[fpc-pascal] Setting bit in array of byte

[Koenraad Lelong]

Hi,
I have following code (almost literally translated from C) :
//--------------------------------------------------------------------------
// Bit utility to read and write a bit in the buffer 'buf'.
//
// 'op'    - operation (1) to set and (0) to read
// 'state' - set (1) or clear (0) if operation is write (1)
// 'loc'   - bit number location to read or write
// 'buf'   - pointer to array of bytes that contains the bit
//           to read or write
//
// Returns: 1   if operation is set (1)
//          0/1 state of bit number 'loc' if operation is reading
//
type
  PByteArray = ^array of byte;

function bitacc(op, state, loc : byte; buf : PByteArray) : byte;
{C-code : static SMALLINT bitacc(SMALLINT op, SMALLINT state, SMALLINT 
loc, uchar *buf)}

var
  nbyt,
  nbit : byte;

begin

nbyt := loc div 8;
nbit := loc mod 8;
if (op = WRITE_FUNCTION) then
  begin
   if state=1 then
    buf^[nbyt] := buf^[nbyt] or ($01 << nbit)
   else
    buf^[nbyt] := buf^[nbyt] and not ($01 << nbit);
   result:=1;
  end
else
  result:=(buf^[nbyt] >> nbit) and $01;
end;

It's called with this code :

bitacc(WRITE_FUNCTION, bitacc(READ_FUNCTION,0,i, at SerialNum[portnum][0]), 
(i * 2 + 1), @sendpacket[pos])
{C-code :
bitacc(WRITE_FUNCTION, bitacc(READ_FUNCTION,0,i,&SerialNum[portnum][0]),
  (short)(i * 2 + 1), &sendpacket[pos]);}

Executing the code gives an an access violation exception. What am I 
doing wrong ?
I included some debugging code and it seems accessing buf^[nbyt] is the 
problem.

TIA
Koenraad Lelong.