  [D2]
Most efficient addressing mode because it requires no memory access to fetch the operand.
Uses: loop counters, accumulators, operation results of all kinds.
The data is already in your mailbox, you just need to get it.

12. Addressing Modes: Register Direct Examples
Registers D2 $ D3
$XXXX XX78 A0 $
Register Direct: directly accesses the contents of the indicated register.
MOVE.B D2, D3
Registers D2 $ D3
$XXXX XXXX A0 $
Registers D2 $ D3
$XXXX 5678 A0 $
MOVE.W D2, D3
Memory
$002000
$1234
$002002
$5678
$002004
$ABCD
Registers D2 $ D3 A0 $
MOVE.L D2, D3

13. Addressing Modes: Absolute Long
MOVE $001020, D2
Absolute Long: accesses the contents of the indicated memory location.
RTN:
D2  [$001020]
Motorola 68k also provides an Absolute Short addressing mode, but we will not be using it.
Uses: moving stored variables from memory into registers for processing, storing results back to memory.
You know the actual address ($001020) of the data, so you need to get it from there.

14. Addressing Modes: Absolute Long Examples
Registers D2
$XXXX 1234 D3
$XXXX XXXX A0 $
Absolute Long: accesses the contents of the indicated memory location.
MOVE.W $002000, D2
Registers D2
$XXXX XXXX D3 A0 $
Registers D2
$XXXX XX12 D3
$XXXX XXXX A0 $
MOVE.B $002000, D2
Memory
$002000
$1234
$002002
$5678
$002004
$ABCD
Registers D2 $ D3
$XXXX XXXX A0 $
MOVE.L $002000, D2

15. Addressing Modes: Immediate
MOVE #X, D2
Immediate: an actual number X is provided.
RTN:
D2  X
Immediate value is assumed to be decimal unless indicated otherwise (ie by $ for hexadecimal for octal).
Uses: incrementing loop counters, working with immediate values.
You know the actual value of the data

16. Addressing Modes: Immediate Examples
Registers D2
$XXXX XX0C D3
$XXXX XXXX A0 $
Immediate: an actual number X is provided.
MOVE.B #12, D2
Registers D2
$XXXX XXXX D3 A0 $
Registers D2
$XXXX 0012 D3
$XXXX XXXX A0 $
MOVE.W #$12, D2
Memory
$002000
$1234
$002002
$5678
$002004
$ABCD
Registers D2
$ C D3
$XXXX XXXX A0 $
MOVE.L #12, D2

17. Addressing Modes: Register Indirect
MOVE (A0), D2
Register Indirect: accesses the contents of the memory location in the indicated register.
Effective Address: [A0]
RTN:
D2  [[A0]]
Uses: repeated access to the same memory location
You have a friend (A0) who knows the address of the data. You can ask her where it is in memory, then get it from that location in memory.

18. Addressing Modes: Register Indirect Examples
Registers D2
$XXXX XX12 D3
$XXXX XXXX A0 $
Register Indirect: accesses the contents of the memory location in the indicated register.
MOVE.B (A0), D2
Registers D2
$XXXX XXXX D3 A0 $
Registers D2
$XXXX 1234 D3
$XXXX XXXX A0 $
MOVE.W (A0), D2
Memory
$002000
$1234
$002002
$5678
$002004
$ABCD
Registers D2 $ D3
$XXXX XXXX A0 $
MOVE.L (A0), D2

19. Addressing Modes: Register Indirect – Indexed Basic
MOVE X(A0), D2
Indexed Basic: An index value X is added to the memory address in the indicated register to form the effective address, then the contents of the effective address are accessed.
Effective Address:
[A0] + X
RTN:
D2  [[A0] + X]
X is a decimal integer index value
Motorola 68k also provides an Indexed Full addressing mode, but we will not be using it.

20. Addressing Modes: Indexed Basic Examples
Registers D2
$XXXX ABCD D3
$XXXX XXXX A0 $
Indexed Basic: An index value is added to the memory address to form the effective address.
MOVE.W 2(A0), D2
Registers D2
$XXXX XXXX D3 A0 $
Registers D2
$XXXX 1234 D3
$XXXX XXXX A0 $
MOVE.W -2(A0), D2
Memory
$002000
$1234
$002002
$5678
$002004
$ABCD
Registers D2 $ D3
$XXXX XXXX A0 $
MOVE.L -2(A0), D2

21. Addressing Modes: Indexed Basic Example
Struct Student {
int grade1;
int grade2;
int grade3; };
Struct Student Joe, Bob, Mary;
Avg_Joe = Joe.grade1 +
Joe.grade2 +
Joe.grade3;
Avg_Joe = Avg_Joe / 3;
Memory
Address Value $ $ $
MOVE.L #002000, A0
CLR.L D1
ADD.W (A0), D1
ADD.W 2(A0), D1
ADD.W 4(A0), D1
DIV.W #3, D1

22. Addressing Modes: Register Indirect – Post-increment
MOVE (A0)+, D2
Post-increment or Autoincrement: Operand is accessed indirectly, then address register is incremented.
Effective Address:
the contents of A0
RTN:
D2  [[A0]]
A0  [A0] + the number of bytes accessed
Increment size: BYTE = 1, WORD = 2, LONG = 4
Uses: moving through an array, popping from stack

23. Addressing Modes: Post-increment Examples
Registers D2
$XXXX XX12 D3
$XXXX XXXX A0 $
Post-increment: Operand is accessed indirectly, then address register is incremented.
MOVE.B (A0)+, D2
Registers D2
$XXXX XXXX D3 A0 $
Registers D2
$XXXX 1234 D3
$XXXX XXXX A0 $
MOVE.W (A0)+, D2
Memory
$002000
$1234
$002002
$5678
$002004
$ABCD
Registers D2 $ D3
$XXXX XXXX A0 $
MOVE.L (A0)+, D2

24. Addressing Modes: Register Indirect – Pre-decrement
MOVE.W –(A0), D2
Pre-decrement or Autodecrement: Address register is decremented, then operand is accessed indirectly.
Effective Address:
(the contents of A0) – (number of bytes accessed)
RTN:
A0  [A0] – (number of bytes accessed)
D2  [[A0]]
Decrement size: BYTE = 1, WORD = 2, LONG = 4
Uses: moving through an array, pushing onto stack

25. Addressing Modes: Pre-decrement Examples
Registers D2
$XXXX XX78 D3
$XXXX XXXX A0 $
Pre-decrement: Address register is decremented, then operand is accessed indirectly.
MOVE.B -(A0), D2
Registers D2
$XXXX XXXX D3 A0 $
Registers D2
$XXXX 5678 D3
$XXXX XXXX A0 $
MOVE.W –(A0), D2
Memory
$002000
$1234
$002002
$5678
$002004
$ABCD
Registers D2 $ D3
$XXXX XXXX A0 $
MOVE.L –(A0), D2

26. Addressing Modes: Post-increment/Pre-decrement
In the 68000, the increment/decrement depends on the operand size
Suppose A0 = $
MOVE.B (A0)+,D0  A0 = $
MOVE.W (A0)+,D0  A0 = $
MOVE.L (A0)+,D0  A0 = $

27. Assembly Language: In-Class Exercises
Assembly Language and Addressing Modes
Exercise
Big Endian (Memory) vs Little Endian (Registers)

28. Intro to Assembly Language: You Should Know…
Introduction to Assembly language
Basic instructions: MOVE, ADD, EXT, etc.
Operand size (B, W, L)
Register Transfer Notation
Addressing Modes
Register direct, immediate, absolute long, register indirect, indexed basic, autoincrement, autodecrement
When to use the various modes
Assigned Reading
HVZ Chapter 2.4, 2.5, 2.6, 3.8, 3.9, 3.10, 3.11
Reference: HVZ Appendix C