9/20/6Lecture 3 - Instruction Set - Al1 Program Design Examples

9/20/6Lecture 3 - Instruction Set - Al2 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

9/20/6Lecture 3 - Instruction Set - Al3 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

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. 9/20/6Lecture 3 - Instruction Set - Al4

9/20/6Lecture 3 - Instruction Set - Al5 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

9/20/6Lecture 3 - Instruction Set - Al6 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

9/20/6Lecture 3 - Instruction Set - Al7 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.

9/20/6Lecture 3 - Instruction Set - Al8 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

9/20/6Lecture 3 - Instruction Set - Al9 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

9/20/6Lecture 3 - Instruction Set - Al10 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

9/20/6Lecture 3 - Instruction Set - Al11 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.

9/20/6Lecture 3 - Instruction Set - Al12 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

9/20/6Lecture 3 - Instruction Set - Al13 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

9/20/6Lecture 3 - Instruction Set - Al14 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

9/20/6Lecture 3 - Instruction Set - Al15 Circular Buffer concept

9/20/6Lecture 3 - Instruction Set - Al16 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

9/20/6Lecture 3 - Instruction Set - Al17 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

9/20/6Lecture 3 - Instruction Set - Al18 PDL – level 2 continued  Initialize Count=0 In_pointer := Start Out_pointer := Start End Initialize

9/20/6Lecture 3 - Instruction Set - Al19 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

9/20/6Lecture 3 - Instruction Set - Al20 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)

9/20/6Lecture 3 - Instruction Set - Al21 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.