16 bit; 8 bit; other things

1 files changed, 25 insertions, 0 deletions

diff --git a/portability b/portability
index fcb4736..263c9a8 100644
--- a/portability
+++ b/portability
@@ -31,3 +31,28 @@
 	  locate labels based on their names
 
 	  fix up jumps by calling out to 'patch'
+
+
+Notes about x86:
+
+How to call a statically known function (e.g, memcpy() or malloc())
+from JIT compiled code:
+
+- In 64 bit mode:
+
+  The JIT compiled code can end up more than 4GB away from the
+  function, which means a standard call with 32-bit offset is not good
+  enough. Instead we have to do "call RIP_REL(label)", where label
+  refers to a place where the address of the function in question is
+  stored.
+
+  It might be possible to detect at compile time whether the final
+  generated code would be less than 4GB away, which would allow a call
+  rel32 to be used. It gets a little bit tricky though because the
+  rel32 has a different length, and in general we don't know the final
+  address of the jit compiled code until later.
+
+- In 32 bit mode:
+
+  This is easy: Just do <call rel32> and have the linker fix it up
+  before generating the final code.