mpvecdigmuladd.s

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

/*
 *	mpvecdigmuladd(mpdigit *b, int n, mpdigit m, mpdigit *p)
 *
 *	p += b*m
 *
 *	each step looks like:
 *		hi,lo = m*b[i]
 *		lo += oldhi + carry
 *		hi += carry
 *		p[i] += lo
 *		oldhi = hi
 *
 *	the registers are:
 *		b = R1
 *		n = R4
 *		m = R5
 *		p = R6
 *		i = R7
 *		hi = R8		- constrained by hardware
 *		lo = R9		- constrained by hardware
 *		oldhi = R10
 *		tmp = R11
 *		
 */
TEXT	mpvecdigmuladd(SB),$0

	MOVW	n+4(FP),R4
	MOVW	m+8(FP),R5
	MOVW	p+12(FP),R6


	MOVW	R0, R10		/* oldhi = 0 */
	BEQ	R6, _muladd1
_muladdloop:
	MOVW	0(R1), R9	/* lo = b[i] */
	ADDU	$4, R1
	MOVW	0(R6), R11	/* tmp = p[i] */
	MULU	R9, R5
	MOVW	HI, R8		/* hi = (b[i] * m)>>32 */
	MOVW	LO, R9		/* lo = b[i] * m */
	ADDU	R10, R9		/* lo += oldhi */
	SGTU	R10, R9, R2
	ADDU	R2, R8		/* hi += carry */
	ADDU	R9, R11		/* tmp += lo */
	SGTU	R9, R11, R2
	ADDU	R2, R8		/* hi += carry */
	MOVW	R11, 0(R6)	/* p[i] = tmp */
	ADDU	$4, R6
	MOVW	R8, R10		/* oldhi = hi */
	SUBU	$1, R4
	BNE	R4, _muladdloop

_muladd1:
	MOVW	0(R6), R11	/* tmp = p[i] */
	ADDU	R10, R11	/* tmp += oldhi */
	MOVW	R11, 0(R6)	/* p[i] = tmp */

	RET