Using the student_input.s ARM code provided in the Files --> ARM Programs on Can

Question

Using the student_input.s ARM code provided in the Files --> ARM Programs on Canvas and discussed in class expand the code to do all the following:

1. Prompt the user for an input number (integer). [Already in program]

2. Print the input number back to the screen. [Already in program]

3. Prompt the user for an input character. [New function]. Use "%s" instead of "%c" for the input. This will keep from reading the CR from the previous input.

4. Print the input character back to the screen. [New function]. Use "%c" for the output.

5. Return control back to the operating system. [Already in program]

student_input.s File as follows:

@ Filename: student_input.s

@ Author:
@ Purpose: show how to read user inputs from KB.

@ Use these command to assemble, link and run this program

@ as -o student_input.o student_input.s

@ gcc -o student_input student_input.o

@ ./student_input ;echo $?
@ Have to use C compiler to link because of C library uses.

.global
main @ Have to use main because of C library uses.

main:

prompt:

@ Ask the user to enter a number.

ldr r0, =strInputPrompt @ Put the address of my string into the first parameter

bl printf @ Call the C print to display input prompt.

get_input:

@ Set up r0 with the address of input pattern

ldr r0, =numInputPattern @ Setup to read in one number.

@ scanf puts the input value at the address stored in r1. We are going

@ to use the stack (sp) for that address. But first we have to push the

@ stack to get a new pointer to an unused location. After that call scanf.

@ The value read from the input is stored at the address pointed to by

@ sp. Load the value into r1 and pop the stack to what is was before.

sub sp, sp, #4 @ Update stack pointer to new loc.
mov r1, sp @ Put address into r1 for read.

bl scanf @ scan the keyboard.

ldr r1, [sp, #0] @ The character is on the stack.

add sp, sp, #4 @ Reset the stack.

@ print the input out as a number.

printinputasNum:

@ r1 is already loaded with the number to print.

ldr r0, =strOutputNum

bl printf

@End of my code. Force the exit and return control to OS

myexit:

mov r7, #0x01 @SVC call to exit

svc 0 @Make the system call.

.data

@ Declare the strings and data needed

.balign 4

strInputPrompt: .asciz "Input the number: "

.balign 4

strOutputNum: .asciz "The number value is: %d "

@ Format pattern for scanf call.
.balign 4

numInputPattern: .asciz "%d"

@ Let the assembler know these are the C library functions.

.global printf

.global scanf

@end of code.

Explanation / Answer

.global
main @ Have to use main because of C library uses.

main:

prompt:

@ Ask the user to enter a number.

ldr r0, =strInputPrompt @ Put the address of my string into the first parameter

bl printf @ Call the C print to display input prompt.

get_input:

@ Set up r0 with the address of input pattern

ldr r0, =numInputPattern @ Setup to read in one number.

@ print the input out as a number.

printinputasNum:

@ r1 is already loaded with the number to print.

ldr r0, =strOutputNum

bl printf

@End of my code. Force the exit and return control to OS

myexit:

mov r7, #0x01 @SVC call to exit

svc 0 @Make the system call.

.data

@ Declare the strings and data needed

.balign 4

strInputPrompt: .asciz "Input the number: "

.balign 4

strOutputNum: .asciz "The number value is: %d "

@ Format pattern for scanf call.
.balign 4

numInputPattern: .asciz "%d"

@ Let the assembler know these are the C library functions.

.global printf

.global scanf

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