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

As to the historic rationale, I can't say for certain (because I didn't design them). My thoughts on the matter are that early CPUs got their original program counter set to 0 and it was a natural desire to start the stack at the other end and grow downwards, since their code naturally grows upward.

As an aside, note that this setting of the program counter to 0 on reset is not the case for all early CPUs. For example, the Motorola 6809 would fetch the program counter from addresses 0xfffe/f so you could start running at an arbitrary location, depending on what was supplied at that address (usually, but by no means limited to, ROM).

One of the first things some historical systems would do would be to scan memory from the top until it found a location that would read back the same value written, so that it would know the actual RAM installed (e.g., a z80 with 64K address space didn't necessarily have 64K or RAM, in fact 64K would have been massive in my early days). Once it found the top actual address, it would set the stack pointer appropriately and could then start calling subroutines. This scanning would generally be done by the CPU running code in ROM as part of start-up.

With regard to the stacks growth, not all of them grow downwards, see this answer for details.

Answer 2

One good explanation I heard was that some machines in the past could only have unsigned offsets, so you'd want to the stack to grow downward so you could hit your locals without having to lose the extra instruction to fake a negative offset.

Answer 3

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 4

IIRC the stack grows downwards because the heap grows upwards. It could have been the other way around.

Answer 5

Stanley Mazor (4004 and 8080 architect) explains how stack growth direction was chosen for 8080 (and eventually for 8086) in "Intel Microprocessors: 8008 to 8086":

The stack pointer was chosen to run "downhill" (with the stack advancing toward lower memory) to simplify indexing into the stack from the user's program (positive indexing) and to simplify displaying the contents of the stack from a front panel.

Answer 6

I believe it's purely a design decision. Not all of them grow downward -- see this SO thread for some good discussion on the direction of stack growth on different architectures.

Answer 7

I believe the convention began with the IBM 704 and its infamous "decrement register". Modern speech would call it an offset field of the instruction, but the point is they went down, not up.

Answer 8

Just 2c more:

Beyond all the historic rationale mentioned, I'm quite certain there's no reason which is valid in modern processors. All processors can take signed offsets, and maximizing the heap/stack distance is rather moot ever since we started dealing with multiple threads.

I personally consider this a security design flaw. If, say, the designers of the x64 architecture would have reversed the stack growth direction, most stack buffer overflows would have been eliminated - which is kind of a big deal. (since strings grow upward).

Answer 9

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.