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/*
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 *	mpvecsub(mpdigit *a, int alen, mpdigit *b, int blen, mpdigit *diff)
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 *
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 *		diff[0:alen-1] = a[0:alen-1] - b[0:blen-1]
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 *
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 *	prereq: alen >= blen, diff has room for alen digits
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 */
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TEXT	mpvecsub(SB),$0
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	MOVL	a+0(FP),SI
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	MOVL	b+8(FP),BX
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	MOVL	alen+4(FP),DX
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	MOVL	blen+12(FP),CX
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	MOVL	diff+16(FP),DI
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	SUBL	CX,DX
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	XORL	BP,BP			/* this also sets carry to 0 */
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	/* skip subraction if b is zero */
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	TESTL	CX,CX
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	JZ	_sub1
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	/* diff[0:blen-1],borrow = a[0:blen-1] - b[0:blen-1] */
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_subloop1:
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	MOVL	(SI)(BP*4),AX
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	SBBL	(BX)(BP*4),AX
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	MOVL	AX,(DI)(BP*4)
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	INCL	BP
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	LOOP	_subloop1
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_sub1:
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	INCL	DX
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	MOVL	DX,CX
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	LOOP	_subloop2
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	RET
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	/* diff[blen:alen-1] = a[blen:alen-1] - 0 */
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_subloop2:
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	MOVL	(SI)(BP*4),AX
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	SBBL	$0,AX
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	MOVL	AX,(DI)(BP*4)
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	INCL	BP
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	LOOP	_subloop2
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	RET
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