registers:
	IP	VM instruction pointer
	SP	VM stack pointer (top down)
	LP	VM local variables base pointer
	EP	VM exception catch block pointer
	TOP	VM top stack register (void, int, long, float, address)

note:
	a LP is not strictly necessary. if the compiler keeps track of the
	temporary arguments on the stack, then the SP can be used to access local variables.

call proc:
	1st arg is in TOP (void, int, long, float, address)
	2nd++ args are on stack (rtl)
	push IP on stack
	load IP with proc address

start of proc:
	push EP on stack
	push LP on stack
	load LP with current SP
	push TOP on stack (void, int, long, float, address)
	lower SP to make room for local variables

end of proc:
	load SP with current LP
	pop LP
	pop EP
	pop IP

try:
	push "IP": address of catch block
	push EP
	push LP
	load EP with current SP

try end:
	load SP with current EP  (or drop)
	pop LP					 (or drop)
	pop EP
	drop "IP"

throw:
	load TOP with exception (object)
	load SP with current EP
	pop LP
	pop EP
	pop IP

catch:
	LSET or push TOP (exception object)