问题
I'm trying to solve a problem to write assembly language program to detect if a phrase or characters entered by the user is a palindrome.
I've gotten this far, and I believe everything should work, but I'm wondering how I can implement this so that it takes an actual word to test. When I run in MARS, there's simply no input option.
.data
string_space: .space 1024
is_palin_msg: .asciiz "The string is a palindrome.\n"
not_palin_msg: .asciiz "The string is not a palindrome.\n"
.text
main:
la $a0, string_space
li $a1, 1024
li $v0, 8
syscall
la $t1, string_space
la $t2, string_space
length_loop:
lb $t3, ($t2)
beqz $t3, end_length_loop
addu $t2, $t2, 1
b length_loop
end_length_loop:
subu $t2, $t2, 2
test_loop:
bge $t1, $t2, is_palin
lb $t3, ($t1)
lb $t4, ($t2)
bne $t3, $t4, not_palin
addu $t1, $t1, 1
subu $t2, $t2, 1
b test_loop
is_palin:
la $a0, is_palin_msg
li $v0, 4
syscall
b exit
not_palin:
la $a0, not_palin_msg
li $v0, 4
syscall
b exit
exit:
li $v0, 10
syscall
I've tried
string_space: .asciiz "Enter your word:\n"
and also
string_space: .asciiz "racecar"
but I haven't quite been able to get it yet.
Any tips?
回答1:
Ciao,
so - taking also as template this question here [you are strongly invited to check similar questions before posting] - you need to introduce in your code a .data section with an input string to ask the user to enter the string to check
.data
string_space: .space 1024
input: .asciiz "Enter a string: "
is_palin_msg: .asciiz "The string is a palindrome.\n"
not_palin_msg: .asciiz "The string is not a palindrome.\n"
# other strings you may need
Then you can start setting the logic to push them out
.text
main:
li $v0, 4 # system call code for print_str
la $a0, input # address of string to print
syscall # print the input
li $v0, 8 # code for syscall read_string
la $a0, string_space # tell syscall where the buffer is
li $a1, 1024 # tell syscall how big the buffer is
syscall
# double check the buffer content [see the next snippet]
# rest of the code to test
You will start asking the user a string [in this case limited to 1024 bytes/characters]. Reading the "System Calls and I/O" section of this link you will find the same prompt used in the snippet above [search "Print out string (useful for prompts)" within the page]
The table in the same section will explain you the meaning of the li $v0, 4 and li $v0, 8 instructions. This is another good read. The same table will make you understand that before calling a print string you have to set one argument [$a0] while for the read string operation you will need two [$a0 and $a1]
In your code the main begins with a read string operation. But please notice that string_space is used both to allocate the size of the buffer where to read from [in .data section] and for asking to enter the word [you are trying to call string_space: .asciiz "Enter your word:\n"]. This issue has been fixed by the snippets above
In case of troubles don't forget to double check the content of your string_space buffer:
la $a0, string_space # move buffer into a0
li $v0, 4 # print buffer
syscall
Complete code tested and working in MARS 4.5:
.data
string_space: .space 1024
input: .asciiz "Enter a string: "
is_palin_msg: .asciiz "The string is a palindrome.\n"
not_palin_msg: .asciiz "The string is not a palindrome.\n"
.text
main:
li $v0, 4 # system call code for print_str
la $a0, input # address of string to print
syscall # print the input
la $a0, string_space
li $a1, 1024
li $v0, 8
syscall
#la $a0, string_space # move buffer into a0
#li $v0, 4 # print buffer
#syscall
la $t1, string_space
la $t2, string_space
length_loop:
lb $t3, ($t2)
beqz $t3, end_length_loop
addu $t2, $t2, 1
b length_loop
end_length_loop:
subu $t2, $t2, 2
test_loop:
bge $t1, $t2, is_palin
lb $t3, ($t1)
lb $t4, ($t2)
bne $t3, $t4, not_palin
addu $t1, $t1, 1
subu $t2, $t2, 1
b test_loop
is_palin:
la $a0, is_palin_msg
li $v0, 4
syscall
b exit
not_palin:
la $a0, not_palin_msg
li $v0, 4
syscall
b exit
exit:
li $v0, 10
syscall
IMPORTANT: this code logic has a bug. If you enter palindromes whose length is odd it will give the wrong result [e.g. civic is detected as a non-palindrome but actually it is]
来源：https://stackoverflow.com/questions/53678889/how-to-pass-an-input-string-from-user-to-a-mips-program