SDCC and malloc() - allocating much less memory than is available

Question

When I run compile this code with SDCC 3.1.0, and run it on an Amstrad CPC 464 (under emulation, with WinCPC 0.9.26 running on Wine):

void _test_malloc()
{
  long idx = 0;
  while (1)
    {
      if (malloc(5))
    {
      printf("%ld\r\n", ++idx);
    }
      else
    {
      printf("done");
      break;
    }
  }
}

... it consistently taps out at 92 malloc()s. I make that 460 bytes, which leads me to a couple of questions:

• What is malloc() doing on this system? I was sort of hoping for an order of magnitude more storage even on a 64kB system

• The behaviour is consistent on 64kB systems and 128kB systems; do I have to perform some sort of magic to access the additional memory, like manual bank switching?

Answer 1

The answer is that, on Z80 systems, heap size is hard-coded to 1kB. Maarten Brock answered this on the sdcc-user mailing list:

Hello Duncan,

You have to create the heap yourself if the standard 1kB is not enough. Copy heap.s into your project dir and modify it to create your preferred size. Then assemble it and link with your project.

Unlike the mcs51 heap which is defined in _heap.c this is not documented for Z80 in the manual. Feel free to request a documentation update or merge of _heap.c and heap.s in the tracker system.

Maarten

Answer 2

In fact, as Duncan Bayne says, there is a very narrow heap space in the default memory manager that SDCC implements for Z80.

However, before trying to modify SDCC's heap, you should consider if you actually need dynamic memory on an Amstrad CPC. Generally, there is no point on using dynamic memory when you run a stand-alone application, which owns the entire hardware. You can test and know how much memory you have, and you can directly write to memory wherever you wanted. There is no memory protection, and no other applications running on background.

Therefore, is much preferable for you to design your own memory map (where you want your data to be and how much space to use) and then directly manage the memory. Moreover, code optimization is much important in this machine, and manually managing memory is extremelly relevant for optimization.

If your code is running directly in the Amstrad CPC (i.e. not using a modern OS like Symbos), you have to manually deal with bank switching to access memory. Z80 CPU has a 16-bits bus, which can only address 64KB of memory without bank switching.