Assembly Language Tag register mark bit _nv up EI NG NZ AC PE CY

In 8086CPU, there is a marker register with a length of 16bit;

The information stored here is called the program status Word, thePSW, and the Register is abbreviated as flag.

Function: 1) used to store some execution results of related instructions;

2) to provide the basis for the implementation of the relevant instructions for the CPU;

3) to control how the CPU works.

Flag Register Everyone

Note: 16 characters not marked in the 8086CPU are not used, meaningless;

The characters are marked with a special meaning.

Bit symbol English full name description (mathematical or logical operation)

0 CF Carry flag carry flag bit unsigned number operation result whether carry/borrow

2 PF Parity flag Parity operation result binary indicates the number of members 1 (odd-even representation)

4 AF auxiliary carry flag auxiliary carry marker bit af=1, carry or borrow to high or higher byte

6 ZF Zero Flag 0 operation result of 0

7 SF sign flag symbol Mark bit signed number operation result positive and negative

8 TF Trap flag trace flag bit tf=1, resulting in a single-step interrupt, triggering the interrupt process

9 IF interrupt-enable flag interrupt allow Mark bit if=1/0,cpu allow interrupt or shutdown interrupt

DF Direction Flag Direction marker bit string processing instruction DF=0/1, controlling the increment or decrease of si,di per operation

One of Overflow flag overflow flag bit signed number operation result overflow

Note: Mathematical or logical operation instructions will affect flag, including add,sub,mul,div,inc,dec,or,and, etc.;

Mov,push,pop and other delivery instructions have no effect on flag;

TF trace flag (set for debug machine or Debugger discovery failure):

Tf=1,cpu the execution of an instruction will cause a single-step interrupt, and go to execute interrupt handler;

The TF=0,CPU is in continuous working condition.

If interrupt allows the mark bit (can mask interrupt interrupts during CPU detection if determine if masking interrupts):

If=1,cpu after executing the current instruction, the response is interrupted and the interrupt process is triggered;

IF=0,CPU does not respond to interrupts after executing the current instruction.

STI, then if=1;

CTI, then if=0.

Detailed description of each register reference: http://www.cnblogs.com/ant-colonies/p/6008322.html

The representation of the tag register in debug
ax=0000 bx=0000 cx=0000 dx=0000 sp=ffee bp=0000 si=0000 di=0000
ds=**** es=**** ss=**** cs=**** ip=0100NV up EI PL NZ NA PO NC
The bit flag value is1The flag value is0The logo
0 CF CY (CarrY) NC (not CarrY)
2 PF PE (Parity even) PO (Parity Odd)
4 AF AC (auxiliary Carry) NA (No auxiliary Carry)
6 ZF ZR (zero) NZ (not zero)
7 SF NG (negative) PL (Positive)
9 IF DI (Disabled Interrupt) EI (inabled Interrupt)
DF DN (down) up (UP)
One of OV (Overflow) NV (not Overflow)

Mark Bit CF with of:

ADC is an addition instruction with carry
Instruction format: ADC OBJECT1,OBJECT2
Function: OBJECT1=OBJECT1+OBJECT2+CF
Example: ADC ax,bx (AX) = (AX) + (BX) +CF

SBB is a subtraction instruction with borrow.
Instruction format: SBB Object1,object2
Function: OBJECT1=OBJECT1-OBJECT2-CF
Example: ADC ax,bx (AX) = (AX)-(BX)-CF

Note: ADC and SBB can be used to solve the problem that the carry cannot be stored when the add,sub operation;

ADC (add & CF), with carry addition;

SBB (Sub & below), with misplaced subtraction;

The comparison of the CF with the of the mark bit:

1111 1111 8bit indicates that the range corresponding flag bit is out of range
2^8-1 (255) unsigned number [0,255] CF takes place or borrow to a higher position
2^7-1 (127) signed number [ -128~127] of overflow

CMP directives
Instruction format: CMP OBJECT1,OBJECT2
Function: Calculates the Operation Object Object1-object2, but does not save the result, only sets the flag register according to the calculation result
Example:
CMP Ax,ax; (comparison of unsigned numbers)
After executing ax value unchanged, zf=1,pf=1,cf=0,of=0;
MOV ax,8
MOV bx,3
CMP AX,BX
After execution: (AX) =8,zf=0,pf=1,cf=0,of=0.

Instruction CMP AX,BX(comparison between unsigned numbers, reference mark bit ZF,CF)
Zf=1, then (ax) = (BX)
Zf=0, then (AX)! = (BX)
Cf=1, then (AX) < (BX)
Cf=0, then (AX) >= (BX)
Cf=0 and Zf=0, then (AX) > (BX)
Cf=1 or Zf=1, then (AX) <= (BX)

Instruction CMP AH,BH(comparison between signed numbers, reference mark bit SF,OF,ZF)
Sf=1, and of=0, then (AH) < (BH)
Sf=1, and Of=1, then (AH) > (BH)
Sf=0, and Of=1, then (AH) < (BH)
Sf=0, and of=0, then (AH) >= (BH)

Conditional Transfer directives:
The essence of "transfer" is to modify the IP; Condition "refers to the ability to determine whether an IP is modified according to a condition
The transfer displacement range for all conditional transfer directives is [-128,127]
1) Jcxz, that is, when (CX) =0, that is, modify the IP (implement transfer)
2) The CMP directive affects the mark register mark Bit and is used in conjunction with the following conditional transfer directives
A commonly used conditional transfer directive that transfers based on the comparison of unsigned numbers:
directive meaning detection related flag bit
Je equals the transfer zf=1
Jne is not equal to transfer zf=0
JB less than then transfer cf=1
JNB is not less than the transfer cf=0
JA is greater than transfer cf=0 and zf=0
JNA not greater than transfer cf=1 or zf=1

Note: j-jmp
E-equal Ne-not Equal
B-below Nb-not Below
A-above Na-not above

Mark bit DF with string transfer instructions:

DF (Direction flag), directional sign, controls the increase or decrease of si,di after each operation
Df=0,si,di Increment
Df=1,si,di Descending
String Transfer instructions:
Format:MOVSB; MOV string byte
Function: 1) ((es) *16+ (di)) = ((ds) *16+ (SI))
ie: mov es:[di],byte ptr ds:[si]
2) when df=0, (si) = (si) +1
(di) = (DI) +1
That is: Inc si
Inc di
When Df=1, (si) = (si)-1
(di) = (DI)-1
namely: Dec si
Dec di

Format:MOVSW; MOV string word
Function: 1) ((es) *16+ (di)) = ((ds) *16+ (SI))
ie: mov es:[di],word ptr ds:[si]
2) when df=0, (si) = (si) +2
(di) = (DI) +2
That is: Add si,2
Add di,2
When Df=1, (si) = (si)-2
(di) = (DI)-2
That is: Sub Si
Sub di
General MOVSB/MOVSW in conjunction with Rep; rep-repeat
Format:Rep MOVSB
namely: S:MOVSB
loop S; The role of rep is to repeat the string transfer based on CX values

Instructions for setting DF:
CLD; Place DF 0, string Transfer--offset address increment direction clear DF
STD; Place DF 1, string Transfer--offset address descending direction set DF

Assume Cs:code,ds:datadata segment  db ' Welcome to masm! '            ; Copy this string to the same segment address, offset address 16 memory location  db (0)                    data endscode segment  Start:mov ax,data         mov ds,ax         mov es, AX mov si,0 mov di,16 mov cx,8 CLD rep MOVSW  mov ax,4c00h int 21hcode endsend start

PUSHF and Popf:

PUSHF function--the value of the tag register is pressed to stack;

Popf function--out of the stack, the value is fed into the tag register.

Note: This article refers to the network and "Assembly language" (Wang Shuang, second edition)

Assembly Language Tag register mark bit _nv up EI NG NZ AC PE CY