Question

On a raspberry pi, write an assembler program that will calculate the factorial of an integer value inputted by the user. the program should detect if an overflow occurs. If no overflow occurred, then it should print out the value of the factorial. Otherwise print out a message indicating that an overflow occurred.

Answer 1

I am providing an assembler program that will calculate the factorial of an integer value inputted by the user on a raspberry pi

CODE:

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/* -- factorial01.s */ .data message1: .asciz "Type a number: " format: .asciz "%d" message2: .asciz "The factorial of %d is %d\n" .text factorial: str lr, [sp,#-4]! /* Push lr onto the top of the stack */ str r0, [sp,#-4]! /* Push r0 onto the top of the stack */ /* Note that after that, sp is 8 byte aligned */ cmp r0, #0 /* compare r0 and 0 */ bne is_nonzero /* if r0 != 0 then branch */ mov r0, #1 /* r0 ← 1. This is the return */ b end is_nonzero: /* Prepare the call to factorial(n-1) */ sub r0, r0, #1 /* r0 ← r0 - 1 */ bl factorial /* After the call r0 contains factorial(n-1) */ /* Load r0 (that we kept in th stack) into r1 */ ldr r1, [sp] /* r1 ← *sp */ mul r0, r0, r1 /* r0 ← r0 * r1 */ end: add sp, sp, #+4 /* Discard the r0 we kept in the stack */ ldr lr, [sp], #+4 /* Pop the top of the stack and put it in lr */ bx lr /* Leave factorial */ .global main main: str lr, [sp,#-4]! /* Push lr onto the top of the stack */ sub sp, sp, #4 /* Make room for one 4 byte integer in the stack */ /* In these 4 bytes we will keep the number */ /* entered by the user */ /* Note that after that the stack is 8-byte aligned */ ldr r0, address_of_message1 /* Set &message1 as the first parameter of printf */ bl printf /* Call printf */ ldr r0, address_of_format /* Set &format as the first parameter of scanf */ mov r1, sp /* Set the top of the stack as the second parameter */ /* of scanf */ bl scanf /* Call scanf */ ldr r0, [sp] /* Load the integer read by scanf into r0 */ /* So we set it as the first parameter of factorial */ bl factorial /* Call factorial */ mov r2, r0 /* Get the result of factorial and move it to r2 */ /* So we set it as the third parameter of printf */ ldr r1, [sp] /* Load the integer read by scanf into r1 */ /* So we set it as the second parameter of printf */ ldr r0, address_of_message2 /* Set &message2 as the first parameter of printf */ bl printf /* Call printf */ add sp, sp, #+4 /* Discard the integer read by scanf */ ldr lr, [sp], #+4 /* Pop the top of the stack and put it in lr */ bx lr /* Leave main */ address_of_message1: .word message1 address_of_message2: .word message2 address_of_format: .word format

The other option could be:

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(1) MOV AL, 01 MOV BL, X LOOK: MUL BL DEC BL JNZ LOOK MOV FACT, AL END (2) MOV AL, 01 MOV BL, X LOOK: JZ NEXT MUL BL JNZ LOOK NEXT: MOV FACT, AL END (3) MOV AL, 01 MOV CL, X LOOK: JZ NEXT MUL CL LOOP LOOK NEXT: MOV FACT, AL END

Note: The code which I have provided is only for the calculation of the factorial of the given number by the user.

For checking the condition of overflow you can put conditional statement using loop .

For example:

/* for (int i = 100 ; i >= 0; i--) */ mov r1, #100 loop: /* do something */ sub r1, r1, #1 /* r1 ← r1 - 1 */ cmp r1, #0 /* update cpsr with r1 - 0 */ bge loop /* branch if r1 >= 100 */