Assembler Instruction Set for High-Level Language Expressions: Lecture 3, Slides of Microcomputers

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Program Design Examples

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

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 2

Assembler for HLL Expressions 2

• HLL: IF (cond L) THEN S1 ELSE S
  • TST.B D
  • BEQ.S ELSE
  • S1_actions
  • BRA.S EXIT
  • ELSE S2_actions
  • …
  • EXIT 9/20/6 Lecture 3 - Instruction Set - Al 4

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 9/20/6 Lecture 3 - Instruction Set - Al 5

Assembler for HLL Expressions 5

• HLL: REPEAT S UNTIL L
  • NEXT S_actions
  • TST.B D
  • BEQ NEXT
  • EXIT 9/20/6 Lecture 3 - Instruction Set - Al 7

Assembler for HLL Expressions 6

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

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 10

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 11

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 9/20/6 Lecture 3 - Instruction Set - Al 13

Circular Buffer concept

9/20/6 Lecture 3 - Instruction Set - Al 14

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 9/20/6 Lecture 3 - Instruction Set - Al 16

PDL – level 2 continued

• Initialize
  • Count= In_pointer := Start Out_pointer := Start End Initialize 9/20/6 Lecture 3 - Instruction Set - Al 17

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+
    • ELSE
    • BEGIN
    • Set overflow flag
    • Out_pointer := Out_pointer + 1 9/20/6 Lecture 3 - Instruction Set - Al 19

cont

• 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.

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