Fix BYTES_HERE MACRO on MASM 6.1

[640-KB]

BYTES_HERE MACRO does not exit with error if a fixed ORG is overrun as it does on MASM 5.0.

@csoren

[640-KB]

To add more context, the objective is to have a way to ensure that code changes or additions in code do not overrun into adjacent to BIOS CSM fixed offsets. Being a single file project, it's possible for a MACRO to evaluate the offset immediately before an ORG statement and compare to ensure that the code has not already exceeded it. GLaBIOS does this with the BYTES_HERE MACRO, unfortunately this only works on MASM 5. For example:

ORG 100H

0100   MOV  AX, 1234H
0103   ADD  AX, 1
0106   RET

; location is now 0107H

ORG 106H   ; this next code exists at this fixed offset per CSM specification

; location is now 0106H

0106   MOV BX, 4321H

; the previous RET has now been overwritten leaving the exit path of the first code section broken.

The issue arises from the differences in how and when MACRO expressions are evaluated, as mentioned in the MASM 6.1 README.txt file:

     Span-Dependent Expressions used in Macros
     -----------------------------------------
     MASM 6.1x evaluates macro expressions only on the first pass of 
     assembly, but code and data are reevaluated on subsequent passes.
     Because of this, macro expressions which depend on the span between
     two addresses may not evaluate correctly. For instance, the
     following code will not evaluate correctly:

	  Label1:
	       JMP Label2
	  Label2:

	  REPEAT Label2 - Label1  ; Evaluates incorrectly
	       INC AX
	  END

     View the listing file to determine if a questionable macro expression
     was evaluated as desired.

Ultimately, MACRO evaluations of labels only occur once during the first pass and so subsequent offset changes are not reflected and so conditional logic against their final locations cannot be done.

Help or suggestions for a more elegant solution for how this can be verified under MASM 6 would be most appreciated!