A Stack is a very efficient way of allocating and deallocating memory, but it works only with a restricted pattern of usage. Garbage collection is completely flexible but comparatively costly. The implementation of powerful control structures naturally uses memory which usually fits in with stack allocation but must have the flexibility to do otherwise from time to time. How can we manage memory which only once in a while violates stack restrictions, without paying a price the rest of the time? This paper provides an extremely simple way of doing so, in which only the part of the system which actually uses the stack needs to know anything about the stack. We call them Phantom Stacks because they are liable to vanish if subjected to close scrutiny. Phantom Stacks will be used in the next version of the Artificial Intelligence Lab's Scheme microprocessor chip.
