Purpose of the M-bit and FRAME DATUM Fields (66579)

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The M-bit field in the fixup field of the FIXUPP record is described in the "MS-DOS Encyclopedia." Its value determines if the linker should calculate the address from the start of the segment in question (segment-relative), or from the current location in the code/data (self-relative). The following is an example:

 MYSEG Segment

       -->      +-----------------+     <-- Start of segment
      |         |                 |
      |         |                 |
      |         |                 |
      |         |                 |
      |    -->  +-----------------+     <-- Reference to symbol
      |   |     |                 |
          |     |                 |
     (A)  |     |                 |
                /                 \ 
      |  (B)    \                 / 
      |         /                 \ 
      |   |     \                 / 
      |   |     |                 |
      |   |     |                 |
       --> -->  +-----------------+     <-- Symbol
                /                 \ 
                \                 / 
                /                 \ 
                \                 / 
                +-----------------+     <-- End of segment
				

In this example, (A) is the segment-relative address and (B) is the self- relative address. The self-relative calculation is used when you want to jump forward a certain number of bytes in the code (for example, a short jump). The calculation is fairly straightforward. Calculate the segment- relative address for the symbol. Subtract the segment-relative address of the reference, and you have the self-relative address for the symbol from the reference.

The FRAME DATUM field is used in almost exactly the same manner. It might seem that the fields TARGET DATUM and TARGET DISPLACEMENT are sufficient to fix up references in an LEDATA or LIDATA record. To understand why it is required, see the following example:

Microsoft C/C++ compilers (16-bit) generate a group called DGROUP that holds data segments _NULL, _DATA, _CONST, and so forth. The fixups that it generates are similar to the following: For the average MS-DOS program, this is fine. We could, eliminate the FRAME DATUM field and change target to DGROUP, right? Wrong. Examine ROMable code; they use the same OBJ. In that case, the linker must know if a reference to _CONST is a far reference (based on DGROUP) or a near reference (based on CONST). If we only told it to use DGROUP, it would have no idea how to resolve the references to _CONST after it moved _CONST to another segment (for example, ROM based). If we left in the target _CONST and removed the frame DGROUP, we would have the wrong address for MS-DOS, right? Hence, FRAME DATUM *is* important.

The way the FRAME DATUM field affects the fixup makes sense, given the example above for the M-bit. Just calculate the offset based on the TARGET segment and add in the offset from the FRAME.