Computing Machinery I Assignment 5 Solution



Part A: Global Variables and Separate Compilation

Reverse Polish (or postfix) notation can be used to represent arithmetic expressions. For example, the infix expression

(1 – 2) * (4 + 5) =

is the following using reverse Polish notation:

1 2 – 4 5 + * =

Hewlett-Packard calculators often use this form of entry. The following is a C program that emulates such a calculator:

#include <stdio.h>

#include <stdlib.h>

/* Constants */

#define MAXOP 20

#define NUMBER ‘0’

#define TOOBIG ‘9’

/* Function prototypes */

int push(int f);

int pop();

void clear();

int getop(char *s, int lim);

int getch();

void ungetch(int c);

int main()


int type;

char s[MAXOP];

int op2;

while ((type = getop(s, MAXOP)) != EOF) {

switch (type) {

case NUMBER:



case ‘+’:

push(pop() + pop());


case ‘*’:

push(pop() * pop());


case ‘-‘:

op2 = pop();

push(pop() – op2);


case ‘/’:

op2 = pop();

if (op2 != 0)

push(pop() / op2);


printf(“zero divisor popped\n”);


case ‘=’:

printf(“\n%d\n\n”, push(pop()));


case ‘c’:



case TOOBIG:

printf(“%.20s … is too long\n”, s);



printf(“unknown command %c\n”, type);




return 0;


#define MAXVAL 100

int sp = 0;

int val[MAXVAL];

int push(int f)


if (sp < MAXVAL)

return val[sp++] = f;

else {

printf(“error: stack full\n”);


return 0;



int pop()


if (sp > 0)

return val[–sp];

else {

printf(“error: stack empty\n”);


return 0;



void clear()


sp = 0;


int getop(char *s, int lim)


int i, c;

while ((c = getch()) == ‘ ‘ || c == ‘\t’ || c == ‘\n’)


if (c < ‘0’ || c > ‘9’)

return c;

s[0] = c;

for (i = 1; (c = getchar()) >= ‘0’ && c <= ‘9’; i++)

if (i < lim)

s[i] = c;

if (i < lim) {


s[i] = ‘\0’;

return NUMBER;

} else {

while (c != ‘\n’ && c != EOF)

c = getchar();

s[lim-1] = ‘\0’;

return TOOBIG;



#define BUFSIZE 100

char buf[BUFSIZE];

int bufp = 0;

int getch()


return bufp > 0 ? buf[–bufp] : getchar();


void ungetch(int c)


if (bufp > BUFSIZE)

printf(“ungetch: too many characters\n”);


buf[bufp++] = c;


Translate all functions except main() into ARMv8 assembly language, and put them into a separate assembly source code file called a5a.asm. These functions will be called from the main() function given above, which will be in its own C source code file called a5aMain.c. Also move the global variables into a5a.asm. Your assembly functions will call the library routines printf() and getchar(). Be sure to handle the global variables and format strings in the appropriate way. Input will come from standard input; the program is terminated by typing control-d. Run the program to show that it is working as expected, capturing its output using the script UNIX command, and name the output file script1.txt. Use a variety of input expressions to show that your program is calculating correctly.

Part B: External Pointer Arrays and Command-Line Arguments

Given the following declaration in C:

char *month[] = {“January”, “February”, “March”, “April”, “May”,

“June”, “July”, “August”, “September”, “October”,

“November”, “December”};

create an ARMv8 assembly language program to accept as command line arguments three strings representing a date in the format mm dd yyyy. Your program will print the date with the name of month as well as the correct suffix. For example:

./a5b 9 11 1990

September 11th, 1990

Be sure to use the proper suffix for the day of the month. For example, one should distinguish the 11th from the 1st, 21st, and 31st. Don’t forget the comma after the day. Your program should exit, printing this error message, if the user does not supply three command-line arguments:

usage: a5b mm dd yyyy

You will need to call atoi() to convert strings to numbers, and printf() to produce the output. Be sure to do range checking for the day, month, and year. Name your source code file a5b.asm. Run your program three times with different input to illustrate that it works; capture the output using the script UNIX command. Name the output file script2.txt.

New Skills need for this Assignment:

  • Understanding and use of external variables in assembly

  • Separate compilation

  • Calling assembly functions from main()

  • Calling library functions from assembly routines

  • External arrays of pointers

  • Command line arguments

Submit the following:

  1. Your source code and 2 scripts via electronic submission. Use the Assignment 5 Dropbox Folder in D2L to submit electronically. Your TA will assemble and run your programs to test them. Name your files a5aMain.c and a5a.asm for Part A, and a5b.asm for Part B, and the scripts as script1.txt and script2.txt.

Computing Machinery I

Assignment 5 Grading


Part A:

Correct use of external variable(s) 4 ______

push() function in assembly 4 ______

pop() function in assembly 4 ______

clear() function in assembly 2 ______

getop() function in assembly 6 ______

getch() function in assembly 3 ______

ungetch() function in assembly 3 ______

Linking of separate source code modules 2 ______

Correct evaluation of expressions 4 ______

Part B:

Command line arguments 4 ______

Correct use of external pointer arrays 4 ______

Calls to library functions 2 ______

Range checking 3 ______

Correct output 4 ______

2 Scripts showing I/O 4 ______

Complete documentation and commenting 4 ______

Formatting (use of columns and white space) 4 ______

Design quality 4 ______

Total 65 ______ _____%