131 lines
3.2 KiB
NASM
Executable File
131 lines
3.2 KiB
NASM
Executable File
; How string comparison works
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; DI => scasb compares value stored in DI which is 's' with 's' stored in AX register
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; and then increments DI if DF is 0
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; v
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; memory address 1: |s|n|a|t|0|
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; memory address 2: |s|n|a|k|e|0|
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; ^
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; SI => lodsb loads value stored at SI to AX and then increments SI if DF is 0
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; Additionaly, if the di string ends and the relevant character in si contains a space, it will still return
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; the strings as being equal, this is to allow for command line arquments
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os_compare_strings:
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cld
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.compare:
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mov al, 0
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scasb
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je .di_ended
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dec di
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lodsb
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scasb ; Compare di to si
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jne .unequal ; If they are not equal, jump to .unequal
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jmp .compare ; Finally, repeat
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.unequal:
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mov cl, 0 ; Change to 0 if unquality is proven
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ret
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.di_ended:
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lodsb
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cmp al, 20h ; 20h = space
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je .equal
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cmp al, 0
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je .equal
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jmp .unequal
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.equal:
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mov cl, 1
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ret
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; Get the length of a string
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; 'hello world', 0 is 11 characters long (excluding the terminator)
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; input: si points to the string to be counted
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; output: cl holds the length
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os_string_length:
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push si
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xor cl,cl ; Clear the al register
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.loop:
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lodsb
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cmp al, 0
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je .finish
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inc cl
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jmp .loop
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.finish:
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pop si
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ret
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; convert a string to fat's filename format
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; It will be capitalised and 11 characters long,
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; 8 for the filename, 3 for the extension
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; eg: 'file.txt' -> 'FILE TXT'
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; input: si points to filename, di points to a free 11 bytes in memory
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; output: di points to the fat formatted filename
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os_format_fat_filename:
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pusha
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call os_upper_case
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call os_string_length ; Stores the length of the string in cl
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mov bl, 11
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sub bl, cl ; 11 - string_length
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.character_loop:
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lodsb
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cmp al, 0
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je .finish
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cmp al, 2Eh ; 2Eh
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je .add_spaces ; This will end up back at .character_loop
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stosb
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jmp .character_loop
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.add_spaces: ; Add the number of spaces as bl holds
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mov al, ' ' ; 20h = space
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.spaces_loop:
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stosb
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cmp bl, 0
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je .character_loop
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dec bl
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jmp .spaces_loop
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.finish:
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popa
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ret
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; Convert a string to all upper/lower case
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; INPUT: si pointing to a string
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; OUPUT: the same string in memory will now be capitalised/decapitalised
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os_upper_case: ; to upper case
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pusha
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mov di, si
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.loop:
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lodsb ; Load the character into al
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inc di
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cmp al, 0
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je .finish ; If it's null then the string is finished
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cmp al, 7Ah ; 7Ah = 'z'
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jns .loop ; Ignore if it's more than 'z'
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cmp al, 61h ; 61h = 'a'
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js .loop ; Ignore if it's less than 'a'
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sub al, 20h ; Otherwise subtract 20h to capitalise it
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dec di
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stosb ; Store the new value
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jmp .loop ; Next character
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.finish:
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popa
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ret
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os_lower_case: ; to lower case
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pusha
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mov di, si
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.loop:
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lodsb ; Load the character into al
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inc di
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cmp al, 0
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je .finish ; If it's null then the string is finished
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cmp al, 5Ah ; 5Ah = 'Z'
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jns .loop ; Ignore if it's more than 'Z'
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cmp al, 41h ; 41h = 'A'
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js .loop ; Ignore if it's less than 'A'
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add al, 20h ; Otherwise subtract 20h to capitalise it
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dec di
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stosb ; Store the new value
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jmp .loop ; Next character
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.finish:
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popa
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ret
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