I am creating an ARM Assembly Language framework to implement a SELECT/CASE construct.
The format is:
SELECT number
CASE 1:
<< statements if number is 1 >>
CASE 2:
<< statements if number is 2 >>
CASE ELSE:
<< statement if not any other case >>
This is what I have tried but the program does not output the messages based on the entered number:
// Declare program entry point
.global _start
// Messages to be displayed
.data
prompt: .asciz "Input a number: " // Prompt message
case1_msg: .asciz "You entered 1\n" // Case 1 message
case2_msg: .asciz "You entered 2\n" // Case 2 message
else_msg: .asciz "You entered a value other than 1 and 2\n" // Else message
.text
_start:
// Display prompt and get user input
mov x0, #0 // Set output to stdout
adr x1, prompt // Load prompt address
mov x2, #17 // Set prompt length
mov x8, #64 // Call Linux write function
svc #0 // Execute system call
mov x0, #0 // Set input to stdin
mov x8, #63 // Call Linux read function
svc #0 // Execute system call
// Check user input against cases
cmp w0, #1 // Compare input with case 1
beq case1 // Branch to case 1 if equal
cmp w0, #2 // Compare input with case 2
beq case2 // Branch to case 2 if equal
b else_case // Branch to else case if not equal to either case
case1:
// Perform actions for case 1
adr x0, case1_msg // Load case 1 message address
mov x8, #64 // Call Linux write function
svc #0 // Execute system call
b end_select // Branch to end of case1
case2:
// Perform actions for case 2
adr x0, case2_msg // Load case 2 message address
mov x8, #64 // Call Linux write function
svc #0 // Execute system call
b end_select // Branch to end of case2
else_case:
// Perform actions for else case
adr x0, else_msg // Load else message address
mov x8, #64 // Call Linux write function
svc #0 // Execute system call
end_select:
// Terminate program
mov x0, #0 // Set return code to 0
mov x8, #93 // Call Linux exit function
svc #0 // Execute system call
I managed to solve the problem. The issue was that I was comparing an integer with a character. I converted the character into an integer and the compared and everything worked as expected. I appreciate everyone who contributed to help me figure out what was the problem. Below is the code I wrote.
// Assembler program to implement a SELECT/CASE construct
.global _start // Provide program starting address
_start:
//Display a message and prompt for input
mov X0, #0
ldr X1, =prompt // Load address of prompt message
mov X2, #74 // length of the prompt message
mov X8, #64 // call Linux
svc 0 //display the message
//Reading the user input
mov X0, #0
ldr X1, =input // Load address of input buffer
mov X2, #10 // input size
mov X8, #63 // system call
svc 0 // read input from keyboard
// From ASCII to integer conversion
ldrb W0, [X1] // load the character
sub W0, W0, #'0'// convert to integer
cmp W0, #1 // check if the value is 1
b.eq print_one // if true print message for 1
cmp W0, #2 // check if it is 2
b.eq print_two // print message for two
// This will execute if the input is not 1 or 2
ldr X1, =unknown// "unknown" message string
mov X2, #49 // length of the string
mov X0, #1 // 1 = StdOut
mov X8, #64 // write system call
svc 0 // system call to print
b exit // move to exit
//this option will be executed if the user enters 1
print_one:
//print "one" message
ldr X1, =one // message string
mov X2, #29 // length of the string
mov X8, #64 // write system call
svc 0 // call to output message
b exit // move to exit
print_two:
// Print "two" message
ldr X1, =two // message string
mov X2, #29 // lenth of the string
mov X8, #64 // write system call
svc 0 // call to output message
b exit // move to exit
exit:
// Exiting program
mov X0, #0 // Use 0 return code
mov X8, #93 // Service code 93 terminates
svc 0 // Call Linux to terminate
.data
//different messages to be displayed
prompt: .ascii "Implement a Select/Case structure:\n1. Case 1 actions\n2. Case 2 actions\n"
input: .skip 10 // Input buffer for read system call
one: .ascii "case 1 actions will be taken\n"
two: .ascii "case 2 actions will be taken\n"
unknown: .ascii "input does not match with any case. Else actions\n"
//end of the program