SingOS/IntToString.nasm

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; Puts the stringified version of eax on the stack
; IN: eax = number to write
; IN: edi = base of where to write string
; OUT: ecx = number of characters written to buffer
IntToString: ; (eax = number, edi = buffer base) -> ecx = number of characters
	push eax
	push edi
	;push ecx
	push edx
	push esi
	push ebp
	
	mov	ecx, edi		; store negative original buffer base
	neg	ecx		;    (used for counting amount of characters)

	test	eax, eax
	jns	.not_negative
	
	mov	BYTE [edi], '-'
	neg	eax
	add	edi, 1

.not_negative:
	mov	esi, edi		; store pointer after potential negative sign character
	mov	ebp, 0xcccccccd	; ebp = 32 bit multiplicative reciprocal of 10

.divide_loop:

	mov	ebx, eax		; save number in ebx

	mul	ebp
	shr	edx, 3		; edx = quotient

	lea	eax, [edx + edx*4]	; eax = quotient * 10 = (number - remainder)
	add	eax, eax		;  |

	sub	ebx, eax		; ebx = number - (number - remainder) = remainder

	add	ebx, '0'		; remainder is our digit, turn it ASCII
	mov	BYTE [edi], bl	; store ASCII digit to buffer
	lea	edi, [edi+1]

	mov	eax, edx		; We need quotient in eax for 'mul'

	test	eax, eax
	jne	.divide_loop

	add	ecx, edi		; Update amount of characters

	mov	byte [edi],  0	; Null terminator

	lea 	edi, [edi-1]	; We incremented the buffer pointer once too much

.swap_loop:
	cmp	esi, edi
	jae	.done_swapping
	movzx	eax, BYTE [edi]
	movzx	edx, BYTE [esi]
	mov	BYTE [edi], dl
	mov	BYTE [esi], al
	sub	edi, 1
	add	esi, 1
	jmp	.swap_loop

.done_swapping:

	pop ebp
	pop esi
	pop edx
	;pop ecx
	pop edi
	pop eax
	ret