Flag bits in assembly language: CF, PF, AF, ZF, SF, TF, if, DF, of

One 、 Operation structure flag bit

1.CF( Carry flag bit )： Mainly used to reflect whether the operation produces carry or borrow , Generating carry or borrow is CF=1, otherwise CF=0.

2.PF( Parity flag bit )： Used to reflect the results of operations “1” The parity of the number of , If “1” The number of is even , be PF=1, otherwise ,PF=0.

3.AF( Auxiliary carry flag bit )： Look at the last four digits during the operation , Whatever the length , The last four digits carry or borrow forward , be AF=1, otherwise ,AF=0.

4.ZF( Zero flag bit )： It is used to reflect whether the operation result is 0, If the result is 0, be ZF=1, otherwise ZF=0.

5.SF( Symbol sign bit )： The sign bit used to reflect the positive and negative of the operation result , When the operation structure is positive ,SF=0, otherwise SF=1.

6.OF( overflow flag )： Used to reflect whether the result of signed number addition and subtraction operation overflows , If the operation result exceeds the range represented by the current operation digit , Then overflow ,OF=1, otherwise ,OF=0.

Two 、 Status control flag bit

The status control flag bit is used to control CPU Operation of the , They can only be changed by special instructions .
1.TF( Tracking flag bit )： When TF Be set to 1 when ,CPU Enter single step execution mode , That is, every instruction executed , Generate a single step interrupt request . This method is mainly used for program debugging .
Be careful ： There is no special instruction in the instruction system to change the flag bit TF Value , But programmers can change its value in other ways .

2.IF( Interrupt allow flag bit )： Used to decide CPU Whether to respond to CPU External maskable interrupt requests , When IF=1 when ,CPU Respond to CPU External maskable interrupt requests , When IF=0 when ,CPU Do not respond to CPU External maskable interrupt requests .
Be careful ： Regardless of the value of this flag ,CPU Must respond to CPU An interrupt request made by an external non maskable interrupt , as well as CPU Internally generated interrupt requests .

3.DF( Direction sign bit )： In a string processing instruction , After each operation , If DF=0, be si、di Increasing , If DF=1, be si、di Decline .
Be careful ：DF The value of is set by the programmer .（cld The order is to DF Set as 0,std The order is to DF The value is 1）.