The 15mar93jfb release moves Scheme->C to additional platforms that
differ from the previous platforms in that they may not have UNIX-like
operating systems nor assume that sizeof(int) and sizeof( int* ) is 32.

Users on existing platforms will see a few bugs fixed, but otherwise the
system looks pretty much the same.  If you were one of the few users of
SAVE-HEAP or OPEN-PROCESS, you'll be disappointed as those procedures have
been dropped as too operating system dependent.

Users who have done ports to other platforms will find mixed news.  While
the source has been reorganized to centralize machine specific items and
provide a system independent interface layer between Scheme->C and the
host system, you're old patches won't drop right in to the new source.
However, new ports should be a lot easier, and ports to non-UNIX like systems
are now much easier.

Finally, the system may now be configured as a Scheme interpreter (perhaps
with your extensions) that can be safely embedded inside an event driven
system.  The embedded server does not require operating system signals nor an
I/O system.  It supports time slicing and checks for stack overflow so
it is safe to evaluate: (let loop () (loop)) or  (let loop () (loop) #t)
within the server.

N.B.  Making Scheme->C more portable has required changes in the runtime
      environment.  It is not possible to mix code compiled by earlier
      versions of the compiler with code compiled by this compiler.


Documentation changes since the 01nov91jfb release:

1.  Added PostScript for R4RS.

2.  Updated index to reflect changes.

3.  Added the document "Embedded Scheme->C" explaining how to embed
    Scheme in an event sensative server.


Compiler changes since the 01nov91jfb release:

1.  Added the syntax (define-external variable TOP-LEVEL module) to define
    a variable that is addressed via the top level that lives in a known
    module.

2.  Change LETREC macro expansion to enclose the body in a BEGIN to prevent
    incorrect evaluation of internal defines.

3.  Change LISTIFY-OPTIONAL-ARGUMENTS to not allocate a temporary when the
    list is composed of a single item.

4.  Constants defined by DEFINE-CONSTANT are now defined for use in compile
    time expressions.

5.  When a module form is not supplied, the module name defaults to the source
    file name (less the .sc suffix).

6.  Forms like:

	(let ()
  	  (define x 1)
  	  (set! foo 2) )
	(let ()
	  (define x 3)
	  (set! bar 4) )

    containging let's whose only purpose is to introduce a new lexical scope
    are now correctly compiled when they appear at the top level.

7.  Lambda expressions containing arguments that are not symbols now result
    in an error message rather than a compiler error.

8.  Compiled code now supports optional stack overflow checks and timeslicing.

9.  A number of changes were made in the emitted C code to handle architecture
    and C compiler differences across the platforms now supported.  For
    example, 32-bit int's are no longer assumed, nor are 32-bit pointers.

10. External procedure declarations now declare the type of their arguments
    when the C compiler allows it.

11. The compiler no longer uses a saved heap image.  This was eliminated to
    make the compiler more portable and allow one compiler to compile for a
    variety of configurations.

12. The compiler no longer has any direct calls to the C runtime environment.

13. When a module name is not defined, the module name defaults to the source
    file name (less the ".sc" suffix).


Runtime changes since the 01nov91jfb release:

1.  If the string passed to SYMBOL->STRING and SYMBOL->UNINTERNED-STRING is
    later mutated, it does not effect the symbol.

2.  Zero argument values no longer cause GCD to trap.

3.  NUMBER->STRING does not return radix prefix in the string.

4.  Change the expansion of LETREC to correctly handle internal defines.
    Previously:

	(letrec ((x 1)) (define x 2) x)
	= ((LAMBDA (X) ((LAMBDA (X) (SET! X 2) (SET! X 1) X) 0)) 0)
	=> 1

    Now:

	(expand '(letrec ((x 1)) (define x 2) x))
	= ((LAMBDA (X) (SET! X 1) (BEGIN ((LAMBDA (X) (SET! X 2) X) 0))) 0)
	=> 2

5.  LOAD, LOADE, and LOADQ no longer cause READ-EVAL-PRINT to assume Scheme
    is interactive.

6.  Correct token size computation when output to a port is being pretty
    printed.

7.  Correct comment on getmem in scinit.c.

8.  EVAL now verifies that the optional environment is an a-list.

9.  Added DEBUG-OUTPUT-PORT, TRACE-OUTPUT-PORT, WEAK-CONS, RENAME-FILE,
    REMOVE-FILE, additional C and Scheme structure access procedures,
    procedures to set/get the time slice, stack size, and garbage
    collector parameters, and TIME-OF-DAY.

10. Deleted OPEN-PROCESS, OPEN-INPUT-PROCESS, OPEN-OUTPUT-PROCESS, SAVE-HEAP,
    and MY-RUSAGE as they were too system specific.

11. The system may be easily compiled to build an interpreter that can be
    embedded inside an event driven server.  When compiled this way, it does
    not require access to operating system signals or an I/O system.  The
    embedded server supports time slicing and checks for stack overflow so
    it is safe to evaluate the following:  (let loop () (loop)) and
    (let loop () (loop) #t).

12. The bulk of the operating system specific interfaces were moved into
    cio.c.  This has resulted to significant changes to heap.c and scinit.c.
    There are now no direct calls to the C runtime environment from Scheme
    code.  All access is via routines in cio.c.

13. The default maximum heap size is now five times the initial heap size.

14. The file names scexpand1.sc and scexpand2.sc were changed to scexpnd1.sc
    and scexpnd2.sc to be compatible with MS-DOS.

15. WHEN-UNREFERENCED has been recoded to reduce the amount of debris left
    on the stack that used to cause objects to be unnecessarily retained.

16. Code moved from scrt5.sc to scrt6.sc to fit in MS Windows' code segment
    limitations.

17. Added a dummy module scrtuser.sc to as an aid to inserting additional
    user code.

18. The compiler no longer generates direct calls to
    call-with-current-continuation.  As a result, user code may redefine
    it without having to change the underlying Scheme->C system.

19. Added a primitive record type, %record, that users can use to implement
    their desired record system.

20. Control-C during a system file task is now correctly handled.


C declaration compiler changes since the 01nov91jfb release:

1.  Added a new declaration type "sizeof" that defines the size and alignment
    characteristics of the base C types.  Alignment information is used to
    generate correct access procedures.

2.  Added the C program "sizeof" that creates sizeof declarations for a the
    host system.


Xlib changes since the 01nov91jfb release:

1.  Add support for some procedures added in X11R4.

2.  Module initialization problems for a few structure access modules were
    corrected.

3.  Do not assume that long int's and int's are the same size.

4.  Remove assumptions about base C type sizes from xwss.sc.

4/28/2008: Changed to MIT license and HP Copyright.
