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是proteus的51单片机仿真，刚把两种代码生成的汇编代码给看了看，应该是这里的区别：

第一种的汇编：
_Delay_Ms:
USING 0

 C0001:
MOV   A,R7
ORL   A,R6
JZ   C0007

MOV   R5,#078H
MOV   R4,#00H
C0004:
MOV   A,R5
DEC   R5
JNZ   C0011
DEC   R4
C0011:
MOV   A,R5
ORL   A,R4
JNZ   C0004
C0003:
MOV   A,R7
DEC   R7
JNZ   C0001
DEC   R6
C0012:
SJMP C0001
C0007:
RET

第二种的汇编：
_Delay_Ms:
USING 0

 C0001:
MOV   A,R7
ORL   A,R6
JZ   C0007

MOV   R5,#078H
 C0004:
DJNZ R5, C0004
 C0003:
MOV   A,R7
DEC   R7
JNZ   C0001
DEC   R6
 C0011:
SJMP C0001

 C0007:
RET

玩了一下51的仿真，发现8位的单片机处理char型和int型时间相差比较大