README.macos

It doesn't work on MacOS X on x86, but it looks like it shouldn't be
hard to port.  Here are the problems I know about:

- The MacOS assembler doesn't support the -a flag that gas does to
  generate a listing file.
- The MacOS assembler has a different format for .section directives,
  and no .bss directive.  I don't know if the semantics of sections
  are the same or not.
- There seems to be some other syntactic incompatibility in the
  assembler that I can't figure out; there are about 3500 "junk
  character valued 95" errors in the file.  This might be spurious.
  The assembler's error messages have the wrong line numbers, which
  makes it unnecessarily difficult to track down.
- To pass arguments to MacOS system calls, according to rumor, you
  push the parameters on the stack, with the system call number last,
  rather than putting them in registers; and then you call int $0x80
  as in Linux.  As documented in /usr/include/sys/syscall.h, the
  system call numbers for the system calls used by Ur-Scheme are the
  same as in Linux: 1 for exit(2), 3 for read(2), 4 for write(2).
  They're probably the same as in 6th Edition Unix from 1976,
  actually.

It's probably actually more work to find a way to tell the compiler
whether to generate MacOS or Linux code than to actually make the
change.

Needless to say, porting to MacOS on any other CPU architecture, such
as PowerPC, is likely to be rather more of a hassle, since it would
involve rewriting all of the assembly code.