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I have the following code to convert a command line argument (null terminated string) to integer from hex. I have trouble finding the correct algorithm to implement this. I know it's a sloppy code, but it helps me understand. Any pointers?
I know something is off because the max is 250 and entering FA gives an error. :(

	mov	ebx, dword	; 2nd arg

loop:	

	mov	al, byte

	

	cmp	al, "0"

	jl	erronR1Value

	cmp	al, "9"

	jg	checkletters

	jmp	wasdigit

checkletters:

	cmp	al, "A"

	jl	erronR1Value

	cmp	al, "F"

	jg	checklower

	jmp	wasupper

checklower:

	cmp	al, "a"

	jl	erronR1Value

	cmp	al, "f"

	jg	erronR1Value

	sub	al, 61h

	add	al, 10

	jmp	convloop



wasdigit:

	sub	al, 30h

	jmp	convloop

wasupper:

	sub	al, 41h

	add	al, 10

	jmp	convloop

convloop:

	mov	edx, dword 	; radius 1 address. must store int value in .

	shl	, 4		; multiply by 16

	add	, al

	inc	ecx

	add	ebx, 1	

	cmp	byte, NULL

	jne	loop

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