Oppolzer - Informatik / Stanford Pascal Compiler

Pointer arithmetic - new functions ADDR, PTRADD, PTRDIFF, SIZEOF, PTR2INT

When I started to code the LE inspired storage allocation routines, I soon discovered that some more compiler features are needed to make this task more fun. In fact, I borrowed some features from the C language and added them to Stanford Pascal in a sensible way (I hope), so that the overall spirit of the language will not be compromised.

The features added in this sprint include

• new function ADDR to get the address of any variable; the result of this function is a pointer without type, similar to the NIL pointer, which is compatible with every other pointer type

• new function PTRADD to add an integer expression to a pointer (of any type) - this adds addresses in contrast to C, where element sizes are added

• new function PTRDIFF, that subtracts two pointers (of any type), giving an integer result

• new function SIZEOF, which works much the same as the C function of the same name; as with C, you can specify a variable as argument or a type identifier

• new function PTR2INT, which converts a pointer to an integer value; this is not really needed, but it was good for testing the other functions - in fact there were some problems due to some optimizations that lead to wrong results in the first shot; I had to implement two new P-Code instructions ADA and SBA to overcome these problems (ADA = add integer to address, SBA = subtract addresses)

Here is an example program that shows the new features:

program TESTADDR ( OUTPUT ) ; (********) (*$A+ *) (********) type CHAR10 = array [ 1 .. 10 ] of CHAR ; SHOWPTR = record case BOOLEAN of TRUE : ( X1 : -> CHAR ) ; FALSE : ( X2 : INTEGER ) end ; var X : INTEGER ; Y : -> INTEGER ; FELD : array [ 1 .. 10 ] of CHAR ; CHP : -> CHAR ; CHP2 : -> CHAR ; Z : SHOWPTR ; begin (* HAUPTPROGRAMM *) X := 25 ; Y := ADDR ( X ) ; WRITELN ( 'vergleich: ' , X , Y -> ) ; FELD := 'OPPOLZER' ; CHP := ADDR ( FELD ) ; while CHP -> <> ' ' do begin WRITELN ( 'chp -> = ' , CHP -> ) ; CHP := PTRADD ( CHP , 1 ) ; end (* while *) ; CHP2 := ADDR ( FELD ) ; X := PTR2INT ( CHP ) ; WRITELN ( 'ptr2int (chp) = ' , X ) ; Z . X1 := CHP ; WRITELN ( 'chp via showptr = ' , Z . X2 ) ; X := PTR2INT ( CHP2 ) ; WRITELN ( 'ptr2int (chp2) = ' , X ) ; X := PTR2INT ( ADDR ( FELD ) ) ; WRITELN ( 'ptr2int (addr(feld)) = ' , X ) ; WRITELN ( 'ptr2int (addr(feld)) = ' , PTR2INT ( ADDR ( FELD ) ) ) ; X := PTRDIFF ( CHP , CHP2 ) ; WRITELN ( 'ptrdiff = ' , X ) ; X := PTRDIFF ( CHP , ADDR ( FELD ) ) ; WRITELN ( 'ptrdiff = ' , X ) ; X := PTRDIFF ( ADDR ( FELD ) , CHP ) ; WRITELN ( 'ptrdiff = ' , X ) ; WRITELN ( 'sizeof (feld) = ' , SIZEOF ( FELD ) ) ; WRITELN ( 'sizeof (integer) = ' , SIZEOF ( INTEGER ) ) ; WRITELN ( 'sizeof (char10) = ' , SIZEOF ( CHAR10 ) ) ; end (* HAUPTPROGRAMM *) .

Note: the type SHOWPTR shows how you could do pointer arithmetic before my extensions (it is indeed done this way in SNAPSHOT and other system related routines). I used it to test the results.

The A+ switch tells the P-Code translator to output ASSEMBLER code in addition to the binary 370 code to file ASMOUT, which proved to be very useful when doing these changes and implementing the new P-Code instructions.

When adding the new functions, I also improved the compiler (PASCAL1), so that future additions of new library functions are still easier (improved table layout of the internal compiler tables).