Why do stacks typically grow downwards?

Question

I know that in the architectures I'm personally familiar with (x86, 6502, etc), the stack typically grows downwards (i.e. every item pushed onto the stack results in a decremented SP, not an incremented one).

I'm wondering about the historical rationale for this. I know that in a unified address space, it's convenient to start the stack on the opposite end of the data segment (say) so there's only a problem if the two sides collide in the middle. But why does the stack traditionally get the top part? Especially given how this is the opposite of the "conceptual" model?

(And note that in the 6502 architecture, the stack also grows downwards, even though it is bounded to a single 256-byte page, and this direction choice seems arbitrary.)

Answer 1

One possible reason might be that it simplifies alignment. If you place a local variable on the stack which must be placed on a 4-byte boundary, you can simply subtract the size of the object from the stack pointer, and then zero out the two lower bits to get a properly aligned address. If the stack grows upwards, ensuring alignment becomes a bit trickier.

Answer 2

I'm not certain but I did some programming for the VAX/VMS back in the days. I seem to remember one part of memory (the heap??) going up and the stack going down. When the two met, then you were out of memory.