Objective

1. Enter your program. Use the following code as a skeleton. The comments indicate what each missing instruction needs to do:
   

   	TITLE	Program counts the negative numbers stored in an "array"
   	.model	small		;start all programs you will write 
   	.stack	100h		;for CS2011 with these directives
   
   	.data			;the program's variables go here
   ourdat 	dw		400, 16, -25, -49, -160, 18
   datlen = ($ - ourdat)/2		    ; datlen = length of the data
   
   	.code			    ;the code starts here
   	.startup		    ;TASM directive that sets up stack
   				    ;segment and data segment
   	
   	mov	bx, offset ourdat   ;initialize bx to point to our data
   
   	;now write the code for the following instructions (down until the nop)
   
   				   ;store the data length in CX
   
       				   ;move the word pointed to by BX 
   				   ;   to AX
   
   				   ;add 2 to BX
   
   				   ;subtract 1 from CX

label1:  				;move (copy) the word pointed to by BX to DX

					;compare AX to DX

     					;if AX <= DX then jump forward to the  
					;  instruction at label2

					;move DX to AX

label2:	  				;add 2 to BX

					;subtract 1 from CX

					;compare CX to 0

     					;if CX > 0 then jump back to the
					;  instruction at label1

label3:	nop				;return to DOS - the .exit takes care 
					;of that!
	.exit				;a TASM directive that returns 
					;control to the calling program.
					;Meant to be used in conjunction
					;with .startup
	end				;end of source code

1. Using the instructions found at:
   
   http://www.cs.wpi.edu/~jburge/courses/c02/cs2011/tasm_instructions.html  
   
   assemble and link your program.  You may want to save a copy to  a floppy disk to reference it later.
   
   

2.  Once you have successfully built the program, bring up the turbo debugger.  Then,

   • set a breakpoint (using F2) after  your first compare instruction (on the first jump).   Set a breakpoint on the nop instruction (at label3) as well.
     
     
   • the "view register" command will bring up the registers window in the turbo debugger. The left column gives the registers. The right column gives the values for the flags. The first four are the ones that you'll be using the most:  c for carry, z for zero, s for sign, and o for overflow. Run to the first breakpoint using F9. 
     
     What are the values of the flags the first time you hit the breakpoint after comparing AX to DX?
     carry = 
     zero =
     sign =
     overflow =
     
     Based on the flags, do you expect the jump to occur? ________ (Tables giving the conditional jumps and the flags they jump on are in Irvine, starting on p. 193).
     
      Single step to the next instruction (F8) to see if you were right.
     
     
   • Run until you hit the breakpoint on the nop instruction (if you want to save time you can remove the earlier breakpoint by putting your cursor on it and hitting F2 again). What is the result in AX? _____________________. It should be equal to the smallest number (the largest negative number).
     
     What are the values of the flags this time?
     carry = 
     zero =
     sign =
     overflow =
     
     
   • Why did you need to add two to BX each time through the loop?__________________________________________________
     
   • You should have used the variable datlen to initialize CX at the start of your program. What does the $ in the datlen definition mean? Why do you divide the result by 2? _______________________
     ________________________________

   You will need to turn in this handout at the end of your lab session to receive credit for the lab.