MOVE <EA>,<EA> * Copy Src to Destination
^ ^
| |
| +--- Destination
+-------- Source operand
|
- Syntax
to specified the
indexed mode:
x(An, Dm) where An = A0, A1, A2, A3, A4, A5, A6, or A7 Dm = D0, D1, D2, D3, D4, D5, D6, or D7 x = a number between -128 and 127 Examples: 0(A1,D5) -8(A3,D7) 4(A5,D4) - Semantics (meaning):
- The operand
specified by x(An, Dm)
is located in
memory
at the
location (address)
given by
the value of the expression
x + An (32 bits) + Dn (32 bits)
The address used by the indexed mode is equal to:
- x + the 32 bit value in register An + the 32 bit value in register Dm.
Note:
- The registers
An and
Dm used in the
addressing mode will
not be altered
- The sum:
x + An (32 bits) + Dn (32 bits)is stored in a special Memory Address Register (MAR) and used to access the memory
- The operand
specified by x(An, Dm)
is located in
memory
at the
location (address)
given by
the value of the expression
- Example:
MOVEA.L #1000,A1 (set up address register) MOVE.L #4000,D4 (set up data register) MOVE.L #34, 4(A1, D4), D0 will store 34 (as binary number) in memory location at address 5004 Beecause: A0 contains 1000, D0 = 4000, So: 1000 + 4000 + 4 = 5004
- Some demo programs may use:
x(An, Dm.W)Then I am using the 16 bit number inside the register Dm
(That's because in my demo program, I used a very small value that can be represented by 16 bits)
- Most common situation to use the
index mode:
- Accessing an array variable such as: A[i], A[i+j], etc.
- Example 1: accessing elements in
an int array
B
High level programming language: int B[10]; int k; // k has been initialized B[k] = 34;
Assembler code: MOVEA.L #B,A0 * A0 = base address of array B MOVE.L k, D0 * D0 = k * k is the index in the array * You need to multiply by the size * of each array element to get * the offset ! MULS #4, D0 * This instruction multiply D0 by 4 * and store the result in D0 * --- Now D0 contains the offset of B[k] * --- from the base address of array B MOVE.L #34,0(A0,D0) * Move 34 into element B[k] How to defined the array B and k:: B: DS.L 10 An integer array: int B[10] k: DS.L 1 An integer variable: int k (used as index into array A)
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: as255 indexed
- To run, use: m68000
- Example 2: accessing elements in
a short array
B
High level programming language: short B[10]; int k; // k has been initialized B[k] = 34;
Assembler code: MOVEA.L #B,A0 * A0 = base address of array B * The address of any array type is 32 bits !! MOVE.L k, D0 * D0 = k * k is the index in the array * You need to multiply by the size * of each array element to get * the offset ! MULS #2, D0 * D0 = 2*D0 * (because a short occupies 2 bytes memory) * --- Now D0 contains the offset of B[k] * --- from the base address of array B MOVE.W #34,0(A0,D0) * Move 34 into element B[k] * We need to use .W because B[k] is a short How to defined the array B and k: B: DS.W 10 A shirt array: short B[10] k: DS.L 1 An integer variable: int k (used as index into array A)
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: as255 indexed1
- To run, use: m68000
- Quiz:
why
is the following program
not correct ???
High level programming language: short B[10]; int k; // k has been initialized B[k] = 34;
Assembler code: MOVEA.L #B,A0 * A0 = base address of array B * The address of any array type is 32 bits !! MOVE.L k, D0 * D0 = k * k is the index in the array * You need to multiply by the size * of each array element to get * the offset ! MULS #2, D0 * D0 = 2*D0 * (because a short occupies 2 bytes memory) * --- Now D0 contains the offset of B[k] * --- from the base address of array B MOVE.L #34,0(A0,D0) * Error !!! * Can you predict what this instruction will do ???
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: as255 indexed2
- To run, use: m68000