How do I validate a user input in MIPS instruction set using MARS? In my following program I want to know how I can add validation to user input for example when asking the user if they want to decrypt or encrypt a message they need to input either 'E' or 'D' (case sensitive due to ascii values). As well as an input from (1-9) which is the encryption or decryption key. If they enter anything other than 'E', 'D' and a number from 1-9 how do I ask the user to enter a valid input using a subroutine for each prompt?
Code:
.data
encrypt_decrypt: .asciiz "Type(E)ncrypt/(D)ecrypt:"
key: .asciiz "\nEnter encryption key (1-9): "
message_prompt: .asciiz "\nEnter message: "
output: .asciiz ""
re_enter: .asciiz "\nWrong input"
buffer: .space 50
.text
main:
# Prints the prompt asking the user if they want to encrypt or decrypt
li $v0, 4
la $a0, encrypt_decrypt # Loads address from the user prompt
syscall
# Takes in E OR D based on user choice
li $v0, 12
syscall
add $t6, $v0, $0
bne $t6, 69, ELSE # Branch to else if inputed value is not E
# Prints the prompt asking the user for a message to either decrypt or
encrypt based on the users previous choice
li $v0, 4
la $a0, message_prompt # Loads address from the users message
syscall
# Takes the users message
li $v0, 8
la $a0, buffer
lb $t0, 0($a0)
li $a1, 48
syscall
la $s2, buffer
# Asks the user for a key from 1-9
li $v0, 4
la $a0, key
syscall
# Takes the users key value from 1-9
li $v0, 5
syscall
add $s0, $v0, $0
la $t0, ($s0) # Loads key
li $t1, 0 # Lower order bit to be toggled
la $t2, ($s2) # Load user message
beq $t6, 69, encrypt # 69 Is the printable ASCII value of "E"
beq $t6, 68, decrypt # 68 is the printable ASCII value of "D"
ELSE:
bne $t6, 68, E_D_VALIDATION # Branch to E_D_VALIDATION if user input is not D
E_D_VALIDATION:
li $v0, 4
la $a0, re_enter
syscall
j main
encrypt:
encrypt_loop:
lb $t3, 0($t2) # Loads character bytes
beq $t3, 10, encrypted # Branches to "encrypted" if we reach the end of a character
add $t3, $t3, $t0
addi $t4, $0, 1
sllv $t4, $t4, $t1
xor $t3, $t3, $t4 # Lower order bit toggle
sb $t3, 0($t2) # Stored encrypted character
addi $t2, $t2, 1
j encrypt_loop # Loops through the list of encrypted characters
encrypted:
li $v0, 4 # Print out encrypted message
la $a0, output
syscall
la $a0, ($s2)
syscall
j exit # Exits the program
decrypt:
decrypted_loop:
lb $t3, 0($t2) # Loads character bytes
beq $t3, 10, decrypted # Branches to "decrypted" if we reach the end of a character
addi $t4, $0, 1
sllv $t4, $t4, $t1
xor $t3, $t3, $t4
sub $t3, $t3, $t0
sb $t3, 0($t2) # Stored decrypted character
addi $t2, $t2, 1
j decrypted_loop # Loops through the list of decrypted characters
decrypted:
li $v0, 4 # Print out decrypted message
la $a0, output
syscall
la $a0, ($s2)
syscall
j exit # Exit the program
exit:
