memmove.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

	TEXT	memmove(SB), $0

	JMP	move

	TEXT	memcpy(SB), $0
move:
	MOVW	R1, s1+0(FP)

	MOVW	n+8(FP), R3		/* R3 is count */
	MOVW	R1, R4			/* R4 is to-pointer */
	SGT	R0, R3, R5
	BEQ	R5, ok
	MOVW	(R0), R0		/* abort if negative count */
ok:
	MOVW	s2+4(FP), R5		/* R5 is from-pointer */
	ADDU	R3,R5, R7		/* R7 is end from-pointer */
	ADDU	R3,R4, R6		/* R6 is end to-pointer */

/*
 * easiest test is copy backwards if
 * destination string has higher mem address
 */
	SGT	$4,R3, R2
	SGTU	R4,R5, R1
	BNE	R1, back

/*
 * if not at least 4 chars,
 * don't even mess around.
 * 3 chars to guarantee any
 * rounding up to a word
 * boundary and 4 characters
 * to get at least maybe one
 * full word store.
 */
	BNE	R2, fout


/*
 * byte at a time to word align destination
 */
f1:
	AND	$3,R4, R1
	BEQ	R1, f2
	MOVB	0(R5), R8
	ADDU	$1, R5
	MOVB	R8, 0(R4)
	ADDU	$1, R4
	JMP	f1

/*
 * test if source is now word aligned
 */
f2:
	AND	$3, R5, R1
	BNE	R1, fun2
/*
 * turn R3 into to-end pointer-15
 * copy 16 at a time while theres room.
 * R6 is smaller than R7 --
 * there are problems if R7 is 0.
 */
	ADDU	$-15,R6, R3
f3:
	SGTU	R3,R4, R1
	BEQ	R1, f4
	MOVW	0(R5), R8
	MOVW	4(R5), R9
	MOVW	R8, 0(R4)
	MOVW	8(R5), R8
	MOVW	R9, 4(R4)
	MOVW	12(R5), R9
	ADDU	$16, R5
	MOVW	R8, 8(R4)
	MOVW	R9, 12(R4)
	ADDU	$16, R4
	JMP	f3

/*
 * turn R3 into to-end pointer-3
 * copy 4 at a time while theres room
 */
f4:
	ADDU	$-3,R6, R3
f5:
	SGTU	R3,R4, R1
	BEQ	R1, fout
	MOVW	0(R5), R8
	ADDU	$4, R5
	MOVW	R8, 0(R4)
	ADDU	$4, R4
	JMP	f5

/*
 * forward copy, unaligned
 * turn R3 into to-end pointer-15
 * copy 16 at a time while theres room.
 * R6 is smaller than R7 --
 * there are problems if R7 is 0.
 */
fun2:
	ADDU	$-15,R6, R3
fun3:
	SGTU	R3,R4, R1
	BEQ	R1, fun4
	MOVWL	0(R5), R8
	MOVWR	3(R5), R8
	MOVWL	4(R5), R9
	MOVWR	7(R5), R9
	MOVW	R8, 0(R4)
	MOVWL	8(R5), R8
	MOVWR	11(R5), R8
	MOVW	R9, 4(R4)
	MOVWL	12(R5), R9
	MOVWR	15(R5), R9
	ADDU	$16, R5
	MOVW	R8, 8(R4)
	MOVW	R9, 12(R4)
	ADDU	$16, R4
	JMP	fun3

/*
 * turn R3 into to-end pointer-3
 * copy 4 at a time while theres room
 */
fun4:
	ADDU	$-3,R6, R3
fun5:
	SGTU	R3,R4, R1
	BEQ	R1, fout
	MOVWL	0(R5), R8
	MOVWR	3(R5), R8
	ADDU	$4, R5
	MOVW	R8, 0(R4)
	ADDU	$4, R4
	JMP	fun5

/*
 * last loop, copy byte at a time
 */
fout:
	BEQ	R7,R5, ret
	MOVB	0(R5), R8
	ADDU	$1, R5
	MOVB	R8, 0(R4)
	ADDU	$1, R4
	JMP	fout

/*
 * whole thing repeated for backwards
 */
back:
	BNE	R2, bout
b1:
	AND	$3,R6, R1
	BEQ	R1, b2
	MOVB	-1(R7), R8
	ADDU	$-1, R7
	MOVB	R8, -1(R6)
	ADDU	$-1, R6
	JMP	b1

b2:
	AND	$3, R7, R1
	BNE	R1, bun2

	ADDU	$15,R5, R3
b3:
	SGTU	R7,R3, R1
	BEQ	R1, b4
	MOVW	-4(R7), R8
	MOVW	-8(R7), R9
	MOVW	R8, -4(R6)
	MOVW	-12(R7), R8
	MOVW	R9, -8(R6)
	MOVW	-16(R7), R9
	ADDU	$-16, R7
	MOVW	R8, -12(R6)
	MOVW	R9, -16(R6)
	ADDU	$-16, R6
	JMP	b3
b4:
	ADDU	$3,R5, R3
b5:
	SGTU	R7,R3, R1
	BEQ	R1, bout
	MOVW	-4(R7), R8
	ADDU	$-4, R7
	MOVW	R8, -4(R6)
	ADDU	$-4, R6
	JMP	b5

bun2:
	ADDU	$15,R5, R3
bun3:
	SGTU	R7,R3, R1
	BEQ	R1, bun4
	MOVWL	-4(R7), R8
	MOVWR	-1(R7), R8
	MOVWL	-8(R7), R9
	MOVWR	-5(R7), R9
	MOVW	R8, -4(R6)
	MOVWL	-12(R7), R8
	MOVWR	-9(R7), R8
	MOVW	R9, -8(R6)
	MOVWL	-16(R7), R9
	MOVWR	-13(R7), R9
	ADDU	$-16, R7
	MOVW	R8, -12(R6)
	MOVW	R9, -16(R6)
	ADDU	$-16, R6
	JMP	bun3

bun4:
	ADDU	$3,R5, R3
bun5:
	SGTU	R7,R3, R1
	BEQ	R1, bout
	MOVWL	-4(R7), R8
	MOVWR	-1(R7), R8
	ADDU	$-4, R7
	MOVW	R8, -4(R6)
	ADDU	$-4, R6
	JMP	bun5

bout:
	BEQ	R7,R5, ret
	MOVB	-1(R7), R8
	ADDU	$-1, R7
	MOVB	R8, -1(R6)
	ADDU	$-1, R6
	JMP	bout

ret:
	MOVW	s1+0(FP), R1
	RET
	END