Difference between instructions, pseudo-instructions and assembly directives

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Instruction: 
No need to say more. It is a kind of mnemonic which can be mapped into binary code directly by translating opcode and operand.

Pseudo-instruction: 
Pseudo-instruction is not a real instruction related to ISA(Instruction Set Architecture). It exists in assembler which can produce corresponding real instructions after assembling. For example, LDR R0, = immediate is a pseudo-instruction. It may produce one MOV or several instructions which depends on the size of the immediate.
PS: LDR R0, [indirect address] is not a pseudo-instruction. It is a real instruction in ISA of ARM.
Pseudo instructions are essentially pre-defined macros, so you can implement your own pseudo instructions if you want.

Directive: 
Also not a real instruction, and only exists in assembler. But after assembling, it does not produce any code. It just indicates the assembler how to configure the processor, for example: changing modes(ARM or Thumb-2), code location (AREA NAME, CODE(DATA), READONLY(READWRITE))
So in 8051, DB, DS, EQU, DSEG, CSEG and others similar are all direcitives

Reference:http://www.embeddedrelated.com/usenet/embedded/show/25889-1.php, accessed on Dec 8th, 2011

Comments

  1. Unknown15 June 2016 at 12:36

    (Y)

    ReplyDelete
  2. Muhammed Yasinhan Yaşar1 August 2021 at 18:35

    yeah, thanks for explanation but, I want to ask that, Are they assembled before the actual instructions? for example:
    ```
    LEA R0 , DAYS
    ADD R3 , R3 , x0
    LOOP
    BRz DISPLAY
    ADD R0 , R0 , #10
    ADD R3 , R3 , #−1
    BR LOOP
    DISPLAY PUTS
    ...
    HALT
    ...
    DAYS
    .STRINGZ ” Sunday
    ”
    .STRINGZ ” Monday
    ”
    .STRINGZ ” T u e s d a y ”
    .STRINGZ ” Wednesday ”
    .STRINGZ ” T h u r s d a y ”
    .STRINGZ ” F r i d a y ”
    .STRINGZ ” S a t u r d a y ”
    ```

    in this example we had to know
    where is the address of 'DAYS' to run LEA R0, DAYS
    So there are so much work for the assembler right?

    ReplyDelete

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