commentary about extern register
R=rsc
DELTA=16 (12 added, 0 deleted, 4 changed)
OCL=33030
CL=33035
diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 88e5161..c9b6a26 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -64,10 +64,22 @@
typedef struct Hchan Hchan;
/*
- * per cpu declaration
+ * per-cpu declaration.
+ * "extern register" is a special storage class implemented by 6c, 8c, etc.
+ * on machines with lots of registers, it allocates a register that will not be
+ * used in generated code. on the x86, it allocates a slot indexed by a
+ * segment register.
+ *
+ * amd64: allocated downwards from R15
+ * x86: allocated upwards from 0(FS)
+ * arm: allocated upwards from R9
+ *
+ * every C file linked into a Go program must include runtime.h
+ * so that the C compiler knows to avoid other uses of these registers.
+ * the Go compilers know to avoid them.
*/
-extern register G* g; // R15
-extern register M* m; // R14
+extern register G* g;
+extern register M* m;
/*
* defined constants
