Detailed explanation of pseudo instructions in assembly language (with examples)

Pseudo instructions are not real instructions , There is no corresponding machine code , Will not be executed .

The function of pseudo instruction is mainly to control the assembly process .

Common pseudo instructions are as follows ：

Catalog

（1）ORG——— Assembly start instruction

（2）END—— Assembly end command

（3）EQU—— Assignment instruction

（4）DB—— Byte definition pseudo instruction

（5）DW—— Define word command

（6）DS—— Define storage space instructions

（7）BIT—— Bit address symbol command

（1）ORG——— Assembly start instruction

     

function ： Specify the starting address of the target program under the pseudo instruction

     

Format ：

ORG  16 Bit address

    

  give an example ：

           ORG 0080H    ; Indicates the following program from the address 0080H Start

     ST：……

           ORG 0100H

            ……

• Be careful ：ORG The address after the command must be increased in the order it appears , And cannot overlap .
  Otherwise, the compiler may report an error . For example, in the above example 0100H Greater than 0080H.

（2）END—— Assembly end command

         

This pseudo instruction marks the end of the assembly language , For in END Subsequent assembly instructions are not processed .

An assembler can have multiple ORG Pseudo instruction , But there can only be one END Instructions , Otherwise, the compiler will report an error .

              ORG 0000H    ; After the MCU is reset, it starts from 0000H Take command

              AJMP START   ; Jump to the real start address of the program

              ORG 0003H    ; External interrupt 0 Entry address

              AJMP SER_INT0   ; This instruction is used to jump to an external interrupt 0 Interrupt service routine

              ORG 000BH  ; Timer interrupt entry

              AJMP SER_TIMER0 ; Jump to timer T0 Service interruption procedure of

             ………………..

              ORG 0030H

   START： …………  ; Beginning of program

                …………

   SER_INT0:

                …………

              RETI      ; Used for interrupt return

   SER_TIMER0:

                …………

              RETI      ; Used for interrupt return

              END       ; Assembly instruction end flag

（3）EQU—— Assignment instruction

     Format ： Character name    EQU   Numbers or specific symbols

     function ： Assign a constant or a specific symbol to a specified character name . When the assembler encounters EQU After the character name in front , I will use EQU The number behind （ Or specific symbols ） Instead of .

         for example ： CC EQU R1

                  DD EQU #11H

                  MOV CC,DD ; This instruction is related to MOV R1,#11H Agreement .

Be careful ： The character name is not a label , There is no need for “：”.

（4）DB—— Byte definition pseudo instruction

• Format ：
    label ：DB  Data item      ; Data items are separated by commas

• function ：
  From that address , Define a string in the program memory
  byte
  unit , And assign values with data items .

         example ：       ORG 0400H

           

 TAB：DB 80H,95H,74H

      After compilation , In program memory ：（0040H）=80H,（0041H）=95H,（0042H）=74H.

• This instruction is often placed at the end of the program , Used to open tables .

（5）DW—— Define word command

• Format ：
  label ： DW  Data item

• function ：
  From that address , Define a string in the program memory
  Word unit
  , And assign values with data items .
  Save high byte first , Save low byte after
  , namely ：
  Put the high byte in the low address , Put the low byte in the high address
  .

           example ：   ORG 0080H

          TAB：DW 08H,7799H,1234H

        After compilation , In program memory ：

        （0080H）=00H,（0081H）=08H,

        （0082H）=77H,（0083H）=99H,

        （0084H）=12H,（0085H）=34H.

（6）DS—— Define storage space instructions

• Format ：
  DS　 expression

• function ：
  Start with the instruction address , Retain DS The storage unit specified by the value of the expression , For later use .

     example ：   ORG 0090H

             DS  5

             DB 50H

  After compilation , from 0090H Start keeping 5 Storage unit , and （0095H）=50H

（7）BIT—— Bit address symbol command

• Format ：
    Character name   BIT  Bit address

• function ：
  Assign the bit address to the specified character name .

        example    

LEFT    BIT  P1.0

              RIGHT BIT  P1.1