1. 68000 Architecture, Data Types and Addressing Modes
9/20/6
Lecture 2 - Prog Model

2. Lecture Overview Assembler Language Programmers Model Addressing Modes
Registers
Addressing Modes
9/20/6
Lecture 2 - Prog Model

3. Assembler Language
Programming language very close to the machine code for a language
Can be considered a native language for the architecture targeted.
Assembler languages have a 1-to-1 translation to machine code
All assemblers have two types of statements
Executable instructions
Assembler directives
9/20/6
Lecture 2 - Prog Model

4. Assembler Directives TTL EQU
“Title” – gives the program a user-defined name
EQU
**Links a pneumonic name to a value. Very valuable in making the program “readable”.
Example to reserve 128 bytes for a stack.
STACK_FRAME EQU Define a stack frame of 128 bytes
***
LINK A1,# -STACK_FRAME
9/20/6
Lecture 2 - Prog Model

5. Assembler Directives (2)DC
Define constant. Allows the programmer to set memory locations to specific values.
Qualifier specifies data type: .B, .W, .L
ORG $001000
FST DC.B 10, Set constants of 10 and 66
DC.L $0A Store $000A1234 in memory
STR DC.B ‘April’ Store ASCII values
9/20/6
Lecture 2 - Prog Model

6. Assembler Directives (3)DS
Reserve bytes, words, or longwords of storage
ORG
Define the value of the location counter. Sets the memory address where the code is placed.
SECTION
Set the address counter to zero for position independent code.
END
9/20/6
Lecture 2 - Prog Model

7. Assembler Code Example
9/20/6
Lecture 2 - Prog Model

8. Programmers Model
When looking at any architecture what does the programmer see?
REGISTERS (General purpose)
ADDRESS REGISTERS
SPECIAL PURPOSE REGISTERS
MEMORY
Allowable operations and data transfers supported
9/20/6
Lecture 2 - Prog Model

9. DATA REGISTERS A 32-bit machine 8 general purpose Called D0 to D7
Any operation permitted on Di can also be done on Dj
Data access 8,16,32 bits
9/20/6
Lecture 2 - Prog Model

10. Address Registers 8 Address Regs Labeled A0 to A7
Entire 32 bits is single entity value
Any operation permitted on Ai is also allowed on Aj
2’s complement value treatment
Used in instructions to address memory
9/20/6
Lecture 2 - Prog Model

11. Special Purpose Registers
2 such registers
PC – Program Counter
Contains the address of the next instruction to be executed.
SR – Status Register
16 bits divided into two bytes
MSB – System byte – control operation mode
LSB – Condition codes
9/20/6
Lecture 2 - Prog Model

12. Status Word
9/20/6
Lecture 2 - Prog Model

13. Addressing Modes
Modes supported are reflective of even the most modern architecture
Some Notation
[Dx] means the “contents of”
% num means binary
$ num means hexadecimal
 indicates transfer of information
Table on page 27 lists meaning of symbols and notation
9/20/6
Lecture 2 - Prog Model

14. Immediate Addressing
ACTION: The value of the operand (data) is loaded into the specified register at the time the program runs.
The value of the operand is used in the operation
Note that general form of instructions is
Operation #Source, Destination
MOVE.W #$8123,D3 [D3(0:15)]$8123
Fill the low order bytes of De
MOVE.L #$8123,D3 [D3(0:31)]$
Fills all 32 bits of register D3
9/20/6
Lecture 2 - Prog Model

15. 9/20/6
Lecture 2 - Prog Model

16. Absolute Addressing Sometimes called “Direct Addressing”
The address in memory being referenced by the instruction is given in the instruction. Used when the address of the operand is known when the program is written.
The instruction contains the operand’s address
FORM Operation source,destination
MOVE.L D3,$ [M($1234)][D3(16:31)]
[M($1236)][D3(0:15)]
MOVE.W $1234,D3 [D3(0:15)][M($1234)]
9/20/6
Lecture 2 - Prog Model

17. Register Direct Addressing
Register to register operation
The assembler instruction lists both register which will be used for operand of the operation to be performed
The 2nd register listed is both the 2nd operand of the operation and gets the results of the operation.
Example: MOVE.L D0,D3
Effect: [D3]  [D0]
9/20/6
Lecture 2 - Prog Model

18. Address Register Indirect
Register Indirect means that the address of the operand is in a register, the address registers.
Ex:
9/20/6
Lecture 2 - Prog Model

19. Forms of Address Reg. Indirect
Address Register Indirect with postincrement
The contents of the address register are incremented by the appropriate amount after use
That is, the data is referenced with the value currently in the address register. Then the contents of the register are incremented by the size referenced.
Address Register Indirect with predecrement
Contents are decremented and then used
What use may these have?
9/20/6
Lecture 2 - Prog Model

20. Forms of Address Reg. Indirect (2)
Address Register Indirect with displacement
A 16-bit displacement is specified in instruction
Displacement added to address register to obtain the effective address of the operand
Register Indirect with Index Addressing
Effective address is calculated by adding the address register + 8-bit signed displacement + contents of general purpose register
Ref fig 2.15 of text, pg.39
9/20/6
Lecture 2 - Prog Model

21. PC Relative
Similar to register indirect except that the address of the operand is specified with respect to the PC contents.
Allows data to be located relative to the location of the program in memory, i.e., relative to the Program Counter – allows WHAT????
Restricted to only the source operand
9/20/6
Lecture 2 - Prog Model

22. PC Relative
Allows data to be located relative to the location of the program in memory, i.e., relative to the Program Counter – allows position-independent code.
What is position-independent code?
9/20/6
Lecture 2 - Prog Model

23. PC Relative
Allows data to be located relative to the location of the program in memory, i.e., relative to the Program Counter – allows position-independent code.
What is position-independent code?
Code that can be located anywhere (some caveats) in memory and executes the same.
9/20/6
Lecture 2 - Prog Model

24. The Stack
With autoincrementing/autodecrementing of address register you have essentially 8 stacks.
In this way register A0 through A7 can become user defined stacks
For Jump to Subroutine, register A7 is used
So you may not want to use address register A7 for programs that calls to subroutines
The stack grows down and the register points to the TOS element and has valid contents.
9/20/6
Lecture 2 - Prog Model

25. Stack Operations PUSH data onto stack POP data from stack
Stack grows down.
TOS has valid data.
Use the predecrement form
MOVE.W $4234, -(A7)
POP data from stack
Use the postincrement form
MOVE.W (A7)+,D4
9/20/6
Lecture 2 - Prog Model

26. Assignment From Text HW1 Problem 2-1 parts a,b,d,g Problem 2-5
9/20/6
Lecture 2 - Prog Model