ARM条件码与CPSR标志位的关系

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：
[align=left]l           0000 = EQ - Z set (equal)[/align]
[align=left]l           0001 = NE - Z clear (not equal)[/align]
[align=left]l           0010 = CS - C set (unsigned higher or same)[/align]
[align=left]l           0011 = CC - C clear (unsigned lower)[/align]
[align=left]l           0100 = MI - N set (negative)[/align]
[align=left]l           0101 = PL - N clear (positive or zero)[/align]
[align=left]l           0110 = VS - V set (overflow)[/align]
[align=left]l           0111 = VC - V clear (no overflow)[/align]
[align=left]l           1000 = HI - C set and Z clear (unsigned higher)[/align]
[align=left]l           1001 = LS - C clear or Z set (unsigned lower or same)[/align]
[align=left]l           1010 = GE - N set and V set, or N clear and V clear (greater or equal)[/align]
[align=left]l           1011 = LT - N set and V clear, or N clear and V set (less than)[/align]
[align=left]l           1100 = GT - Z clear, and either N set and V set, or N clear and V clear (greater than)[/align]
[align=left]l           1101 = LE - Z set, or N set and V clear, or N clear and V set (less than or equal)[/align]
[align=left]l           1110 = AL - always[/align]
[align=left]l           1111 = NV - never[/align]
[align=left]怎样去理解这些设置呢？拿1001 = LS - C clear or Z set (unsigned lower or same)来说：[/align]
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：
[align=left]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.[/align]
[align=left]Z flag: Is set to 1 if the result of the instruction is zero (which often indicates an equal result from[/align]
[align=left]a comparison), and to 0 otherwise.[/align]
[align=left]         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。[/align]
[align=left]In either case, the new condition code flags (after the instruction has been executed) usually mean:[/align]
[align=left]N Is set to bit 31 of the result of the instruction. If this result is regarded as a two's complement[/align]
[align=left]signed integer, then N = 1 if the result is negative and N = 0 if it is positive or zero.[/align]
[align=left]Z Is set to 1 if the result of the instruction is zero (which often indicates an equal result from[/align]
[align=left]a comparison), and to 0 otherwise.[/align]
[align=left]C Is set in one of four ways:[/align]
[align=left]• For an addition, including the comparison instruction CMN, C is set to 1 if the addition[/align]
[align=left]produced a carry (that is, an unsigned overflow), and to 0 otherwise.[/align]
[align=left]• For a subtraction, including the comparison instruction CMP, C is set to 0 if the[/align]
[align=left]subtraction produced a borrow (that is, an unsigned underflow), and to 1 otherwise.[/align]
[align=left]• For non-addition/subtractions that incorporate a shift operation, C is set to the last bit[/align]
[align=left]shifted out of the value by the shifter.[/align]
[align=left]• For other non-addition/subtractions, C is normally left unchanged (but see the[/align]
[align=left]individual instruction descriptions for any special cases).[/align]
[align=left]V Is set in one of two ways:[/align]
[align=left]• For an addition or subtraction, V is set to 1 if signed overflow occurred, regarding the[/align]
[align=left]operands and result as two's complement signed integers.[/align]
[align=left]• For non-addition/subtractions, V is normally left unchanged (but see the individual[/align]
[align=left]instruction descriptions for any special cases).[/align]
 
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）