Update information for how to port to a new architecture
diff --git a/klibc/arch/README b/klibc/arch/README
index b97aaca..eceb23d 100644
--- a/klibc/arch/README
+++ b/klibc/arch/README
@@ -1,8 +1,16 @@
-Each architecture needs a crt0.S file. The crt0.S assembly routine
-typically corresponds to the following pseudo-C code. In addition,
-each architecture needs any support routines that gcc-generated code
-expects to find in the system library -- Alpha, for example, needs
-divide subroutines.
+To port klibc to a new architecture, you need:
+
+a) A directory structure
+
+Each archtecture has an arch/ directory, which should include an
+MCONFIG and a Makefile.inc file.
+
+b) Startup code (arch/*/crt0.S)
+
+The crt0.S assembly routine typically corresponds to the following
+pseudo-C code. In addition, each architecture needs any support
+routines that gcc-generated code expects to find in the system library
+-- Alpha, for example, needs divide subroutines.
The "getenvtest" test program is a very good test for proper crt0.S
functionality.
@@ -22,5 +30,38 @@
}
+c) A setenv implementation (arch/*/setjmp.S, arch/*/include/klibc/archsetjmp.h)
-
+On most (but not all!) architectures, this entails creating a setjmp
+buffer big enough to hold all callee-saved registers, plus the stack
+pointer and the return address. In setjmp.S you have:
+
+* A "setjmp" function that writes out the callee-saved registers, the
+ stack pointer and the return address to the buffer pointed to by the
+ first argument, and then returns zero normally.
+
+ On some architectures you need to take some kind of action to make
+ sure the contents of the stack is actually manifest in memory and
+ not cached in the CPU. In some cases (e.g. on SPARC) this will
+ automatically spill the registers onto the stack; then they don't
+ need to be spilled into the jmp_buf.
+
+* A "longjmp" function that read back these same registers from the
+ jmp_buf pointed to by the first argument, and returns the second
+ argument *to the address specified in the jmp_buf*.
+
+ On some architectures you need to take some kind of action to flush
+ any cached stack data or return stack.
+
+
+d) Any support functions needed by gcc, *unless* they are in libgcc
+ *and* libgcc is usable for klibc on your particular platform. If
+ libgcc isn't usable for klibc (on MIPS, for example, libgcc is
+ compiled in a way that is not compatible with klibc) there are
+ reasonably good clones of most of the libgcc functions in the libgcc
+ directory. To use them, add them to ARCHOBJS in
+ arch/*/Makefile.inc.
+
+
+e) A link location for the shared klibc. This should be specified in
+ SHAREDFLAGS in arch/*/MCONFIG.
