#######################################
# PS 4 Fill Level - PS_DIAG
# Indentify start of next read address (either last read or start of PS)
if NCDD0AE9.raw < 1 then
   # This PS has not been read
   #
   if NCDD0AEC.raw < 1 then
        # Nor has it been written to - use the PS start address
        VAR_PS4_ReadPtr :=  NCDD0AE8.raw;
   else
       # There's a valid packet though, use the FVPP
   	VAR_PS4_ReadPtr :=  NCDD0AEC.raw;
   endif;
else
  VAR_PS4_ReadPtr := NCDD0AE9.raw;
endif;
#
# Get write address
VAR_PS4_WritePtr := NCDD0AEB.raw;
#
# Difference between them is the amount of unread data
if VAR_PS4_WritePtr >= VAR_PS4_ReadPtr then
   VAR_PS4_UnreadBytes := VAR_PS4_WritePtr - VAR_PS4_ReadPtr;
else
   VAR_PS4_UnreadBytes := (NCDD0AE7.raw + VAR_PS4_WritePtr) - VAR_PS4_ReadPtr; 
endif;
#
# Return amount of data stored in this packet store
return (VAR_PS4_UnreadBytes);