2.4. Checking for Stack Overflow

The compiler requires a stack to support procedure calls. Each stack frame contains the link address, a pointer to the previous frame, and any local data that has not been allocated a register and is not static.

Usually Coral 66 programs require very little stack space, and the default size of 64K (as set in the linker script file) is more than enough. However, programs that include recursive procedures may require large numbers of stack frames, and each frame will contain any non-preset arrays and tables declared locally.

Use the compile-time option -fstack-check to get the compiler to generate additional code each time a stack allocation is made. The stack limit, which is the lowest address in the stack, is held in a global variable. Here is an example that overflows the 64K byte main program stack:

$ more biggy.cor
'begin'
   'recursive' Biggy;
   'begin'
      'byte''array' S [1 : 100000];
   'end' Biggy;

   Biggy;
'end'

The generated code for the recursive procedure Biggy is as follows. Note that if Biggy were a procedure and not recursive then the large array would be allocated statically and not on the stack. In this case stack overflow would not be relevant.

$ m68k-coff-gcc -Wa,-a -c biggy.cor
...
   6                    Biggy:
   7 0000 4E56 0000             link.w  %a6,#0
   8 0004 207C FFFE             move.l  #-100000,%a0
   8      7960 
   9 000a D1CF                  add.l   %sp,%a0
  10 000c B1F9 0000             cmpa.l  _stack_limit,%a0
  10      0000 
  11 0012 6D00 0002             blt     .+4
  12 0016 4E49                  trap    #9
  13 0018 4E5E                  unlk    %a6
  14 001a 4E75                  rts
...

The program will fail at run time having raised trap 9. You may attach a handler to trap 9 and take appropriate action.

$ m68k-coff-run biggy
Stopped at PC = 16#000006DE#: Exit code = 256
Unhandled interrupt 0x29: TRAP #9 Instruction Vector (Stack Overflow)
art0.S:521