6502 MCU Programming--Pseudo Instruction 2

** Macro Instructions ** What makes me extremely excited is that 6502_Macroassembler & Simulator has powerful macro functions . This undoubtedly makes program writing much simpler and more convenient . Let's talk about the format of macros first . Macro name : .MACRO  macro virtual parameter 1, macro virtual parameter 2, macro virtual parameter 3..... macro body.ENDM Here , .MACRO and .ENDM must appear in pairs . The format of macro calls : macro name macro actual parameter 1, macro actual parameter 2, macro parameter 3 When the source program with macro calls is compiled , each macro call will be expanded by the compiler . Macro expansion is actually the macro body designed during macro definition to replace the corresponding macro instruction name , and the actual parameters are used to replace the virtual parameters to form actual code that meets the function and can be executed. Let's give an example : Implement the interrupt instruction INT. As we all know , the current 6502 compiler does not support the INT instruction , which causes inconvenience to writing , but now we can use macros to implement it . ORG $0000 INT: .MACRO INT_PARAM .DB $00 .DW INT_PARAM .ENDM indicates that INT is the macro name . Note that it should be followed by : , INT_PARAM is a virtual parameter . Since the machine code of INT is 00, we .DB $00 in the macro body . For example , INT $C008 has a machine code of 00 08 C0, so we .DW INT_PARAM. In this way , we implement the INT instruction . For example, if we input INT $C008 in the program , it has the same function as INT $C008 in Wenquxing . Let me give you another example . In 6502 , there is no instruction to directly push or pop the stack with register X. Here we use macros to implement it . ORG $0000 PHX: .MACRO TXA PHA .ENDM PLX: .MACRO PLA TAX .ENDM MAIN: PHX ... PLX RTS See , we can use PHX and PLX in the program . Although macros have a lot of functions , we cannot use them indiscriminately , because macros are different from subroutines . Subroutines are called by JSR and returned by RTS , so the machine code of the subroutine after assembly only occupies one program segment , no matter how many times it is called , which saves memory . Each time a macro instruction is called , it occupies a program segment. The more times it is called , the more memory is occupied . Therefore, in terms of overhead , subroutines are better than macro instructions . However, from the perspective of program execution time , each time a subroutine is called, the return address must be protected and restored , which takes some time. Macro instruction calls do not require this process .
   

   

     
    







   

   
   
   
   
    
   

    
    
   

    
    
   

    
    
    

   The execution time is shorter , so from the execution time point of view , macro instructions are better than subroutines . Therefore , when a program segment that needs to be accessed multiple times is long and the number of accesses is not too many , it is better to use a subroutine ; when a program segment that needs to be accessed multiple times is short and the number of accesses is very frequent , it is better to use a macro instruction .