Assignment 9, due Oct 31

On every assignment, write your name legibly as it appears on your University ID card! Homework is due on paper Homework is due on paper at the start of class on the day indicated (usually Friday). 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: Consider these definitions of symbols that you might find in a program:

           EXT     A
           INT     B
   B:      W       5
   C       =       5
   D       =       .
           COMMON  E,4
   

   Some of these symbols are absolute, some are relocatable. In order to answer the following, questions, you will need to work out which is which.

   For each of the following expressions, is it legal? Yes or no. If it is legal and absolute, what is the value of the expression? If it is legal and relocatable, what is its value relative to what relocation base? If it is illegal, what is the problem? (0.2 points each)

   a) A + 1

   b) B + A

   c) C - B

   d) D + C

   e) E - D

   Note: This topic is discussed in Chapters 5 and 10 of the notes, and in Sections 2.3 and 4 of the SMAL manual.

2. Background: Suppose you had an application that required extensive use of 4-word (128 bit) integers. To support this, you write write a package of subroutines to perform add, subtract, multiply and divide operations. Here is the header file for that package:

   ; bigint.h
   ;   interface specification for bigint.a, high precision integer arithmetic
   
   BIGINTSZ =      16      ; size in bytes of an bigint object
                           ; note:  this is set for 128 bit objects,
   			; but this can be changed for other versions
   
           EXT     BIGADD  ; bigadd(a,b,c): a = b + c
           EXT     BIGSUB  ; bigsub(a,b,c): a = b - c
           EXT     BIGMUL  ; bigmul(a,b,c): a = b * c
           EXT     BIGDIV  ; bigdiv(a,b,c): a = b / c
   ;               ; uses R3 = pointer to operand a
   ;               ; uses R4 = pointer to operand b
   ;               ; uses R5 = pointer to operand c
   	
   

   Note that one thing you probably want to be able to do is write applications that use the bigint package and then, later, change BIGINTSZ when you discover that the size you were using wasn't big enough (or perhaps it was too big).

   a) Write SMAL code to declare a global variable called X that is a bigint. (0.5 points)

   b) Here is the SMAL code for an activation record of a subroutine:

   ;RETAD  =       0       ; the return address
   INDEX   =       4       ; a 32 bit integer
   ARSIZE  =       8
   

   Rewrite the activation record to include two local variables that hold bigints, one called Y and the other called Z. Your code should be resiliant enough that the activation record automatically rearranges itself if someone changes BIGINTSZ. (0.5 points)

   c) Write SMAL Hawk code to call bigsub(x,y,z). Assume that R2 usage has not been optimized. The variables x, y and z are the variables you defined in parts a and b above. (0.5 points)

3. Background: We have been writing activation records like this:

   ;RETAD  =       0       ; the return address
   INDEX   =       4       ; a 32 bit integer
   ARSIZE  =       8
   

   With a bit of cleverness, we could solve problem 2b above by writing activation records like this, instead:

           NEWAR
           LOCAL   INDEX,INTSIZE
           LOCAL   Y,BIGINTSZ
           LOCAL   Z,BIGINTSZ
   

   The net effect of these macros should be that ARSIZE is set correctly and the identifiers INDEX, Y and Z are defined just as they were by your answer to problem 2b.

   A Problem: Write the code for the macros NEWAR and LOCAL. (0.5 points)