; How string comparison works
;                   DI => scasb compares value stored in DI which is 's' with 's' stored in AX register
;                         and then increments DI if DF is 0
;                    v
; memory address 1: |s|n|a|t|0|       
; memory address 2: |s|n|a|k|e|0|     
;                    ^
;                   SI => lodsb loads value stored at SI to AX and then increments SI if DF is 0
os_compare_strings:	
	cld

.compare:
	lodsb
	scasb			; Compare di to si
	jne .unequal		; If they are not equal, jump to .unequal
	cmp al, 0		; Check if string is finished
	je .equal		; If it has, return true
	jmp .compare		; Finally, repeat

.unequal:
	mov cl, 0		; Change to 0 if unquality is proven
	ret

.equal:
	mov cl, 1
	ret

; Get the length of a string
; 'hello world', 0 is 11 characters long (excluding the terminator)
; input: si points to the string to be counted
; output: cl holds the length
os_string_length:
   push si
   xor cl,cl ; Clear the al register
.loop
   lodsb
   cmp al, 0
   je .finish
   inc cl
   jmp .loop
.finish
   pop si
   ret

; convert a string to fat's filename format
; It will be capitalised and 11 characters long,
; 8 for the filename, 3 for the extension
; eg: 'file.txt' -> 'FILE    TXT'
; input: si points to filename, di points to a free 11 bytes in memory
; output: di points to the fat formatted filename
os_format_fat_filename:
   pusha
   call os_upper_case
   call os_string_length ; Stores the length of the string in cl
   mov bl, 11
   sub bl, cl ; 11 - string_length
   .character_loop:
	  lodsb
	  cmp al, 0
	  je .finish
	  cmp al, 2Eh ; 2Eh
	  je .add_spaces ; This will end up back at .character_loop
	  stosb
	  jmp .character_loop

   .add_spaces: ; Add the number of spaces as bl holds
	  mov al, ' ' ; 20h = space
	  .spaces_loop:
		 stosb
		 cmp bl, 0
		 je .character_loop
		 dec bl
		 jmp .spaces_loop
   .finish:
	  popa
	  ret

; Convert a string to all upper/lower case
; INPUT: si pointing to a string
; OUPUT: the same string in memory will now be capitalised/decapitalised
os_upper_case:
    pusha
    mov di, si
.loop:
    lodsb ; Load the character into al
    inc di
    cmp al, 0
    je .finish ; If it's null then the string is finished
    cmp al, 7Ah ; 7Ah = 'z'
    jns .loop ; Ignore if it's more than 'z'
    cmp al, 61h ; 61h = 'a'
    js .loop ; Ignore if it's less than 'a'
    sub al, 20h ; Otherwise subtract 20h to capitalise it
    dec di
    stosb ; Store the new value
    jmp .loop ; Next character
.finish:
    popa
    ret

os_lower_case:
    pusha
    mov di, si
.loop:
    lodsb ; Load the character into al
    inc di
    cmp al, 0
    je .finish ; If it's null then the string is finished
    cmp al, 5Ah ; 5Ah = 'Z'
    jns .loop ; Ignore if it's more than 'Z'
    cmp al, 41h ; 41h = 'A'
    js .loop ; Ignore if it's less than 'A'
    add al, 20h ; Otherwise subtract 20h to capitalise it
    dec di
    stosb ; Store the new value
    jmp .loop ; Next character
.finish:
    popa
    ret