CS212 Spring 2002
2/20/2002
Lecture 5: Introduction to Part 2 of the Project (continued)
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[0] Announcements:
+ policy reminder: read online announcements every day
+ online submission: alive and working!
+ partners: sent out names already -- contact TAs if you need to find one(s)
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[1] Summary from last time:
+ new language: Bali
+ sam-code for Bali
  - basics (declarations, arithmetic, logic, assignment, return)
  - control structures (just labels, jumps)
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[2] Overview:
+ more about selection and repetition in SaM
+ compiler
  - overall goal
  - file I/O
  - expression/statement parsing
  - style issues
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[3] sam-code: selection

+ Bali example: 
     main()
	int x;
	bool flag;
	x := 3;						
	flag := 0;     % giving flag a dummy value
	if ( (x > 2) ) % remember that expressions must be surrounded by ()
	   flag = true;
	else
	   flag = false;
	return flag;

+ sam-code instructions:

  labels and jumps:
 
    JUMP label
    label:
    instruction

  JUMPC label

    If the top of the stack is true (1) go to label.
    Otherwise, go to instruction just below JUMPC label instruction.

+ Basics in sam-code (sam3.txt):

  ADDSP  ___	// adjust SP to account for rv, x, and flag
  PUSHIMM __	// push value of 3
  STOREOFF __	// store the 3 in for x
  PUSHIMM __	// push the value of 0 (false)
  STOREOFF __	// store the 0 for flag
  PUSHOFF __	// push the value stored in V[1+FBR] (the address of x)
  PUSHIMM __	// push the value with to compare x
  GREATER	// find Vbelow > Vtop and push the result to the top


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+ Now what?!?
  - Jump to a block of sam-code that handles the case of the
    condition being true; or,
  - Continue executing sam-code because the condition was false.

+ To test if the last condition was true: JUMPC label
  - If Vtop is true, PC<-label. 
    So, SaM will jump to the instruction with label.
  - Otherwise, PC<-PC+1.
    So, SaM will execute the next instruction underneath the JUMPC instruction.

  JUMPC correct	// check if the result of GREATER is true (1) or false (0)
  PUSHIMM 1	// false: push the value 0 (false)
  STOREOFF 2	//        store the value false for flag
  JUMP continue	//        continue with remaining program
  correct:	// true:
  PUSHIMM 1	//        push the value 1 (true)
  STOREOFF 2	//        store the value true for flag
  JUMP continue	//        continue with remaining program
  continue:	// continue with program:
  PUSHOFF 2	//        push the value of flag
  STOREOFF 0	//        store the value of flag in rv
  ADDSP -2	//        reset the SP
  STOP		//        done with the program
  
  redundant code: 
     JUMP continue	//        continue with remaining program
     continue:		// continue with program:

  The correct block will "fall through" to the remaining program.
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[4] sam-code: do-while

+ Example:
  main()
     int x;
     x := 1;
     do
	x := (x + 1);
     while ( (x < 5) );
     return x;

+ what's "while"?

  begin:
  if c then
    do stuff
    goto begin
  else             Note: Bali requires the else-clause, we need a goto here.
    goto done
  done:
  remaining program









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+ sam-code (sam5.txt) for example:

  ADDSP __	   // leave space for x and rv
  PUSHIMM __	   // push 1 on the stack
  STOREOFF __	   // store the value 1 for x
  looplabel:	   // label the loop starting at the "do"
  PUSHOFF __	   // retrieve the current value of x
  PUSHIMM __	   // push 1 onto the stack
  ADD		   // add 1 to the current value of x
  STOREOFF __	   // store the new value of x
  PUSHOFF __	   // retrieve the new value of x
  PUSHIMM __	   // the value x is compared to, which is 5
  LESS		   // compare x
  JUMPC looplabel  // repeat the loop if the comparison succeeded
  PUSHOFF __	   // push the current value of x
  STOREOFF __	   // store the current value of x to return
  ADDSP __	   // adjust the SP to return x
  STOP		   // stop the program

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[5] Compiler:

+ overall goal:

  Compiler converts ______________________ into ______________________ .
  (for SaM)

  Ultimately, you will build an EXPRESSION PARSER
  (convert one expression into another)

  Diagram:

  
   
            


			      (I->C->O)

+ File I/O (streams, Java)








+ Expression parsing
  (see also CS211 notes; specifics in written assignment)







+ General advice
  - software engineering (top-down, bottom-up, stubbing)


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