Added lowercase function, string length function formating to fat12

function

source/kernel/features/strings.asm

@@ -25,24 +25,95 @@ os_compare_strings:
 	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 all upper case
+; 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
+    lodsb ; Load the character into al
     inc di
     cmp al, 0
-    je .finish
-    cmp al, 7Ah
-    jns .loop
-    cmp al, 61h
-    js .loop
-    sub al, 20h
+    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
-    jmp .loop
+    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