Spring 2001 CSCI 255 Lab 12

This lab is scheduled for the week of 16 April - 19 April.

Goals and Methods

This week we'll look at:

• Translating C code to LC-2 assembler
• Using pointers and arrays

Getting started

Begin by executing the following commands to create a directory csci/255/lab12 and copy one file into it.

• mkdir -p csci/255/lab12
• cd csci/255/lab12
• cp /usr/local/csci/255/lab12/lab12.asm .

The copied file is the skeleton of an LC-2 program. You'll finish it during the lab.

The program

By the end of this lab you should have a LC-2 program, stored in lab12.asm, that prompts the user five times for a single hexadecimal digit ('0' to '9', 'a' to 'f', or 'A' to 'F') and then prints the English and French (Anglais et Français) words for that number. Here's an example of what should happen when you give the program three of those five numbers:

Enter the hexadecimal number: C
twelve
douze

Enter the hexadecimal number: b
eleven
ouze

Enter the hexadecimal number: 0
zero
zéro

And, here's an example of a C program to solve the problem:

#include <stdio.h>

char *English[16] = {
	"zero",   " one",       "two",      "three",
	"four",     "five",     "six",      "seven",
	"eight",    "nine",     "ten",      "eleven",
	"twelve",   "thirteen", "fourteen", "fifteen"
} ;

char Francais[16][9] = {
	"z\xe9ro",  "un",       "deux",     "trois",
	"quatre",   "cinq",     "six",      "sept",
	"huit",     "neuf",     "dix",      "onze",
	"douze",    "treize",   "quatorze", "quinze"
} ;
	

int ah2i(int x) {
	if ('0' <= x && x <= '9')
		return x - '0' ;
	if ('A' <= x && x <= 'F')
		return x - ('A' - 10) ;
	if ('a' <= x && x <= 'f')
		return x - ('a' - 10) ;
	return -1 ;

}

main(int argc, char *argv[]) {
	int i, c, b ;
	for (i=5; i != 0; --i) {
		fputs("\nEnter the hexadecimal number (0-9,A-F): ", stdout) ;
		c = getc(stdin) ;
		b = ah2i(c) ;
		if (b == -1)
			fputs("\n?\n", stdout) ;
		else {
			fputc('\n', stdout) ;
			fputs(English[b], stdout) ;
			fputc('\n', stdout) ;
			fputs(Francais[b], stdout) ;
			fputc('\n', stdout) ;
		}
	}

}

The starting point

You've copied an LC-2 program into the file lab12.asm in your directory. This program contains definitions for the English and Francais data structures, named ENG and FRN for the LC-2, along with the code for the ah2i routine.

You are left with writing the code for the loop of the main routine. However, there is one problem. Note the slight, but significant, difference between the types of the English and Francais arrays:

• char *English[16]
• char Francais[16][9]

The C variable English is an array of sixteen pointers to strings. The C variable Francais is a sixteen by nine array of characters. This difference is reflected in the LC-2 variables ENG and FRN. ENG is the address of an array containing sixteen strings. The address of the English word for the number 5 can be found in the 5'th entry of ENG.

On the other hand, FRN is a two-dimensional array. It contains sixteen entries, but each entry is nine characters long. The address of the French word for the number 5 is 45 (or 5 times 9) memory locations from the beginning of FRN.

You must keep this difference in mind when you are generating the addresses sent to the PUTS trap routine to print the English and French words. By the way the easiest way to compute 9*X on the LC-2 is as X<<3+X.

Going home

Write and test lab12.asm and don't delete your file!