the function calling convention

%ECX is used for a count of function argument words, represented as a
fixnum, so it can also be thought of as a count of function argument
bytes.

The first three arguments are stored in registers. The remaining
arguments are stored on the stack.

The comments at the head of DEFINE-VOP (MORE-ARG) explain that
;;; More args are stored contiguously on the stack, starting immediately at the
;;; context pointer. The context pointer is not typed, so the lowtag is 0.

?? Once we switch into more-arg arrangement, %ecx no longer seems to be 
   used for argument count (judging from my walkthrough of kw arg parsing
   code while troubleshooting cold boot problems)