MIPS Assembly chastelib

I have been learning the Assembly language of MIPS recently, just for fun. I have been learning from this book:

I learned enough to translate my Intel functions into the language of MIPS. My code runs in both the spim and mars simulators.

Also, this reference was helpful for understanding the division instructions which were not covered by the book written by Robert Winkler:

I don’t expect to have much use for these functions, but theoretically, I could write nearly anything with them if I put enough time with them. Most of the time I will stick with the Intel Assembly because it runs natively on my computer. I am very glad these simulators exist and were so easy to use. Unfortunately I haven’t found the equivalent simulators I was hoping to for 6502.

# This is the source of the MIPS version of chastelib
# The four basic functions have been translated from Intel x86 Assembly
# They are as follows
#
# putstring (prints string pointed to by $s0 register)
# intstr (converts integer in $s0 register to a string)
# putint (prints integer in $s0 register by use of the previous two functions)
# strint (converts a string pointed to by $s0 register into an integer in $s0)
#
# Most importantly, the intstr and strint functions depend on a global variable (radix)
# In fact, these two functions are the foundation of everything.
# They can convert to and from any radix from 2 to 36

.data
title: .asciiz "A test of Chastity's integer and string conversion functions.\n"

# test string of integer for input
test_int: .asciiz "10011101001110011110011"

#this is the location in memory where digits are written to by the putint function
int_string: .byte '?':32
int_newline: .byte 10,0
radix: .byte 2
int_width: .byte 4

.text
main:

la $s0,title
jal putstring

li $s0,0 #we will load the $s0 register with the number we want to convert to string

loop:
jal putint
addi $s0,$s0,1
blt $s0,16,loop

la $s0,test_int # convert string to integer
jal strint

li $t0,10    #change radix
sb $t0,radix

li $t0,8    #change width
sb $t0,int_width

jal putint

li   $v0, 10     # exit syscall
syscall


putstring:
li $v0,4      # load immediate, v0 = 4 (4 is print string system call)
move $a0,$s0  # load address of string to print into a0
syscall
jr $ra


#this is the intstr function, the ultimate integer to string conversion function
#just like the Intel Assembly version, it can convert an integer into a string
#radixes 2 to 36 are supported. Digits higher than 9 will be capital letters

intstr:

la $t1,int_newline # load target index address of lowest digit
addi $t1,$t1,-1

lb $t2,radix     # load value of radix into $t2
lb $t4,int_width # load value of int_width into $t4
li $t3,1         # load current number of digits, always 1

digits_start:

divu $s0,$s0,$t2 # $s0=$s0/$t2 (divide s0 by the radix value in $t2)
mfhi $t0        # $t0=remainder of the previous division
blt $t0,10,decimal_digit
bge $t0,10,hexadecimal_digit

decimal_digit: # we go here if it is only a digit 0 to 9
addi $t0,$t0,'0'
j save_digit

hexadecimal_digit:
addi $t0,$t0,-10
addi $t0,$t0,'A'

save_digit:
sb $t0,($t1) # store byte from $t0 at address $t1
beq $s0,$0,intstr_end
addi $t1,$t1,-1
addi $t3,$t3,1
j digits_start

intstr_end:

li $t0,'0'
prefix_zeros:
bge $t3,$t4,end_zeros
addi $t1,$t1,-1
sb $t0,($t1) # store byte from $t0 at address $t1
addi $t3,$t3,1
j prefix_zeros
end_zeros:

move $s0,$t1

jr $ra

#this function calls intstr to convert the $s0 register into a string
#then it uses a system call to print the string
#it also uses the stack to save the value of $s0 and $ra (return address)

putint:
sw $ra,0($sp)
sw $s0,4($sp)
jal intstr
#print string
li $v0,4      # load immediate, v0 = 4 (4 is print string system call)
move $a0,$s0  # load address of string to print into a0
syscall
lw $ra,0($sp)
lw $s0,4($sp)
jr $ra











strint:

move $t1,$s0 # copy string address from $s0 to $t1
li $s0,0

lb $t2,radix     # load value of radix into $t2

read_strint:
lb $t0,($t1)
addi $t1,$t1,1
beq $t0,0,strint_end

#if char is below '0' or above '9', it is outside the range of these and is not a digit
blt $t0,'0',not_digit
bgt $t0,'9',not_digit

#but if it is a digit, then correct and process the character
is_digit:
and $t0,$t0,0xF
j process_char

not_digit:
#it isn't a digit, but it could be perhaps and alphabet character
#which is a digit in a higher base

#if char is below 'A' or above 'Z', it is outside the range of these and is not capital letter
blt $t0,'A',not_upper
bgt $t0,'Z',not_upper

is_upper:

sub $t0,$t0,'A'
addi $t0,$t0,10
j process_char

not_upper:

#if char is below 'a' or above 'z', it is outside the range of these and is not lowercase letter
blt $t0,'a',not_lower
bgt $t0,'z',not_lower

is_lower:

sub $t0,$t0,'a'
addi $t0,$t0,10
j process_char

not_lower:

#if we have reached this point, result invalid and end function
#this is only reached if the byte was not a valid digit or alphabet character
j strint_end

process_char:

bgt $t0,$t2 strint_end #;if this value is above or equal to radix, it is too high despite being a valid digit/alpha

mul $s0,$s0,$t2 # multiply $s0 by the radix
add $s0,$s0,$t0     # add the correct value of this digit

j read_strint # jump back and continue the loop if nothing has exited it

strint_end:

jr $ra