pcode.txt

The Clipper OBJ and pcode model (GNU|Open|Clipper project)
==========================================================

Let's consider the following Clipper sample test.prg:


FUNCTION Main()

   ? "Hello world!"

   RETURN NIL


Once it gets compiled into a OBJ, what is there inside it?
In fact, what we get is the equivalent to the following C language
application:


SYMBOL symbols[] = { ... };

void MAIN( void )
{
   BYTE pcode[] = { ... };

   VirtualMachine( pcode, symbols );
}


Basically, test.prg source code has been converted into a sequence
of pcode bytes contained in the array pcode[] = { ... }.  All our MAIN()
function does is invoke, at run-time, a Clipper VirtualMachine() that will
process those pcode bytes.

Let's review the test.prg pcode structure in more detail:

0000 (2A) LINE 0                    2A 00 00
0003 (2A) LINE 3                    2A 03 00
0006 (13)   SYMF [QOUT]             13 02 00
0009 (01)   PUSHC "Hello world!"    01 ...
0018 (27)   DO(1)                   27 01 00
001B (2A) LINE 5                    2A 05 00
001E (7B)   UNDEF                   7B
001F (79)   SAVE_RET                79
0020 (1E)   JMP 0023                1E 00 00
0023 (60) ENDPROC                   60


We could define a hbpcode.h file to better read that pcode:

hbpcode.h

#define  LINE    0x2A
#define  SYMF    0x13
#define  PUSHC   0x01
#define  DO      0x27
#define  UNDEF   0x7B
...


So finally it will look like:

BYTE pcode[] = { LINE, 0, 0,
                 LINE, 3, 0,
                 SYMF, 2, 0,
                 PUSHC, 'H', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', '!', '0',
                 DO,   1, 0,
                 LINE, 5, 0,
                 UNDEF,
                 SAVE_RET,
                 JMP, 0, 0,
                 ENDPROC };


And what is SYMBOL symbols[] ?  Clipper creates a symbol table in
the OBJ that later on will be used to create a dynamic symbol table
shared by the entire application. Each of those symbols has the following
structure:

   typedef struct
   {
      char * szName;   // Clipper in fact keeps an array here (11 bytes).
      BYTE   bScope;
      LPVOID pVoid;
   } SYMBOL;

#define PUBLIC 0    // the scope of the function!

SYMBOL symbols[] = { { "MAIN",  PUBLIC, MAIN },
                     { "QQOUT", PUBLIC, QQOUT } };


Let's remember that the name of a function (MAIN, QQOUT) is the address of the
function, so our symbol table will be ready to use it to jump and execute any
linked function.

In fact, the pcode SYMF 2, 0 in our sample, will instruct the VirtualMachine()
to use the 2 symbol, which is QQOUT.

Let's read the pcode:

LINE 0, 0   =>  We are located at line 0
LINE 3, 0   =>  We are located at line 3
SYMF 2, 0   =>  We are going to call QQOUT from our symbol table
PUSHC ...   =>  This string is going to be used as a parameter
DO   1, 0   =>  ok, jump to QQOUT and remember we have just supplied 1 parameter
LINE 5, 0   =>  We are back from QQOUT and we are located at line 5
UNDEF       =>  we are going to return this value (NIL)
SAVE_RET    =>  Ok, return it
JMP  0      =>  We don't jump to elsewhere, just continue to next pcode byte
ENDPROC     =>  This is the end. We have completed this function execution

All these instructions will be evaluated from our VirtualMachine() function
(Clipper names it _plankton()). All functions end using ENDPROC, so when
the VirtualMachine() finds ENDPROC it knows it has reached the end of a
function pcode.

Now that we clearly understand this basic model we are ready to start
implementing 'production rules' on our yacc (clipper.y) syntax to generate
the specific output file (test.c) with the above structure (or we could
easily just generate the OBJ file for it).

to be continued...

Antonio Linares
www.fivetech.com