Most modern programmers assume that “call” implies a stack but calling is older than the stack. I think the stack was an pure win for most language designs. The stack is more flexible than what went before, and more efficient. Keykos chose the older Fortran paradigm except what was called was not the routine, but a dynamically created object. The Keykos mechanism is described here. An early Keykos design decision was to remove implicit storage allocation from the foundations, which was the kernel.