奇偶標誌得到混合與「1和0」

Question

檢查這個代碼出

include 'win32ax.inc' 

;.data 


.code 

start: 

mov al,00000001b 

add al,00000001b ;even =2  pf = 0 
add al,00000001b ;odd =3  pf = 1 
add al,00000001b ;even =4  pf = 0 
sub al,00000001b ;odd =3  pf = 1 
sub al,00000001b ;even =2  pf = 0 
sub al,00000001b ;odd =1  pf = 0 
sub al,00000001b ;even =0  pf = 1 

.end start 

最後兩個減法指令應設定PF = 1的結果，則PF = 0，那麼，爲什麼不呢？

另外我使用FASM，我正在用ollydbg調試器進行調試。

Answer 1

從維基百科：

在x86處理器，奇偶標誌反映了奇偶只的結果的最低顯著字節，並且被設定，如果那些的集合的位的數目是偶數。

所以

result = 0 an even number of ones are set so pf = 1 is the right answer 
result = 1 an odd number of ones are set so pf = 0 is the right answer 
result = 2 an odd number of ones are set so pf = 0 is the right answer 
result = 3 an even number of ones are set so pf = 1 is the right answer 

1 = 0b00000001 one bit is set an odd number of bits set pf = 0 
3 = 0b00000011 two bits are set pf = 1 
7 = 0b00000111 three bits are set pf = 0 
12 = 0b00001100 two bits are set pf = 1