Creating Large Memory Arrays

The C18 compiler does not support data buffers over 256 bytes without a modification to the linker script to define a larger bank of microcontroller ram.

Copy the default linker script for your PIC18 device from:

C:\Program Files (x86)\Microchip\MPASM Suite\LKR

into your projects directory.  Add the file to your MPLAB project so that it is used in place of the default file.

Find the section listing each of the databanks for your device, e.g.

DATABANK   NAME=gpr0       START=0x60              END=0xFF
DATABANK   NAME=gpr1       START=0x100             END=0x1FF
DATABANK   NAME=gpr2       START=0x200             END=0x2FF
DATABANK   NAME=gpr3       START=0x300             END=0x3FF
DATABANK   NAME=gpr4       START=0x400             END=0x4FF
DATABANK   NAME=gpr5       START=0x500             END=0x5FF
DATABANK   NAME=gpr6       START=0x600             END=0x6FF
DATABANK   NAME=gpr7       START=0x700             END=0x7FF
DATABANK   NAME=gpr8       START=0x800             END=0x8FF
DATABANK   NAME=gpr9       START=0x900             END=0x9FF
DATABANK   NAME=gpr10      START=0xA00             END=0xAFF
DATABANK   NAME=gpr11      START=0xB00             END=0xBFF
DATABANK   NAME=gpr12      START=0xC00             END=0xCFF
#IFDEF _DEBUGDATASTART
  DATABANK   NAME=gpr13      START=0xD00             END=_DATAEND
  DATABANK   NAME=dbgspr     START=_DEBUGDATASTART   END=_DEND           PROTECTED
#ELSE //no debug
  DATABANK   NAME=gpr13      START=0xD00             END=0xDFF
#FI
DATABANK   NAME=gpr14      START=0xE00             END=0xE40

The number will relate to the number of gpr banks your PIC18 device has.

Now select as many consecutive databanks as you will need to be able to hold your array from somewhere in the middle of the available banks. If you need a multiple of 256bytes then you will use the complete banks, or otherwise you will use just part of the final bank. You want to delete those databanks (and just edit the START address for the final bank if not using a 256byte multiple) and then add your own new databank which will hold your array:

DATABANK NAME=my_ram_section START=0x#00 END=0x#FF

Where ‘0x#00’ is the start address of the first removed bank and ‘0x#FF ‘ is the end address of the last removed bank.  E.g. for a 512 byte databank in this example:

DATABANK   NAME=gpr0       START=0x60              END=0xFF
DATABANK   NAME=gpr1       START=0x100             END=0x1FF
DATABANK   NAME=gpr2       START=0x200             END=0x2FF
DATABANK   NAME=gpr3       START=0x300             END=0x3FF
DATABANK   NAME=gpr4       START=0x400             END=0x4FF
DATABANK   NAME=gpr5       START=0x500             END=0x5FF
DATABANK   NAME=gpr6       START=0x600             END=0x6FF

DATABANK NAME=dmx_512_byte_ram_section START=0x700 END=0x8FF

DATABANK   NAME=gpr9       START=0x900             END=0x9FF
DATABANK   NAME=gpr10      START=0xA00             END=0xAFF
DATABANK   NAME=gpr11      START=0xB00             END=0xBFF
DATABANK   NAME=gpr12      START=0xC00             END=0xCFF
#IFDEF _DEBUGDATASTART
  DATABANK   NAME=gpr13      START=0xD00             END=_DATAEND
  DATABANK   NAME=dbgspr     START=_DEBUGDATASTART   END=_DEND           PROTECTED
#ELSE //no debug
  DATABANK   NAME=gpr13      START=0xD00             END=0xDFF
#FI

Save the linker script.  This defines the new block of memory and you can now use the name you assigned to declare your array:

#pragma udata dmx_512_byte_ram_section				//This is the PIC C18 compiler command to use the specially defined section in the linker script (uses a modified linker script for this)
BYTE dmx_tx_buffer[512];			//(C18 large array requirement to use a special big section of ram defined in the linker script)
#pragma udata

Accessing The Array

Its OK to use pointers like this:

	BYTE *buffer_pointer;
	buffer_pointer = &my_array[0] + some_value;
	*buffer_pointer = 0x01;