Assignment 5, due Feb 20

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. Background: The mp1test program could have looked something like this if it were written in the C language:

       /* mp1test.c */
       #include <monitor.h>
   
       typedef struct { /* definition of the record type */
           char * text; /* pointer to the start of the text string */
           int16t x;    /* the X coordinate */
           int16t y;    /* the Y coordinate */
       } record;
   
       /* linkage to the array of pointers to records in mp1.a */
       extern record * array[]
   
       main() {
           int i = 0;
           while (array[i] != NULL) {
               putat( array[i]->x, array[i]->y );
               puts( array[i]->text );
               i = i + 1;
           }
       }
   

   You are not expected to know C, but most of the above syntax should look natural. The following notes about pointers in C may help:

   1. The declaration record * x declares x to be of type "pointer to an object of type record."
   2. The expressio x->y means "given that x is a pointer to an object of some type, use the field named y of that object.

   In the following, assume that this SMAL declaration for the symbol ARRAY has already been given. It allows the linker to link the symbol ARRAY here to the value it was given in the solution to MP1 with which the test program is linked:

       EXT     ARRAY
   

   Assume that register R8 is used for the variable i and note that each array element occupies 4 consecutive bytes; that is, the address of array element array[i] is the address of the first element of the array (array[0]) plus 4 times the value of i. Here, we use the notation array as C code, where the corresponging definition in SMAL is ARRAY.

   a) Write SMAL Hawk code to compute the address of the array element array[i] in register R9. (0.4 points)

   b) Given that the address of array[i] in register R9, write SMAL Hawk code to load the value of array[i] in R10. The value of array[i] is a pointer to a record. (0.4 points)

   c) Given that the value of array[i] is in R10, write SMAL Hawk code to check if this value is null, and if so, branch to the label DONE. (0.4 points)

   d) Given that R10 points to one record, that is, R10 holds the value of array[i], write SMAL Hawk code to load the value of the text field of that record (array[i]->text) into R3. That value is a pointer to a piece of text. (0.4 points)

   e) Given that a pointer to the text has already been loaded in R3, write SMAL Hawk code to call the Hawk monitor puts() routine. This completes the coding of puts(array[i]->text). (0.4 points)

   Note: This does not go so far as giving the entire mp1test.a because we have not discussed how to work with halfwords on the Hawk machine. That will have to wait until early March.

2. Background: In problem 1 above, and in Chapter 5 in general, we assumed that the main program could safely use R8 to R15 for variables.

   A question: How does the main program violate the rules for the standard Hawk calling sequence? (0.5 points)

3. All of the examples in Chapter 5 show calls to Hawk monitor routines that look like this:

           LIL     R1,PUTCHAR
           JSRS    R1,R1           ;   putchar( ch )
   

   All of the examples in Chapter 6 show the same call looking like this:

           ADDI    R2,R2,ARSIZE
           LIL     R1,PUTCHAR
           JSRS    R1,R1           ;   putchar( ch )
           ADDI    R2,R2,-ARSIZE
   

   A question: How did Chapter 5 hide the ARSIZE issue from the novice programmer? (0.5 points)