Assembly Language Programming
Introduction and Addressing Modes
CEG 320/520: Computer Organization and Assembly Language Programming
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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
CEG 320/520: Computer Organization and Assembly Language Programming
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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.
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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.
CEG 320/520: Computer Organization and Assembly Language Programming
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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.
CEG 320/520: Computer Organization and Assembly Language Programming
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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!
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Assembly: Common Instructions - EXT
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
• 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
CEG 320/520: Computer Organization and Assembly Language Programming
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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
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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”
CEG 320/520: Computer Organization and Assembly Language Programming
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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]
CEG 320/520: Computer Organization and Assembly Language Programming
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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.
CEG 320/520: Computer Organization and Assembly Language Programming
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Addressing Modes: Register Direct Examples
•Register Direct:
directly accesses the
contents of the
indicated register.
Registers
D2
$1234 5678
D3
$XXXX XXXX
A0
$0000 2000
Memory
$002000
$1234
$002002
$5678
$002004
$ABCD
MOVE.B D2, D3
MOVE.W D2, D3
Registers
D2
$1234 5678
D3
$XXXX XX78
A0
$0000 2000
Registers
D2
$1234 5678
D3
$XXXX 5678
A0
$0000 2000
Registers
D2
$1234 5678
MOVE.L D2, D3
CEG 320/520: Computer Organization and Assembly Language Programming
D3
$1234 5678
A0
$0000 2000
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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.
CEG 320/520: Computer Organization and Assembly Language Programming
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Addressing Modes: Absolute Long Examples
•Absolute Long:
accesses the contents of MOVE.W $002000, D2
the indicated memory
location.
Registers
D2
$XXXX XXXX
D3
$XXXX XXXX
A0
$0000 2000
Memory
$002000
$1234
$002002
$5678
$002004
$ABCD
MOVE.B $002000, D2
Registers
D2
$XXXX 1234
D3
$XXXX XXXX
A0
$0000 2000
Registers
D2
$XXXX XX12
D3
$XXXX XXXX
A0
$0000 2000
Registers
D2
$1234 5678
MOVE.L $002000, D2
CEG 320/520: Computer Organization and Assembly Language Programming
D3
$XXXX XXXX
A0
$0000 2000
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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
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Addressing Modes: Immediate Examples
•Immediate: an actual
number X is provided.
Registers
D2
$XXXX XXXX
D3
$XXXX XXXX
A0
$0000 2000
Memory
$002000
$1234
$002002
$5678
$002004
$ABCD
MOVE.B #12, D2
MOVE.W #$12, D2
Registers
D2
$XXXX XX0C
D3
$XXXX XXXX
A0
$0000 2000
Registers
D2
$XXXX 0012
D3
$XXXX XXXX
A0
$0000 2000
Registers
D2
$0000 000C
MOVE.L #12, D2
CEG 320/520: Computer Organization and Assembly Language Programming
D3
$XXXX XXXX
A0
$0000 2000
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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.
CEG 320/520: Computer Organization and Assembly Language Programming
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Addressing Modes: Register Indirect Examples
•Register Indirect:
accesses the contents of
the memory location in
the indicated register.
Registers
D2
$XXXX XXXX
D3
$XXXX XXXX
A0
$0000 2000
Memory
$002000
$1234
$002002
$5678
$002004
$ABCD
MOVE.B (A0), D2
MOVE.W (A0), D2
Registers
D2
$XXXX XX12
D3
$XXXX XXXX
A0
$0000 2000
Registers
D2
$XXXX 1234
D3
$XXXX XXXX
A0
$0000 2000
Registers
D2
$1234 5678
MOVE.L (A0), D2
CEG 320/520: Computer Organization and Assembly Language Programming
D3
$XXXX XXXX
A0
$0000 2000
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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.
CEG 320/520: Computer Organization and Assembly Language Programming
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Addressing Modes: Indexed Basic Examples
•Indexed Basic: An index
value is added to the
memory address to form
the effective address.
Registers
D2
$XXXX XXXX
D3
$XXXX XXXX
A0
$0000 2002
Memory
$002000
$1234
$002002
$5678
$002004
$ABCD
MOVE.W 2(A0), D2
MOVE.W -2(A0), D2
Registers
D2
$XXXX ABCD
D3
$XXXX XXXX
A0
$0000 2002
Registers
D2
$XXXX 1234
D3
$XXXX XXXX
A0
$0000 2002
Registers
D2
$1234 5678
MOVE.L -2(A0), D2
CEG 320/520: Computer Organization and Assembly Language Programming
D3
$XXXX XXXX
A0
$0000 2002
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Addressing Modes: Indexed Basic Example
Memory
Address
$002000
$002002
$002004
Value
95
89
83
MOVE.L #002000, A0
CLR.L D1
ADD.W (A0), D1
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;
ADD.W 2(A0), D1
ADD.W 4(A0), D1
DIV.W #3, D1
CEG 320/520: Computer Organization and Assembly Language Programming
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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
CEG 320/520: Computer Organization and Assembly Language Programming
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Addressing Modes: Post-increment Examples
•Post-increment:
Operand is accessed
indirectly, then address
register is incremented.
Registers
D2
$XXXX XXXX
D3
$XXXX XXXX
A0
$0000 2000
Memory
$002000
$1234
$002002
$5678
$002004
$ABCD
MOVE.B (A0)+, D2
MOVE.W (A0)+, D2
Registers
D2
$XXXX XX12
D3
$XXXX XXXX
A0
$0000 2001
Registers
D2
$XXXX 1234
D3
$XXXX XXXX
A0
$0000 2002
Registers
D2
$1234 5678
MOVE.L (A0)+, D2
CEG 320/520: Computer Organization and Assembly Language Programming
D3
$XXXX XXXX
A0
$0000 2004
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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
CEG 320/520: Computer Organization and Assembly Language Programming
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Addressing Modes: Pre-decrement Examples
•Pre-decrement:
MOVE.B -(A0), D2
Address register is
decremented, then operand
is accessed indirectly.
Registers
D2
$XXXX XXXX
D3
$XXXX XXXX
A0
$0000 2004
Memory
$002000
$1234
$002002
$5678
$002004
$ABCD
MOVE.W –(A0), D2
Registers
D2
$XXXX XX78
D3
$XXXX XXXX
A0
$0000 2003
Registers
D2
$XXXX 5678
D3
$XXXX XXXX
A0
$0000 2002
Registers
D2
$1234 5678
MOVE.L –(A0), D2
CEG 320/520: Computer Organization and Assembly Language Programming
D3
$XXXX XXXX
A0
$0000 2000
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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
CEG 320/520: Computer Organization and Assembly Language Programming
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Assembly Language: In-Class Exercises
• Assembly Language and Addressing Modes
– Exercise
• Big Endian (Memory) vs Little Endian (Registers)
– Exercise
CEG 320/520: Computer Organization and Assembly Language Programming
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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
CEG 320/520: Computer Organization and Assembly Language Programming
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