Stack growth towards high addresses?

My beliefs are the code portability and the independence from the underlying hardware.

For example, if I am allocating a stack for a task something like:

task_stack = new u32[stacksize]; task_SP = task_stack + stacksize;

Should the SP be always set to the end of stack or there could be a significant number of cases when the task_SP should be set to the beginning?

On another question, is the SP always pre-decremented or there are architectures where SP is post-decremented?

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

What CPUs, except x51, do have the direction of the stack growth from low to high addresses?

VLV

To quote my favorite philosopher, "Everybody should believe in something and I believe that I'll have another drink."

Seems to me that larwe answered very adequately so I'm at a loss as to why you would ask it again.

Yes!

C'mon, Vladimir, your website claims expertise in AD DSPs and hardware. That's not the kind of question one would expect from someone who has read AD's documentation on their 32-bit chips.

Ken Asbury

Well, this is a semantic question; what, exactly, is "significant"? There are relatively few architectures that have a hardware-enforced "directionness" that makes the stack grow up. The RISC architectures I mentioned are neutral, _BUT_ there are ABI definitions, generally adhered to by compilers, that might "encourage" one direction rather than another. Your life will become consummately interesting if you defy this without modifying the compiler, operating system and libraries with which you intend to interoperate.

This issue has been dealt with extensively by the porters of Linux to eldritch architectures.

I can't think of a useful example off the top of my head, but I've got a vague memory tickling the back of my mind.

From the Intel 80x196kb users' guide:

"On the 8096BH, the address is calculated based on the un-updated version of the stack pointer. The 80C196KB uses the updated version. The offset for POP[SP] and POP nn[SP] instructions may need to be changed by a count of 2."

I am currently working with the 8096 and need to treat the stack pointer as post-decremented.

Regards,

Michael

The context is the following: I am developing the small RTOS keeping in mind the portability to the popular architectures. The RTOS is seen as a library module with the minimum dependencies from the tool chain and the target. So, the question is if it can be assumed that the stacks are always from high to low address, pre-decremented, or it should be defined as the configuration or build option.

I can't recall any except x51, that's why I am addressing the question to the club.

Ditto.

And what conclusion did they came to?

VLV

Then you have a logical inconsistency highlighted by merely asking your original question, viz: if you are targeting "popular" architectures, why are you worrying about obscure, rare, alien architectures not known to the majority of engineers or programmers?

And the answer - which you didn't really need to ask in order to know

- is that it is most flexible if you make it configurable, but most people will be satisfied with the assumption of a downgrowing stack.

Are you asking a question? Well, on ARM, it's normally down, PA-RISC it's up, MIPS it's down, PowerPC it's down, but you can implement it the other way on any of those architectures.

eldritch architectures.

Solutions have been found for processors capable of running Linux. The MCS51 etc obviously do not qualify.

For your application, it's non-obvious why you need to know; your RTOS and libraries would presumably conceal most of this information from the caller (e.g. if your caller needs to start a thread and provide stack space, he passes a pointer to the start of the block, and a size parameter, and your OS handles any required arithmetic and housekeeping).

Why woudl the portable part of your RTOS even need to know? Wouldn't the low-level context handling code for a particular target deal with all of that?

If for some reason it does need to know, it should absolutely be configurable.