9/20/6 Lecture 3 - Instruction Set - Al 1

Program Design Examples

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Lecture Overview Have seen the initial part of

Top down design Modular design Parameter Passing Stack and Local Variables

Today Structured Programming continued HLL Structures to assembler

Example

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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_SEXIT

The BEQ.S is a short 8-bit branch where the target address displacement is <= what can be specified by 8-bits

If the data L to be tested in reg If the data L to be tested is in register D0 and the

test is L = 0 with action S if L=0 Could do

CMPI.W #0, D0 BNE around action S around following_instructions

Note: This does explain how to do a test with the results of the test in D0. That would take a couple of instructions to do. Ref page 66.

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Assembler for HLL Expressions 2 Here D0 has result of test of condition 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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Assembler for HLL Expressions 3 HLL: FOR I = N1 TO N2 DO S

MOVE.B #N1,D1 d1 is loop cntr NEXT S_actions ADDQ.B #1,D1 inc loop cntr CMP.B #N2+1,D1 test BNE NEXT EXIT

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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: A while loop may or may not execute once. Why TST is at start.

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Assembler for HLL Expressions 5 HLL: REPEAT S UNTIL L

NEXT S_actions --and code that puts

result of ‘L’ in D0 as T/F (F=0) TST.B D0 BEQ NEXT EXIT

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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

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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 tested

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Recoverabiltiy Also called 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, such as divide by 0. A poorly designed plan may be far worse than

no plan at all.

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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

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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 $01 0000 and is 1,024 bytes

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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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Circular Buffer concept

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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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PDL Level 2

Module: Circular_buffer Save working registers IF [D1]=0 THEN Initialize END IF IF [D1]=1 THEN Input_char END IF IF [D1]=2 THEN Output_char END IF End Circular_buffer

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PDL – level 2 continued Initialize

Count=0 In_pointer := Start

Out_pointer := Start

End Initialize

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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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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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continued IF Out_pointer>End Then Out_pointer:=Start END IF END

END IF End Input_character

This can then be coded in assembler or even HLL by translating HLL or pseudocode to assembler.