1. Program Design Examples
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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
Parameter Passing
Stack and Local Variables
Today
Structured Programming continued
Example
9/20/6
Lecture 3 - Instruction Set - Al

3. Assembler for HLL Expressions
HLL: IF (cond L) THEN Action_S
Assume data, result of evaluating condition L, to be tested in D0. It has a value of either 1 (true) or 0 (false).
TST.B D0 test low byte DO
BEQ.S EXIT
action_S
EXIT
The BEQ.S is a short 8-bit branch where the target address displacement is <= what can be specified by 8-bits
9/20/6
Lecture 3 - Instruction Set - Al

4. Assembler for HLL Expressions 2
HLL: IF (cond L) THEN S1 ELSE S2
TST.B D0
BEQ.S ELSE
S1_actions
BRA.S EXIT
ELSE S2_actions …
EXIT
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Lecture 3 - Instruction Set - Al

5. Assembler for HLL Expressions 3
HLL: FOR I = N1 TO N2 DO S
MOVE.B #N1,D1 d1 lp cntr
NEXT S_actions
ADDQ.B #1,D1 inc lp cntr
CMP.B #N2+1,D1 test
BNE NEXT
EXIT
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Lecture 3 - Instruction Set - Al

6. Assembler for HLL Expressions 4
HLL: WHILE L DO S
REPEAT TST.B DO
BEQ.S EXIT if 0 quit
S_actions
within actions will perform
a compare that leaves
the TRUE(1) or FALSE(0)
in D0
BRA REPEAT
EXIT
NOTE: B
9/20/6
Lecture 3 - Instruction Set - Al

7. Assembler for HLL Expressions 5
HLL: REPEAT S UNTIL L
NEXT S_actions
TST.B D0
BEQ NEXT
EXIT
9/20/6
Lecture 3 - Instruction Set - Al

8. Assembler for HLL Expressions 6
HLL: FOR I=N DOWNTO -1 DO S
MOVE.W #n,D1
NEXT S_actions
DBRA D1,NEXT
Decrement D1 and loop if not -1
9/20/6
Lecture 3 - Instruction Set - Al

9. Lecture 3 - Instruction Set - Al
Testability
In the real world $$$ come into play
Could test to degree that errors never occur
Test to degree that is rational
Structured code is more testable than alternatives
Modules are units to be tested
Interaction of modules is tests
9/20/6
Lecture 3 - Instruction Set - Al

10. Lecture 3 - Instruction Set - Al
Recoverabiltiy
Also call exception handling
What does the system do with erroneous data
What does system do in response to certain classes or errors.
Decision on what action to take when these conditions occur
A poorly designed plan may be far worse than no plan at all.
9/20/6
Lecture 3 - Instruction Set - Al

11. Lecture 3 - Instruction Set - Al
Pseudocode (PDL)
PDL is a way to write down an algorithm
A compromise between HLL and assembler
Facilitates the production of reliable code
Much like modern HLLs
9/20/6
Lecture 3 - Instruction Set - Al

12. Lecture 3 - Instruction Set - Al
Program Specs
D0 used for character input and output – low order byte of D0
D1 contains code of 0,1, or 2
0 – clear the buffer and reset pointers
1 – place character in D0 into buffer
2 – remove character from buffer to D0
Buffer starts at $ and is 1,024 bytes
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Lecture 3 - Instruction Set - Al

13. Lecture 3 - Instruction Set - Al
Program Specs 2
Scratch storage may be placed after the end of the buffer – up to 32 bytes
When buffer is full oldest data is overwritten – bit 31 of D0 is set to indicate overflow
Underflow action – D0.B set to 0 – msb set
No other register modified
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Lecture 3 - Instruction Set - Al

14. Circular Buffer concept
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Lecture 3 - Instruction Set - Al

15. Lecture 3 - Instruction Set - Al
PDL
Level 1
Module: Circular_buffer
Save working registers
Select one of:
Initialize System
Input a character
Output a character
Restore working registers
End Circular_buffer
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Lecture 3 - Instruction Set - Al

16. Lecture 3 - Instruction Set - Al
PDL
Level 2
Module: Circular_buffer
Save working registers
IF [D1]=0 THEN Initialize END IF
IF [D2]=1 THEN Input_char END IF
IF [D1]=2 THEN Output_char END IF
End Circular_buffer
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Lecture 3 - Instruction Set - Al

17. Lecture 3 - Instruction Set - Al
PDL – level 2 continued
Initialize
Count=0
In_pointer := Start
Out_pointer := Start
End Initialize
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Lecture 3 - Instruction Set - Al

18. Lecture 3 - Instruction Set - Al
PDL – level 2 cont.
Input_character
Store new character
Deal with any overflow
End Input_character
Routine is still a level too high to encode
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Lecture 3 - Instruction Set - Al

19. Lecture 3 - Instruction Set - Al
PDL – level 3
Input_character
Store new character at in_pointer
In_pointer := In_pointer + 1
If In_pointer>End THEN In_pointer:=Start endif
If Count < Max THEN Count:=Count+1
ELSE
BEGIN
Set overflow flag
Out_pointer := Out_pointer + 1
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Lecture 3 - Instruction Set - Al

20. Lecture 3 - Instruction Set - Al
cont
IF Out_pointer>End Then Out_pointer:=Start
END IF
END
End Input_character
This can then be coded in assembler or even HLL.
9/20/6
Lecture 3 - Instruction Set - Al