Currently `sp` requires the user to know the whether stack grows downwards or upwards, but for a specific architecture/platform, you have to write an asm for it, and you know how the stack grows on this architecture/platform, don't you?

Or are those asms independent of how the stack grows?

If the asm knows how the stack grows, you can offset the `sp` internally, leave it transparent to the user.

stackallocator using malloc():
        void * limit = std::malloc( size);
        if ( ! limit) throw std::bad_alloc();
        ctx.size = size;
        ctx.sp = static_cast< char * >( limit) + ctx.size;

if limit is at x and you allocated 64byte of memory for the stack you have to adjust your stack base

for downward growing stacks to address y=x+64, e.g. starting at y your stack has 64bytes to grow.

So how about upward stack, is it x or (x-1)?