Assembly: CPU structure - flag register and related instructions

One 、 summary

FLAG It's a computer terminology , Status flag register .

1、 The function of flag register

1、 be used for Storage Of relevant instructions Some data results .
2、 Used for CPU Execute relevant instructions Provide a basis for behavior .
3、 be used for control CPU Of How it works .

2、8086CPU Structure of flag register

The flag register functions bit by bit , Every one has a special meaning .
The characters marked in the above figure have special meanings , Other bits are not used （ Most of the effects are caused by operation instructions （ Logic / Count ））.

3、debug Look at the flag bit

Before flag bit learning and verification , First, let's see how to debug Look inside at the sign .

Use command

-r

Print the results ：
Print it out in the lower right corner of the screen NV UP EI PL NZ NA PO NC , These symbols represent the values of commonly used flag bits in the flag register .

OF <-----> OV(1) ; NV(0)
DF <-----> DN(1) ; UP(0)
IF <-----> EI(1) ; DI(0)
SF <-----> NG(1) ; PL(0)
ZF <-----> ZR(1) ; NZ(0)
AF <-----> AC(1) ; NA(0)
PF <-----> PE(1) ; PO(0)
CF <-----> CY(1) ; NC(0)

Two 、 Sign a

Flag bit classification ：

1、 Operation result flag bit
2、 Status control flag bit ： Used to control CPU Operation of the , They can only be changed by special instructions ）. The status control flag bits are ：TF、IF、DF.

Display the flag bit list
notes ： The bold part is the status control flag bit

Serial number	Logo name	Abbreviation	english	sign 1	sign 0
1	Overflow sign	OF	Overflow Flag	OV : overflow	NV : no overflow
2	Direction signs	DF	Direction Flag	UP : up	DN :down
3	Interrupt allow flag	IF	Interrupt-enable Flag	EI : enable interrupt	DI : disable interrupt
4	Tracking signs	TF	Trap Flag
5	Symbol sign bit	SF	Sign Flag	PL : plus	NG : negative
6	Zero mark	ZF	Zero Flag	ZR : zero	NZ : no zero
7	Auxiliary carry flag	AF	Auxiliary Carry Flag	AC : assistant carry	NA : no assistant carry
8	Parity mark	PF	Parity Flag	PE : parity even	PO : parity odd
9	Carry mark	CF	Carry Flag	CY : carry	NC : no carry

1、CF（0）： Carry flag bit

Indicates whether the highest bit in addition and subtraction operation has a forward position / Borrow position .
If there is no carry in the operation / Borrow position ：CF == 0( Show as NC)
If the operation has a carry / Borrow position ：CF == 1( Show as CY)

2、PF（2）： Parity flag bit

Record the execution of relevant instructions , Of all the bits of the result 1 Whether the number of is even .
When all binary bits are 1 The number of is odd ：PF == 0( Show as PO)
When all binary bits are 1 The number of is even ：PF == 1( Show as PE)

3、AF（4）： Auxiliary carry flag

stay 8 Bit addition and subtraction operation indicates low 4 Set high 4 Whether the position is advanced or not / Borrow position .
No progress / Borrow position ：AF == 0( Show as NA)
There is progress / Borrow position ：AF == 1( Show as AC)

4、ZF（6）：0 Sign a

Indicates that the result of the operation is 0 Or not 0.
When the result of the operation is not 0：zf == 0( Show as NC)
When the result is 0：zf == 1( Show as CY)

5、SF（7）： Symbol sign bit

Indicates that the result of the operation is negative （ The highest bit is 1） Or nonnegative （ The highest bit is 0）
When the result is nonnegative ：SF == 0( Show as PL)
When the operation result is negative ：SF== 1( Show as NG)

6、TF（8）： Tracking signs

When tracking signs TF == 1 when ,CPU Enter single step execution mode .

7、IF（9）： Interrupt flag bit

Express CPU Can I respond to maskable interrupt requests .IF The status of has no effect on non maskable interrupts and internal interrupts .

8、DF（10）： Direction sign bit

Indicates that the string operation is performed in the way of address subtraction or addition .
Every operation si di Increasing ：df == 0 ( Show as UP)
Every operation si di Decline ：df == 1 ( Show as DN)

9、OF（11）： overflow flag

（ Signed operations ） overflow flag ： When performing signed operations , If the result exceeds the range indicated by the machine , It's called overflow .
8 The range of bit signed numbers is -128 ~ +127,
16 It is -32728 ~ +32767）.

If there is no overflow ：OF==0( Show as NV)
If there is an overflow ：OF == 1( Show as OV)

3、 ... and 、 Instructions

1、adc： Add

Add instruction with carry , utilize CF Value on
Instructions ：adc ax,bx
Calculation ：ax = ax + bx + CF

2、sbb： Subtraction

sbb It's a subtraction instruction with borrow , utilize CF Value on
Instructions ：sbb ax,bx
Calculation ：ax = ax - bx - CF

3、cmp： Compare

cmp It's a comparison command ,cmp The function of is equivalent to the subtraction instruction , Don't save results , But it affects the flag register .
Instructions ：cmp ax,bx
Calculation ： ax - bx Set the flag register input according to the calculation result .

Such as execution ：

mov ax,8
mov bx,3
cmp ax,bx

After execution ：(ax)=8, zf=0, pf=1, sf=0, cf=0, of=0

4、 The conditional transfer instruction that detects the comparison result

Instructions	meaning	Relevant flag bits detected
je	If it is equal to, it will be transferred	ZF=1
jne	If not, transfer	ZF=0
jb	Lower than, transfer	CF=1
jnb	No less than, then transfer	CF=0
ja	If it is higher than, it will be transferred	CF=0 And ZF=0
jna	If it is not higher than, it will be transferred	CF=1 or ZF=1

5、pushf、popf： Push 、 Out of the stack

pushf： Stack the flag register value

popf： Pop data from stack , Enter the flag register