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Catching NULL Pointer Assignments

The nastiest type of bug results from the use of uninitialized or corrupted pointers. The most frequent incorrect value of a pointer is NULL (0). The simplest method to catch these errors is to make the first page of memory inaccessible. Of course, paging must be used to achieve this.

The simplest method is to define a region of 4k size at address 0 and give it NoAccess access rights.

Example:

Region  FirstPage  0     4k  RAM  NoAccess 
Region  LowMem     4k  636k  RAM  Assign 
...

This will guarantee that all references to address 0 will trigger an exception at run-time.

If you want to avoid wasting a whole page of memory, you could just remap it:

Region   FirstPage  0     4k  RAM  Assign 
Region   LowMem     4k  636k  RAM  Assign 
Region   HighMem    1M    3M  RAM  Assign 
FillRAM  HighMem 
...

If you don't allocate anything to region FirstPage, RTLoc will append the physical memory page at address 0 to the end of HighMem, making it available for the heap and stack.

However, you should consider that this approach will destroy the real mode interrupt vector table and the real mode BIOS data area. This is usually no problem, but if your program needs to look up some information in this area at run time, you can't use this approach. Instead, region FirstPage would need ReadOnly or ReadWrite access.

Advanced Topics

Installing Hardware Interrupt Handlers

Catching Stack Overflows