Defining absolute sections in an assembly source file

An absolute section is defined using the ORG directive. In that case, the Macro Assembler generates a pseudo section, whose name is "ORG_<index>", where index is an integer which is incremented each time an absolute section is encountered, as listed in the following listing:

Listing: Defining an absolute section containing data


       ORG   $800    ; Absolute data section.
var:   DS.   1

       ORG   $A00    ; Absolute constant data section.

cst1:  DC.B  $A6

cst2:  DC.B  $BC

In the previous portion of code, the label cst1 is located at address $A00, and label cst2 is located at address $A01.

Listing: Assembler Output


1    1                       ORG   $800
2    2  a000800       var:   DS.B  1

3    3                       ORG   $A00

4    4  a000A00 A6    cst1:  DC.B  $A6

5    5  a000A01 BC    cst2:  DC.B  $BC

Locate program assembly source code in a separate absolute section, as listed in the following listing:

Listing: Defining an absolute section containing code


        XDEF  entry
        ORG   $C00 ; Absolute code section.

entry:

        LDA   cst1 ; Load value in cst1

        ADD   cst2 ; Add value in cst2

        STA   var  ; Store in var

        BRA   entry

In the portion of assembly code above, the LDA instruction is located at address $C00, and the ADD instruction is at address $C03. See the following listing:.

Listing: Assembler Output


    8    8                              ORG   $C00 ; Absolute code
    9    9                       entry:

   10   10  a000C00 C6 0A00             LDA   cst1 ; Load value

   11   11  a000C03 CB 0A01             ADD   cst2 ; Add value

   12   12  a000C06 C7 0800             STA   var  ; Store in var

   13   13  a000C09 20F5                BRA   entry

   14   14

In order to avoid problems during linking or execution from an application, an assembly file should at least:

Initialize the stack pointer if the stack is used.
The RSP instruction can be used to initialize the stack pointer to $FF.
Publish the application's entry point using XDEF.
The programmer should ensure that the addresses specified in the source files are valid addresses for the MCU being used.