Assignment 4, due Sep 19

On every assignment, write your name legibly as it appears on your University ID card! Homework is due on paper at the start of class on the day indicated (usually Friday). Exceptions will be made only by advance arrangement (excepting "acts of God"). Late work must be turned in to the TA's mailbox (ask the CS receptionist in 14 MLH for help). Never push homework under someone's door!

1. A Problem: Consider this SMAL code, and compare it with the code given in the assignment for MP1; you may need to poke into sections 2.2 and 2.6, and chapters 5 and 6 of the SMAL manual to get a handle on it how this works:

           TITLE	"mp1.a by Douglas W. Jones, Sept. 12, 2014"
   ; -- yet another solution to MP1 using a clever macro
                                           ;  3
           MACRO RECORD =x,=y,text         ;  4
             IF LEN(text) > 0              ;  5 (an error was here, now fixed)
               W x, y, TEXT                ;  6
               LCSAVE = .                  ;  7
               . = TEXT                    ;  8
               ASCII text,0                ;  9
               TEXT = .                    ; 10
               . = LCSAVE                  ; 11
             ELSE                          ; 12
               W 0, 0, 0                   ; 13
             ENDIF                         ; 14
           ENDMAC                          ; 15
                                           ; 16
   	INT	ARRAY                   ; 17
   ARRAY:  RECORD  1,1,"Hello"             ; 18
   	RECORD  1,2,"World"             ; 19
   	RECORD  1,3,"-----"             ; 20
   	RECORD                          ; 21
   TEXT	=	.                       ; 22
   	END                             ; 23
   

   a) Does this code load anything in memory that differs from the data loaded by the version given in the assignment for MP1? If so, what? (0.3 points)

   b) In the original code from the assignment for MP1, the programmer had to explicitly include a null terminator on every string. Here, the programmer just typed RECORD 1,1,"Hello" with no following ,0. What part of this code puts the null terminator on the string? (You can refer to parts of the code by the line numbers provided in the comments.) (0.3 points)

   c) How does this code keep the text strings separated from the array of records? In the original code, all the records were given first and then the text strings were all given later. Here, they seem to be mingled. (0.4 points)

2. Background: The mp1test program operated by calling the PUTAT and PUTS routines in the Hawk monitor. Here is some code that could be fixed to serve this purpose:

           TITLE   "mp1test.a by Douglas Jones, broken on Sept. 12, 2014"
           USE     "hawk.h"
   ARSIZE  =       4
           INT     MAIN
           S       MAIN
   MAIN:
           STORES  R1,R2
           ADDI    R2,R2,ARSIZE
   ; ------- start application code
           EXT     ARRAY
           LIL     R8,ARRAY        ; R8 points to an array entry
   LOOP:                           ; do {
           LOADS   R3,R8           ;   load R3 with the X coordinate
           ADDI    R9,R8,4         ;   make R9 point to the Y coordinate ***
           LOADS   R4,R9           ;   load R4 with the Y coordinate
           LIL     R1,PUTAT
           JSRS    R1,R1           ;   putat( X, Y );
           ----    -------         ;   make R9 point to the text pointer
           LOADS   R3,R9           ;   load R3 with the text pointer
           CMPI    R3,0
           BEQ     DONE            ;   if (text == NULL) break
           LIL     R1,PUTS
           JSRS    R1,R1           ;   puts( text );
           ADDI    R8,R8,12        ;   make R8 point to the next array entry
           BR      LOOP            ; } until break
   DONE:
   ; ------- end application code
           ADDI    R2,R2,-ARSIZE
           LOADS   R1,R2
           JUMPS   R1
           END
   

   a) An instruction is missing, it has been replaced with ---- -------. What is it (give the opcode and operands in SMAL Hawk format). (0.5 points)

   b) The instruction on the line marked with *** uses an ADDI to add 4 to the pointer in R8. Why not use an ADDSI instead? (0.5 points)

   c) List all of the instructions in the above code that have 32-bit (long) formats. You don't need to list multiple occurances of each, so if the instruction GREPL is used twice, just say GREPL, don't mention the operands. (0.5 points)

   d) If this program used the opcode BZS instead of BEQ, would the result be any different? Why? (0.5 points)