Virtual Assembler Segments

["Dresner, Norman A." <norman_a_dresner@md.northgrum.com>]

In the Microsoft assembler MASM (or Borland's TASM), I can create a
"virtual" segment (see example below) to specify the complete layout of a
section of memory and to specify exactly what it's addresses are, but no
data is created in the program.  This is  commonly used with memory-mapped
I/O cards where the segment describes the size and location of all of the
registers and any RAM on the card.  An assembly-module with this segment can
be linked with normal .ASM and .C modules in which public symbols declared
in the "virtual" segment appear as external addresses in the code.

I know that I can construct a .C module with a list of explicit declarations
of pointers to the known addresses which can then be dereferenced in code,
but the "virtual" segment technique is more efficient because it creates
addresses which the linker resolves and saves dereferencing time for every
access to the data in these structures.

Can I do this in an assembler which is compatible with gcc?  If so, which
one(s)?
(Does such a capability exist within gcc itself?

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EXAMPLE

			PUBLIC	ByteData,  WordDatum
		IOSpace	SEGMENT	AT	0xD000
				ORG		0x1234
		ByteData	LABEL	BYTE
				DB		24	DUP	(?)
				ORG		0xBEEF
		WordDatum	LABEL	WORD
				DW		3
		IOSpace	ENDS

when I link this module with normal .C code which has declared
	extern		char	ASDF[];
	extern		short	QWERTY
when I reference QWERTY, the linker will insert 0xD000:0xBEEF into the
executable with no pointer-dereferencing; similarly, ByteData appears at
0xD000:0x1234.

------------------------------------------------------------------

	Norm Dresner
	Fellow Systems Engineer
	Radar Systems Engineering Department
	Electronic Systems and Sensors Segment
	Northrop Grumman Corporation
	MS 520
	Box 746
	Baltimore MD 21203

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