5.4. ABSOLUTE: Defining Absolute Labels

The ABSOLUTE directive can be thought of as an alternative form of SECTION: it causes the subsequent code to be directed at no physical section, but at the hypothetical section starting at the given absolute address. The only instructions you can use in this mode are the RESB family.

ABSOLUTE is used as follows:

        ABSOLUTE 0x1A
kbuf_chr        resw 1
kbuf_free       resw 1
kbuf            resw 16

This example describes a section of the PC BIOS data area, at segment address 0x40: the above code defines kbuf_chr to be 0x1A, kbuf_free to be 0x1C, and kbuf to be 0x1E.

The user-level form of ABSOLUTE, like that of SECTION, redefines the __SECT__ macro when it is invoked.

STRUC and ENDSTRUC are defined as macros which use ABSOLUTE (and also __SECT__).

ABSOLUTE doesn’t have to take an absolute constant as an argument: it can take an expression (actually, a critical expression: see Section 3.8) and it can be a value in a segment. For example, a TSR can re-use its setup code as run-time BSS like this:

        org 100h                ; it's a .COM program
        jmp setup               ; setup code comes last
        ; the resident part of the TSR goes here
setup:  ; now write the code that installs the TSR here
        absolute setup
runtimevar1 resw 1
runtimevar2 resd 20

This defines some variables on top of the setup code, so that after the setup has finished running, the space it took up can be re-used as data storage for the running TSR. The symbol tsr_end can be used to calculate the total size of the part of the TSR that needs to be made resident.