1. Lecture 3 - Instruction Set - Al
Program Design (2)
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Lecture 3 - Instruction Set - Al

2. Lecture 3 - Instruction Set - Al
Lecture Overview
Have seen the initial part of
Top down design
Modular design
Now
Parameter Passing
Stack and Local Variables
Structured Programming
9/20/6
Lecture 3 - Instruction Set - Al

3. Parameter Passing to Modules
Modules (subroutines) should have no side effects
Receive (access) only the data passed to it
Modify and store/update only parameters in its formal output list
Some subroutines trigger an event such as sounding an alarm, running a pump, etc.
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Lecture 3 - Instruction Set - Al

4. Parameter passing example
Subroutine PUT_CHAR displays a single character on the monitor.
Main program action
Push character on stack
Call subroutine BSR PUT_CHAR
PUT_CHAR knows the location, and as only 1 byte, the data might be passed in a data register
More formal method places the data on the stack
It can use 4(A7) or 6(A7) to access the character.
9/20/6
Lecture 3 - Instruction Set - Al

5. Parameter passing in Registers
Works when the quantity of data is small or the processor has many, many registers.
Passing arguments in registers is code that is position independent and reentrant.
Position independent as no absolute address location involved in data transfer
Reentrant as the registers would be saved before they are reused.
9/20/6
Lecture 3 - Instruction Set - Al

6. Mechanisms for Parameter Passing
Ways in which parameters are passed.
Pass-by-value: The current value of the variable at the time the subroutine call is made, is passed to the subroutine
Pass-by-reference: The address of the variable is passed to the subroutine, i.e., a pointer. This allows modification of the variable back in the calling routine, by the subroutine.
Mechanisms
Registers - Memory Location - Stack
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Lecture 3 - Instruction Set - Al

7. Lecture 3 - Instruction Set - Al
Pass by Reference
Subroutine to search a region of memory
Text in memory
Want to check for sequence of characters
Info needed by subroutine
Starting and ending address of text
Start and ending address of the string
Subroutine receives addresses when called
Information in call – what string to find
Note: Code is not reentrant and cannot be used by an interrupt routine
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Lecture 3 - Instruction Set - Al

8. Lecture 3 - Instruction Set - Al
Code example
For using the stack to pass by reference.
9/20/6
Lecture 3 - Instruction Set - Al

9. Lecture 3 - Instruction Set - Al
Figures for example
Memory contents and The stack
9/20/6
Lecture 3 - Instruction Set - Al

10. Parameter Passing on the Stack
Can use the stack to transfer the actual data
Can be used to pass the addresses for pass-by-reference
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Lecture 3 - Instruction Set - Al

11. Figures for this example
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Lecture 3 - Instruction Set - Al

12. Stack and Local Variables
Subroutines need access to space for local variables
Possible that they fit in register but may not
Allocate local storage space on stack.
Keeps the subroutine reentrant
Also bind the variables not only to the subroutine, but to the specific invocation
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Lecture 3 - Instruction Set - Al

13. Lecture 3 - Instruction Set - Al
Stack frame
Allocation space on the stack
LEA -200(SP),SP
To restore stack prior to return
LEA 200(SP),SP
Refer to Fig 3.6 of text.
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Lecture 3 - Instruction Set - Al

14. Lecture 3 - Instruction Set - Al
Simplification
LINK and UNLINK
A instruction to specifically do this
MOVE.L D0-D7,/A3-A6, -SP
Save the specified registers on the stack
LINK A1, #-64
Allocate 64 bytes (16 longwords) of on this stack frame
Start subroutine
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Lecture 3 - Instruction Set - Al

15. Lecture 3 - Instruction Set - Al
Figures
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Lecture 3 - Instruction Set - Al

16. Lecture 3 - Instruction Set - Al
Subroutine example
Calculate R=(P2 + Q2)/(P2 – Q2)
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Lecture 3 - Instruction Set - Al

17. Lecture 3 - Instruction Set - Al
Link Example
The stack
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Lecture 3 - Instruction Set - Al

18. Structured Programming
A semiformal method of program development
3 benefits
Inproves programmer productivity
Program is easier to read
Program is more reliable
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Lecture 3 - Instruction Set - Al

19. Three fundamental components
These are the basic elements of all high level functional programming languages.
Sequence
Action A, then action B, then action C, etc.
A looping mechanism
Finite loop – a set number of iterations
Conditional loop – exit when condition occurs
A decision mechanism
If (condition) THEN action A ELSE action B
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Lecture 3 - Instruction Set - Al

20. Lecture 3 - Instruction Set - Al
GO TO????????????????????
All algorithms can be programmed using structured programming techniques without using a GO TO!!!
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Lecture 3 - Instruction Set - Al

21. Lecture 3 - Instruction Set - Al
Assignment 4
Problem 2-62
9/20/6
Lecture 3 - Instruction Set - Al