Homework SourseI have this MIPS Assembly program that checks for palindrome
I have this MIPS Assembly program that checks for palindrome but I dont really understand the steps. If someone could comment some of the code and help me understand that would be greatly appreciated.
Code
.text
main:# SPIM starts by jumping to main.
## read the string S:
la $a0, startPrompt #prompt user to enter a phrase
li $v0, 4
syscall
la $a0, string_space #store phrase
li $a1, 1024 #size of string
li $v0, 8 # load code into $v0.
syscall
la $t1, string_space # A = S.
la $t4, string_space ## we need to move B to the end
length_loop:
lb $t5, ($a0)
beq $t5, 10, checkPali #checking for palindrome/checking if equal
bgt $t5, 47, t1
b negative
t1: blt $t5, 58, positive #ascii
bgt $t5, 64, t2
b negative
t2: blt $t5, 91, positive # less then?
bgt $t5, 96, t3 #greater then? //ascii
b negative
t3: blt $t7, 123, positive #ascii
b negative
positive: #input x is what we store the sanitized string in. Char by Char
bgt $t5, 96, makeLow #ascii
b notUp
makeLow: addi $t5, $t5, +32 #dealing with case sensitivity.
notUp:
sb $t5, ($t1)
addi $a0, $a0, 1 #increment
addi $t1, $t1, 1
b length_loop
negative:
addi $a0, $a0, 1 #increment user input
b length_loop
checkPali:
#t1 = back
#li $a3, 127
sb $zero, ($a0)
addi $t1, $t1, -1 #decrement
loop:
lb $t3, ($t4)
lb $t2, ($t1)
beq $t3, $t2, next
b not_palin
next: jal test_loop #jump and link to test loop.
addi $t4, $t4, 1 # incrememnt
addi $t1, $t1, -1 # decrememnt
b loop # continue to loop
b not_palin
test_loop:
beq $t4, $t1, is_palin
addi $t1, $t1, -1
beq $t4, $t1, is_palin #test to see if we need to check more characters
addi $t1, $t1, 1 #when at the middle, address\'s are going to be either equal
jr $ra #or a == (b-1)
is_palin: ## print the is_palin_msg, and exit.
la $a0, string_space
li $v0, 4
la $a0 is_palin_msg
syscall
b exit
not_palin:
la $a0, not_palin_msg ## print the is_palin_msg, and exit.
li $v0, 4
syscall
b exit
exit: ## exit the program:
li $v0, 10 # load \"exit\" into $v0.
syscall # make the system call.
## Here’s where the data for this program is stored
.data
string_space: .space 1024 # reserve 1024 bytes for the string.
is_palin_msg: .asciiz \"The string is a palindrome.\ \"
not_palin_msg: .asciiz \"The string is not a palindrome.\ \"
startPrompt: .asciiz \"Enter a string to see if it is a palindrome: \"
## end of palindrome.asm
Solution
## In the code following registers, we are assuming.
## $t1 – Register for X.
## $t2 – Register for Y.
## $t3 - the character at address X.
## $t4 - the character at address Y.
## $v0 - syscall parameter / return values.
## $a0 - syscall parameters.
## $a1 - syscall parameters.
.text
main: # SPIM starts by jumping to main.
# read the string S:
la $a0, startPrompt #prompt user to enter string to check for palindrome
li $v0, 4 #print string since 4 is the syscall code for printing strings
syscall
la $a0, string_space #store the user input string at the specified location
li $a1, 1024 #define the maximum size of string to be 1024 bytes
li $v0, 8 #read the user input as 8 is the syscall code for reading string
syscall
la $t1, string_space #load the address of string_space into the register, $t1.
# i.e. X = S
la $t4, string_space ## we need to move Y to the end of string
length_loop:
lb $t5, ($a0) # load the byte at address, $a0, into the register, $t5
beq $t5, 10, checkPali #invoke the checkPali process for each character till end of string.
# 10 is the syscall for exit, here it will be the end of the string
bgt $t5, 47, t1 `#check if the $t5 > 47 then move to t1
b negative
t1: blt $t5, 58, positive
bgt $t5, 64, t2
b negative
t2: blt $t5, 91, positive # less than 91 go to positive
bgt $t5, 96, t3 #greater than 96 go to t3
b negative
t3: blt $t7, 123, positive
b negative
positive: #input x is what we store the sanitized string in. Char by Char
bgt $t5, 96, makeLow
b notUp
makeLow: addi $t5, $t5, +32 #dealing with case sensitivity.
notUp:
sb $t5, ($t1)
addi $a0, $a0, 1 #increment each memory location
addi $t1, $t1, 1
b length_loop
negative: #do not add char to new sanitized string
addi $a0, $a0, 1 #increment user input
b length_loop
checkPali:
#t1 = back
#li $a3, 127
sb $zero, ($a0)
addi $t1, $t1, -1 # #decrement from end of $t1, increment from beginning of $t4
loop: #loop to check whether the strings are same just the strings start from opposite ends
lb $t3, ($t4)
lb $t2, ($t1)
beq $t3, $t2, next #continue the check if bytes are equal
b not_palin
next: jal test_loop #jump and link to test loop. Ensure we are not at last byte
addi $t4, $t4, 1 # move t4 closer to middle
addi $t1, $t1, -1 # move t1 closer to middle but from reverse side
b loop # continue to loop
b not_palin
test_loop:
beq $t4, $t1, is_palin
addi $t1, $t1, -1
beq $t4, $t1, is_palin #test to see if we need to check more characters
addi $t1, $t1, 1 #when at the middle, address\'s are going to be either equal
jr $ra #or a == (b-1)
is_palin: ## print the is_palin_msg, and exit.
la $a0, string_space
li $v0, 4
la $a0 is_palin_msg
syscall
b exit
not_palin:
la $a0, not_palin_msg ## print the is_palin_msg, and exit.
li $v0, 4
syscall
b exit
exit: ## exit the program:
li $v0, 10 # load \"exit\" into $v0.
syscall # make the system call.
## Here’s where the data for this program is stored
.data
string_space: .space 1024 # reserve 1024 bytes for the string.
is_palin_msg: .asciiz \"The string is a palindrome.\ \"
not_palin_msg: .asciiz \"The string is not a palindrome.\ \"
startPrompt: .asciiz \"Enter a string to see if it is a palindrome: \"
## end of palindrome.asm
