Objective

http://www.cs.wpi.edu/~jburge/courses/c01/cs2011/tasm_instructions.html

enter, assemble, and link the following program.  Save your program on diskette (the A: drive) so you'll have it for future reference.

	TITLE	Program subtracts two integers and checks the result

	.model	small		;start all programs you will write 
	.stack	100h		;for CS2011 with these directives

	.data				;the program's variables go here
vara	dw		5
varb	dw		11
res	dw		?

	.code				;the code starts here
	.startup			;TASM directive that sets up stack
					;segment and data segment
	mov		ax,vara		;variable a loaded into AX
	mov		res, 0		;clear result
	
	nop			 	;SET THE FIRST BREAKPOINT HERE
	sub		a, varb
	mov		res, ax		;res = vara – varb

	cmp		res, -1		;SET THE SECOND BREAKPOINT HERE

	je		done		; if the result of the subtraction is zero - jump to the end!
	mov		res, 0		;SET THE THIRD BREAKPOINT HERE

done:   nop				;SET THE FOURTH BREAKPOINT HERE
					;NOP - no operation - it's just a placeholder

	.exit				;a TASM directive that returns 
					;control to the calling program.
					;Meant to be used in conjunction
					;with .startup
	end				;end of source code

You should get an error the first time you try to build the program. What was it?

_____________________________________________________

This is because you can't subtract directly from a memory location. Change the sub vara, varb  to  sub ax, varb (the value in memory location vara has already been loaded into ax). 

Fix the error and build the program again. Once you have successfully built the program, bring up the turbo debugger.  Then,

• set breakpoints at the four  instructions indicated. To set a breakpoint, position the cursor on the desired line of source code and press F2. The line containing the breakpoint will turn red. (Pressing F2 again removes the breakpoint)
  
  
• open an inspectors window for the variable "res." To open an inspectors window, place the cursor in the name of the variable you want to inspect, and press Ctrl-I. The Window pull-down menu can be used to move windows to the front if you need to view a window that is hidden.
  
  
• open the register window. From the View menu item, choose the Registers window.
  
  
• run the program, (Choose Run from the Run menu item, or press F9), to the first breakpoint.
  
  
• Breakpoint 1:  res _______  AX _________ BX______
  
  
• run the program to the second breakpoint (F9).
  
  
• Breakpoint 2: res ________ 
  
  
• Why does "result" have this value (at 2nd BP): ____________
  
  
• Single step (F8) to the next line of the program (the jump instruction).
  
  
• What are the values of the flags? The flags are shown to the right of the register window.
  
  c (carry) = _______ z (zero) = ________ s (sign) = __________ o (overflow) = __________
  
  
• continue running the program (F9)
  
  
  Does the program hit the third breakpoint?___________
  
  
  Why not? ____________________________
  
  
• Exit the turbo debugger

You will need to know how to use the Turbo Assembler tools for Homework 3, so make sure you ask the TA for help if you come across something you do not understand. You will need to hand in this handout at the end of your lab session to receive credit for the lab.