Using variables larger than 256 bytes

Several applications require the use of variables larger than 256 bytes. For example, if you want to build a robot with a camera and process images, or add cool effects to an audio stream. But while the PIC18F4550 of the Dwengo Board contains 2048 bytes of data memory, the default linker script does not allow one to create variables larger than 256. This is fairly easy to fix though.

Creating larger variables is possible in 4 steps:

Step 1. Assign your own section, DATA, to an object that is larger than 256 bytes. In the following example, we declare an array large_table of 768 integers (1536 bytes).

#pragma udata DATA  // section DATA
int large_table[768];

Step 2. You can access the object large_table through a pointer table_ptr. Here, table_ptr points to the first element of the array. You can read and modify elements of the array using table_ptr[ADDRESS].

#pragma udata  // return to default section
int *table_ptr = &large_table[0];

Step 3. In the linker script, the total memory space of the PIC is divided into blocks (DATABANK) that can each be assigned to a specific section (SECTION). Edit the linker script (.lkr file) to assign a sufficiently large memory block. This is done using the keyword DATABANK which you pass the NAME, START and END parameters. In our example, NAME=data starting at address 0x200h until 0x7FFh.
Note that in this example, the memory space we want to use is that large that we need to claim the memory originally used for the USB buffers. This means that, in this program, you can no longer use the USB functionality. Therefore, remove the DATABANK lines in the linker script for USB (addresses 0x400h until 0x7FFh).

Step 4. Finally, the memory space we just created, which is called data (in lowercase), can be assigned to the DATA section (uppercase).

Here the code for the linker script (.lkr file).

// File: 18f4550_bigMem.lkr
// Linker scrip for PIC18F4550 and storing a large array of 1536 bytes
 
LIBPATH .
 
FILES c018i.o
FILES clib.lib
FILES p18f4550.lib
 
CODEPAGE   NAME=page       START=0x0               END=0x7FFF
CODEPAGE   NAME=idlocs     START=0x200000          END=0x200007       PROTECTED
CODEPAGE   NAME=config     START=0x300000          END=0x30000D       PROTECTED
CODEPAGE   NAME=devid      START=0x3FFFFE          END=0x3FFFFF       PROTECTED
CODEPAGE   NAME=eedata     START=0xF00000          END=0xF000FF       PROTECTED
 
ACCESSBANK NAME=accessram  START=0x0            END=0x5F
DATABANK   NAME=gpr0       START=0x60           END=0xFF
DATABANK   NAME=gpr1       START=0x100          END=0x1FF
DATABANK   NAME=data       START=0x200          END=0x7FF          PROTECTED
ACCESSBANK NAME=accesssfr  START=0xF60          END=0xFFF          PROTECTED
 
SECTION    NAME=CONFIG     ROM=config
SECTION       NAME=DATA      RAM=data
 
STACK SIZE=0x100 RAM=gpr1