LBS290F                                                      Fall 1991

                     ASSIGNMENT 18 - MACHINE LANGUAGE - I

Due Date: Before Midnight 11/20/91

This program will implement a simple computer language.  This language is
similar to the lowest level language actually executed by computer hardware.

Your computer has 100 integer variables numbered from 1 through
100.

This language has the following statements:

        Statement            Action
        ---------            ----------------
        * Comment            Simply print out the comment
        E                    Echo every command
        N                    Don't Echo every command
        X or End of file     Exit
        10  S 100            Store 100 into vars[10]
        10  =  20  +  30     vars[10] = vars[20] + vars[30]
        10  =  20  -  30     vars[10] = vars[20] - vars[30]
        10  =  20  *  30     vars[10] = vars[20] * vars[30]
        10  =  20  /  30     vars[10] = vars[20] / vars[30]

The program should catch and print a message for the following errors:

        Z                    First letter illegal
        10 S                 Incorrect number of parameters
        10 >  20  +  30      Illegal Character
        10 =  20  >  30      Illegal operator
        
This program will be extended in assignment 19 so make the program
as clear and clean as possible.

The following is one possible pseudo code for the program:

   while(1)  {
     read data and check for EOF
     check for comment
     check for 'E'
     check for 'N'
     check for 'X'
     check if first character between 0 and 9 - otherwise error
     use sscanf to get second token
     token == 'S'
       use sscanf to parse
       perform store
     else if token == '='
       use sscanf to parse
       process operator '+'
       process operator '-'
       process operator '*'
       process operator '/'
     else 
       error
  }
  dump out memory

An example execution follows:

$ a.out

Enter: * This is a comment
* This is a comment
Enter: 10 S 300
Enter: 11 S 50
Enter: E
Enter: 12 S 65

Command: 12 S 65

Enter: 13 S 1000

Command: 13 S 1000

Enter: 20 = 10 + 11

Command: 20 = 10 + 11

Enter: N
Enter: 21 = 12 - 11
Enter: 22 = 11 * 12
Enter: 23 = 13 / 11
Enter: X

  Address      Value
 --------------------
    10           300
    11           50
    12           65
    13           1000
    20           350
    21           15
    22           3250
    23           20
$