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请将源数据区 Src 中的18个字单元复制到目标数据区 Dst，要求以4个字为单位，最后不足4字时可以以字为单位复制。

Copy_Data.S

num_of_words	EQU	18
 
	AREA	CopyData, CODE, READONLY
	ENTRY
 
start
	LDR	R0, = Src
	LDR	R1, = Dst
	MOV	R2, #num_of_words
	LDR	SP, =0x40001000
	MOVS	R3, R2, LSR #2
	BEQ	Copy_Words
	STMFD	SP!, {R5-R8}
 
Copy_4Words
	LDMIA	R0!, {R5-R8}
	STMIA	R1!, {R5-R8}
	SUBS	R3, R3, #1
	BNE	Copy_4Words
	LDMFD	SP!, {R5-R8}
 
Copy_Words
	ANDS	R2, R2, #3
	BEQ	Stop
Copy_Word
	LDR	R3, [R0], #4
	STR	R3, [R1], #4
	SUBS	R2, R2, #1
	BNE	Copy_Word
 
Stop
	B	Stop
	LTORG
 
Src
	DCD	1,2,3,4,5,6,7,8,9,0xA,0xB,0xC,0xD,0xE,0xF,0x10,0x11,0x12
 
Dst
	DCD	0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
 
	END