在
ARM
体系中，所有
ARM
指令均可条件执行，设置了一些条件执行码，但这些条件码对应设置
CPSR
中的
Z, C, N, V
标志位：
l
0000 = EQ – Z set (equal)
l
0001 = NE – Z clear (not equal)
l
0010 = CS – C set (unsigned higher or same)
l
0011 = CC – C clear (unsigned lower)
l
0100 = MI – N set (negative)
l
0101 = PL – N clear (positive or zero)
l
0110 = VS – V set (overflow)
l
0111 = VC – V clear (no overflow)
l
1000 = HI – C set and Z clear (unsigned higher)
l
1001 = LS – C clear or Z set (unsigned lower or same)
l
1010 = GE – N set and V set, or N clear and V clear (greater or equal)
l
1011 = LT – N set and V clear, or N clear and V set (less than)
l
1100 = GT – Z clear, and either N set and V set, or N clear and V clear (greater than)
l
1101 = LE – Z set, or N set and V clear, or N clear and V set (less than or equal)
l
1110 =
AL
– always
l
1111 = NV – never
怎样去理解这些设置呢？拿
1001 = LS – C clear or Z set (unsigned lower or same)
来说：
是为什么
LS
就对应
C clear and Z set
。看下面的例子：
MOV R0, #5
MOV R1, #6
CMP R0, R1
MOVLS R2, R0 ; if R0 < R1
则
将小值存入
R2
中
在这个例子中，
MOVLS
能够正确执行的条件
C=0 and z=1
成立，是通过
CMP
设置了，看看
reference manual
中的
CMP
设置规则：
C flag:For a subtraction, including the comparison instruction
CMP
, C is set to 0 if the subtraction produced a borrow (that is, an unsigned underflow), and to 1 otherwise.
Z flag: Is set to 1 if the result of the instruction is zero (which often indicates an
equal
result from
a comparison), and to 0 otherwise.
这就不难理解了
LS
与标志位的对应了。下面给出
标志位的设置规则，便于我们从中推导上面每一个例子。
In either case, the new condition code flags (after the instruction has been executed) usually mean:
N
Is set to bit 31 of the result of the instruction. If this result is regarded as a two’s complement
signed integer, then N = 1 if the result is negative and N = 0 if it is positive or zero.
Z
Is set to 1 if the result of the instruction is zero (which often indicates an
equal
result from
a comparison), and to 0 otherwise.
C
Is set in one of four ways:
• For an addition, including the comparison instruction
CMN
, C is set to 1 if the addition
produced a carry (that is, an unsigned overflow), and to 0 otherwise.
• For a subtraction, including the comparison instruction
CMP
, C is set to 0 if the
subtraction produced a borrow (that is, an unsigned underflow), and to 1 otherwise.
• For non-addition/subtractions that incorporate a shift operation, C is set to the last bit
shifted out of the value by the shifter.
• For other non-addition/subtractions, C is normally left unchanged (but see the
individual instruction descriptions for any special cases).
V
Is set in one of two ways:
• For an addition or subtraction, V is set to 1 if signed overflow occurred, regarding the
operands and result as two’s complement signed integers.
• For non-addition/subtractions, V is normally left unchanged (but see the individual
instruction descriptions for any special cases).
来看下
GT (signed greater)
标志位的设置
:
CMP -5, -4 ;
相减为不等于
0
，为负值，且有下溢出，所以
N=1, V=1
CMP 6,5 ;
相减后值为
1
（
N=0
），正值且无下溢（
V=0
）