+ added nil pointer assignment test at the end of program for i8086-msdos

  (works only for small and medium memory model)

git-svn-id: trunk@26888 -

rtl/msdos/prt0stm.asm

@@ -361,6 +361,29 @@ int_number:
         ret 4
 %endif
 
+        global FPC_CHECK_NULLAREA
+FPC_CHECK_NULLAREA:
+%ifdef __TINY__
+        ; tiny model has no nil pointer assignment checking; always return true.
+        mov al, 1
+%else
+        push ds
+        pop es
+        xor di, di
+        mov cx, 32
+        mov al, 1
+        cld
+        repe scasb
+        je .skip
+        dec ax   ; 1 byte shorter than dec al
+.skip:
+%endif
+%ifdef __FAR_CODE__
+        retf
+%else
+        ret
+%endif
+
         segment data
 mem_realloc_err_msg:
         db 'Memory allocation error', 13, 10, '$'

rtl/msdos/system.pp

@@ -116,6 +116,8 @@ procedure MsDos_Carry(var Regs: Registers); external name 'FPC_MSDOS_CARRY';
 procedure InstallInterruptHandlers; external name 'FPC_INSTALL_INTERRUPT_HANDLERS';
 procedure RestoreInterruptHandlers; external name 'FPC_RESTORE_INTERRUPT_HANDLERS';
 
+function CheckNullArea: Boolean; external name 'FPC_CHECK_NULLAREA';
+
 {$I system.inc}
 
 {$I tinyheap.inc}
@@ -279,6 +281,8 @@ begin
          if h>=5 then
            do_close(h);
       end;
+  if not CheckNullArea then
+    writeln(stderr, 'Nil pointer assignment');
   asm
     mov al, byte [exitcode]
     mov ah, 4Ch