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Assembly Language Programming

Learn about basic instructions, operand size indicators, register transfer notation, and addressing modes in assembly language programming. Understand when to use different modes with examples and common instructions.

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Assembly Language Programming

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  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 68000 is capable of performing operations on bytes, words and long words: • In 68000 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. • Destroys contents of D2!

  7. 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 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! • 0000 0011 = 3 • EXT.L : 0000 0000 0000 0011 = 3 • 1111 1101 = 3 • EXT.L : 1111 1111 1111 1101 = 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, $001004 is a memory address. The same notation still suffices: $001004  [$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 • Register Direct: directly accesses the contents of the indicated register. MOVE.B D2, D3 MOVE.W D2, D3 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 • Absolute Long: accesses the contents of the indicated memory location. MOVE.W $002000, D2 MOVE.B $002000, D2 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 or @ for octal). • Uses: incrementing loop counters, working with immediate values. • You know the actual value of the data

  16. Addressing Modes: Immediate Examples • Immediate: an actual number X is provided. MOVE.B #12, D2 MOVE.W #$12, D2 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 • Register Indirect: accesses the contents of the memory location in the indicated register. MOVE.B (A0), D2 MOVE.W (A0), D2 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 • Indexed Basic: An index value is added to the memory address to form the effective address. MOVE.W 2(A0), D2 MOVE.W -2(A0), D2 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 $002000 95 $002002 89 $002004 83 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 • Post-increment: Operand is accessed indirectly, then address register is incremented. MOVE.B (A0)+, D2 MOVE.W (A0)+, D2 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 • Pre-decrement: Address register is decremented, then operand is accessed indirectly. MOVE.B -(A0), D2 MOVE.W –(A0), D2 MOVE.L –(A0), D2

  26. Addressing Modes: Post-increment/Pre-decrement • In the 68000, the increment/decrement depends on the operand size • Suppose A0 = $00002000 • MOVE.B (A0)+,D0  A0 = $00002001 • MOVE.W (A0)+,D0  A0 = $00002002 • MOVE.L (A0)+,D0  A0 = $00002004

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

  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

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