DAS ASSEMBLY DOCUMENTATION


			      Matthew Dillon
			      891 Regal Rd.
			      Berkeley, Ca. 94708
			      USA

			      uunet.uu.net!overload!dillon



    DAS asmfile [-o objectfile]

    DAS is a minimal 68000 assembler designed to assemble the output of DC1

    DAS SHOULD NOT BE USED FOR ASSEMBLY PROJECTS, IT IS MEANT SOLELY TO DEAL
    WITH THE OUTPUT FROM THE COMPILER.

    NOTE:   Das does not implement all instructions and doesn't do the right
	    thing with MOVEM sea,regs when sea required a word or more of
	    extension.

	    Das cannot handle expressions other than simple + and - where
	    only one label in the expression is allowed to be an external
	    reference.

	    Das does not understand integral types other than decimal and hex.

	    Das does not understand strings 'string'.

	    Das cannot export EQU labels (i.e. ABS references).

				    FEATURES

    Das includes optimization features used by the compiler.  Also, Das can
    handle arbitrary forward referenced REG labels for MOVEM which most
    assemblers cannot.

    Das allows and automatically coagulates multiple occurances of section
    directives of the same name and type.

    Das automatically pads CODE sections with NOPs instead of 0's.  However,
    DS.L will always pad with 0.


    DAS handles the following directives:

	DC		declare data
	DS		declare space / align

	SECTION 	declare a section.  Sections may be repeated to flip
			back and forth between them.  A third argument
			may contain a value to be ORd with the hunk
			type field, usually used to specify special
			hunk flags (i.e. hunk to chip memory, etc...)

	XREF		import a label
	XDEF		export a label
	EQU		declare a constant (must be a numerical constant)
	REG		declare zero or more registers in register list

	PROCSTART	used to determine procedure boundries for A5 check/
	PROCEND 	link/unlk removal.  Also automatically sets optimization
			option to 1 if 0.

    DAS handles the following expressions:

	+, -, negation.  Only one relocatable label allowed per expression.
	Any number of EQU'd labels allowed, forward or reverse referenced.

    Optimization level 0 (default unless PROCSTART exists in file)

	* MOVEM's using 'reg' labels are optimized to either a MOVE or by
	  removing the MOVEM entirely (reg labels that specify no registers).

	* BRANCHES to BRAs are optimized (up to 20 hops)
	* unsized BRANCHES are optimized to byte branches if possible
	* BRANCHES to the next instruction are deleted

    Optimization level 1  (automatic if PROCSTART exists in file)

	* LINK/UNLK within PROCSTART/PROCEND bounds is removed if the link
	  register (currently hardwired to be A5) is never referenced AND if
	  no subroutine calls are made within the procedure (JSR's or BSR's)

	  This allows support of stack backtraces while optimizing most low
	  level routines written in C.	Removal of LINK/UNLK in routines that
	  make other subroutine calls does not yield much of an efficiency
	  boost anyway.

				    PROBLEMS

    DAS Uses an incredible amount of memory due to efficiency considerations
    in the optimization and myself wanting to keep it FAST.

    When specifying DIFFERENTLY NAMED CODE SECTIONS in the SAME ASSEMBLY FILE,
    keep in mind that the code sections are not necessarily near to each other
    in the final link and the linker cannot generate a jump table for
    PC-Relative inter-module calls.  Thus, one section calling another should
    not use PC-Relative unless you *know* your final executable is going to
    be less than 32K *AND* you do not use the -frag option to dlink.

    This is why the startup c.a references the autoinit sections with
    absolute calls instead of PC-relative calls.