Assignment 5, due Sept. 25

Part of the homework for 22C:60, Fall 2009
by Douglas W. Jones
THE UNIVERSITY OF IOWA Department of Computer Science

Background: The DSPINI Hawk monitor routine does more than initialize the display, it also returns the height and width of the available output area. DSPAT sets the output display coordinates of the next character to output. DSPCH can be used to output a single character. The instruction SR dst,1 divides an integer in the dst register by 2.
A problem: Take the framework of an empty SMAL Hawk program and fill in code to make it plot an asterisk as close to the center of the available output area as you can. Turn in clean readable source code with appropriate comments. (1.2 points)
Note. You need not run the above code, and if it is correct and readable, you could get full credit for a handwritten submission. You may opt to use the computer to check your syntax and to see how your code works. If you do, note that you can resize the terminal window before you start the Hawk emulator, and it will work in that new size, but you should not resize the window while the emulator is running.
```
                S       START
                USE     "hawk.macs"
                USE     "monitor.h"
	        EXT	UNUSED
        
        START:                          ; begin execution here
                LIL     R2,UNUSED       ; set up the stack
                                        ;  --- begin aplication code
                LIL     R1,DSPINI
                JSRS    R1,R1           ; initialize the display
        
	        SR	R3,1		; divide X (width) by 2
	        SR	R4,1		; divide Y (height) by 2
                LIL     R1,DSPAT
                JSRS    R1,R1           ; call dspat(X,Y)
        
	        LIS	R3,'*'
                LIL     R1,DSPCH
                JSRS    R1,R1           ; call dspch('*')
        
                                        ;  --- end aplication code
                LIL     R1,EXIT
                JSRS    R1,R1           ; call monitor routine to stop!
                END
```
Background: Consider this comment-free SMAL Hawk program:
```
		USE	"hawk.macs"
		S	START
	.	=	#10000
        START:	LEA     R1,NEXT
                LOADS   R2,R1
                ADDSI	R1,4
        NEXT:   STORES  R2,R1
                ADDSI	R1,4
```
a) What value does this program load in R2 and what does this value represent? (0.4 points)

It loads C411A1F2₁₆.
This is the assembled code for the two instruction sequence STORES R2,R1 followed by ADDSI R1,4 that is loaded from the memory location with the label NEXT.

b) Where does the STORES instruction store this value? (0.4 points)

In the location immediately followint the ADDSI at the end of the program.

c) What does this program do? (0.4 points)
Hint: You can actually use the Hawk emulator to help you with this, but you will do better in the long run if you try to figure out the code using only manuals and then use the emulator to check your understanding.

Each time it executes a STORES instruction, it adds another pair of instructions to the end of the program, STORES R2,R1 followed by ADDSI R1,4. The purpose of the ADDSI is to move the address used by the next STORES forward one word, so the program keeps growing.

A problem: Given an unknown signed integer value in R1 write SMAL Hawk code set R2 to the absolute value of whatever is in R1, given that the value in R1 should be interpreted as a signed integer. (0.6 points)

                MOVECC  R2,R1   ; copy and test the value
                BGE     ITSPOS  ; if value was negative
                SUB     R2,R0,R2;   negate R2
        ITSPOS:                 ; now R2 = |R1|