Computer Systems Fundamentals

yeet.c

A simple demonstration of goto in C. 

#include <stdio.h>

int main(void) {

    printf("In this essay I will\n");

    goto signature;

    printf("tell you why I deserve\n");
    printf("a 100 on this assignment.\n");
    printf("\n");

    printf("I have been working very\n");
    printf("hard on this assignment\n");
    printf("and I think I deserve a 100.\n");
    printf("\n");
    
    printf("I have been working on this\n");
    printf("assignment for a long time\n");
    printf("and I think I deserve a 100.\n");
    printf("\n");
    
    printf("It is my humble opinion\n");
    printf("that I deserve a 100 on\n");
    printf("this assignment.\n");
    printf("\n");

signature:
    printf("Kind regards,\n");
    printf("Abiram Nadarajah\n");
    printf("COMP1521 20T2 student\n");

    return 0; 
}
print_if_even.c

Print a message only if a number is even. 

#include <stdio.h>

int main(void) {
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);

    if (n % 2 == 0) {
        printf("even\n");
    }

    return 0;
}
print_if_even.simple.c

Print a message only if a number is even. 

#include <stdio.h>

int main(void) {
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);

    if (n % 2 != 0) goto epilogue;
        printf("even\n");

epilogue:
    return 0;
}
print_if_even.s

Print a message only if a number is even. 

	.text
main:
	# Locals:
	# - $t0: int n
	# - $t1: n % 2

	li	$v0, 4			# syscall 4: print_string
	la	$a0, prompt_msg		#
	syscall				# printf("Enter a number: ");

	li	$v0, 5			# syscall 5: read_int
	syscall				#
	move	$t0, $v0		# scanf("%d", &n);

	rem	$t1, $t0, 2		# if ((n % 2)
	bnez	$t1, epilogue	#     != 0) goto epilogue;

	li	$v0, 4			# syscall 4: print_string
	la	$a0, even_msg		#
	syscall				# printf("even\n");

epilogue:
	li	$v0, 0			#
	jr	$ra			# return 0;

	.data
prompt_msg:
	.asciiz	"Enter a number: "
even_msg:
	.asciiz	"even\n"
sum_100_squares.c

Calculate 1*1 + 2*2 + ... + 99*99 + 100*100 

#include <stdio.h>

int main(void) {
    int sum = 0;

    for (int i = 1; i <= 100; i++) {
        sum += i * i;
    }

    printf("%d\n", sum);

    return 0;
}
sum_100_squares.simple.c

Calculate 1*1 + 2*2 + ... + 99*99 + 100*100. 

#define UPPER_BOUND 100
#include <stdio.h>

int main(void) {
    int sum = 0;

loop_i_to_100__init:;
    int i = 0;
loop_i_to_100__cond:
    if (i > UPPER_BOUND) goto loop_i_to_100__end;
loop_i_to_100__body:
    sum += i * i;
loop_i_to_100__step:
    i++;
    goto loop_i_to_100__cond;
loop_i_to_100__end:

    printf("%d", sum);
    putchar('\n');

    return 0;
}
sum_100_squares.s

Calculate 1*1 + 2*2 + ... + 99*99 + 100*100 

UPPER_BOUND = 100


	.text
main:
	# Locals:
	# - $t0: int sum
	# - $t1: int i
	# - $t2: temporary value

	li	$t0, 0					# int sum = 0;

loop_i_to_100__init:
	li	$t1, 1					# int i = 0;
loop_i_to_100__cond:
	bgt	$t1, UPPER_BOUND, loop_i_to_100__end	# while (i < UPPER_BOUND) {
loop_i_to_100__body:
	mul	$t2, $t1, $t1				#   sum = (i * i) +
	add	$t0, $t0, $t2				#         sum;
loop_i_to_100__step:
	addi	$t0, $t0, 1				#   i++;
	b	loop_i_to_100__cond			# }

loop_i_to_100__end:
	li	$v0, 1					# syscall 1: print_int
	move	$a0, $t0				# 
	syscall						# printf("%d", sum);

	li	$v0, 11					# syscall 11: print_char
	li	$a0, '\n'				#
	syscall						# putchar('\n');

	li	$v0, 0
	jr	$ra					# return 0;
count_to_10.c

Count from 1 to 10 with a loop. 

#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 10; i++) {
        printf("%d\n", i);
    }
    return 0;
}
count_to_10.simple.c

Count from 1 to 10 with a loop. 

#include <stdio.h>

int main(void) {

loop_i_to_10__init:;
    int i = 1;
loop_i_to_10__cond:
    if (i > 10) goto loop_i_to_10__end;

loop_i_to_10__body:
    printf("%d", i);
    putchar('\n');
loop_i_to_10__step:
    i++;                        // i = i + 1;
    goto loop_i_to_10__cond;
   
loop_i_to_10__end:
    return 0;
}
count_to_10.s

Count from 1 to 10 with a loop. 

	.text
main:
	# Locals:
	# - $t0: int i

loop_i_to_10__init:
	li	$t0, 1				# int i = 1;
loop_i_to_10__cond:
	bgt	$t0, 10, loop_i_to_10__end	# if (i > 10) goto loop_i_to_10__end;

loop_i_to_10__body:
	li	$v0, 1				# syscall 1: print_int
	move	$a0, $t0			#
	syscall					# printf("%d", i);

	li	$v0, 11				# syscall 11: print_char
	li	$a0, '\n'			#
	syscall					# putchar('\n');

loop_i_to_10__step:
	addi	$t0, $t0, 1			# i = i + 1;
	b	loop_i_to_10__cond

loop_i_to_10__end:
	li	$v0, 0
	jr	$ra				# return 0;
six.c

Print a message only if a number is divisible by 2 and 3. 

#include <stdio.h>

int main(void) {
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);

    if (n % 2 == 0 && n % 3 == 0) {
        printf("six\n");
    }

    return 0;
}
six.simple.c

Print a message only if a number is divisible by 2 and 3. 


#include <stdio.h>

int main(void) {
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);

    if (n % 2 != 0) goto epilogue;
    if (n % 3 != 0) goto epilogue;
        printf("six-ish\n");

epilogue:
    return 0;
}
six.s

Print a message only if a number is divisible by 2 and 3. 

	.text
main:
	# Locals:
	# - $t0: int n
	# - $t1: n % 2
	# - $t2: n % 3

	li	$v0, 4			# syscall 4: print_string
	la	$a0, prompt_msg		#
	syscall				# printf("Enter a number: ");

	li	$v0, 5			# syscall 5: read_int
	syscall				#
	move	$t0, $v0		# scanf("%d", &n);

	rem	$t1, $t0, 2		# if ((n % 2)
	bnez	$t1, epilogue	        #     != 0) goto epilogue;

	rem	$t2, $t0, 3		# if ((n % 3)
	bnez	$t2, epilogue	        #     != 0) goto epilogue;

	li	$v0, 4			# syscall 4: print_string
	la	$a0, six_msg	        #
	syscall				# printf("six-ish\n");

epilogue:
	li	$v0, 0			#
	jr	$ra			# return 0;

	.data
prompt_msg:
	.asciiz	"Enter a number: "
six_msg:
	.asciiz	"six-ish\n"
two_three.c

Print a message only if a number is divisible by 2 or 3. 


#include <stdio.h>

int main(void) {
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);

    if (n % 2 == 0 || n % 3 == 0) {
        printf("two-three-ish\n");
    }

    return 0;
}
two_three.simple.c

Print a message only if a number is divisible by 2 or 3. 


#include <stdio.h>

int main(void) {
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);

    if (n % 2 == 0) goto two_three_print;
    if (n % 3 == 0) goto two_three_print;
    goto epilogue;
two_three_print:
        printf("two-three-ish\n");

epilogue:
    return 0;
}
two_three.s

Print a message only if a number is divisible by 2 or 3. 

	.text
main:
	# Locals:
	# - $t0: int n
	# - $t1: n % 2
	# - $t2: n % 3

	li	$v0, 4			# syscall 4: print_string
	la	$a0, prompt_msg		#
	syscall				# printf("Enter a number: ");

	li	$v0, 5			# syscall 5: read_int
	syscall				#
	move	$t0, $v0		# scanf("%d", &n);

	rem	$t1, $t0, 2		# if ((n % 2)
	beqz	$t1, two_three_print	#     == 0) goto two_three_print;

	rem	$t2, $t0, 3		# if ((n % 3)
	beqz	$t2, two_three_print	#     == 0) goto two_three_print;

	b epilogue		        # goto epilogue;
	
two_three_print:
	li	$v0, 4			# syscall 4: print_string
	la	$a0, two_three_msg	#
	syscall				# printf("two-three-ish\n");

epilogue:
	li	$v0, 0			#
	jr	$ra			# return 0;

	.data
prompt_msg:
	.asciiz	"Enter a number: "
two_three_msg:
	.asciiz	"two-three-ish\n"
forever_23.c

Example of break/continue use 


#include <stdio.h>

int main(void) {
    for (int n = 0; n < 100; n++) {
        if (n % 3 == 0) {
            continue;
        }
        if (n % 23 == 0) {
            break;
        }

        printf("%d\n", n);
    }

    return 0;
}
forever_23.simple.c

Example of break/continue use 


#include <stdio.h>

int main(void) {
    int n;

    n = 0;
forever_23_loop_top:
    if (n > 100) goto forever_23_loop_end;

        if (n % 3 == 0) goto forever_23_loop_next;

        if (n % 23 == 0) goto forever_23_loop_end;

        printf("%d", n);
        putchar('\n');

forever_23_loop_next:
    n = n + 1;
    goto forever_23_loop_top;

forever_23_loop_end:

    return 0;
}
forever_23.s

Example of break/continue use 

	.text
main:
	# Locals:
	# - $t0: int n
	# - $t1: n % 2
	# - $t2: n % 23

forever_23_loop_init:
	li	$t0, 0				# int n = 0;


forever_23_loop_top:

	rem	$t2, $t0, 3		        # if ((n % 3)
	beqz	$t2, forever_23_loop_next	#     == 0) goto forever_23_loop_next;

	rem	$t1, $t0, 23		        # if ((n % 23)
	beqz	$t1, forever_23_loop_end	#     == 0) goto forever_23_loop_end;

	li	$v0, 1				# syscall 1: print_int
	move	$a0, $t0			# 
	syscall					# printf("%d", n);

	li	$v0, 11				# syscall 11: print_char
	li	$a0, '\n'			#
	syscall					# putchar('\n');

forever_23_loop_next:
	addi	$t0, $t0, 1			#   n++;
        b forever_23_loop_top;	                # goto forever_23_loop_top;
    
forever_23_loop_end:

epilogue:
	li	$v0, 0			        #
	jr	$ra			        # return 0;
do_while.c

Do while example 
#include <stdio.h>

int main(void) {
    int val;
    do {
        printf("val? ");
        scanf("%d", &val);
        printf("%d",val);
        printf("\n");
    } while (val > 0);
}
do_while.s
read/write characters until the user types a '!'
val is represented by $t0 
    .text
main:

loop_start:		# do {

    la   $a0, prompt	#   printf("val? ");
    li   $v0, 4
    syscall

    li   $v0, 5		#   scanf("%d",&val);
    syscall
    move $t0,$v0

    move $a0, $t0	#    printf("%d",val);
    li   $v0, 1
    syscall

    li   $a0, '\n'	#    printf("\n");
    li   $v0, 11
    syscall

    blt  $t0,1,loop_end # } while (val > 0);
    b    loop_start
loop_end:

    li   $v0, 0		# return 0
    jr   $ra

# read 10 numbers into an array then print the 10 numbers

    .data

prompt:
    .asciiz "val? "