Datasheet

PICkit™ 3 Debug Express

2.2.1 Data Memory

Data memory is called “file register” memory in the PIC18XXXX family. It consists of up

to 4096 bytes of 8-bit RAM. Upon power-up, the values in data memory are random.

Data is organized in banks of 256 bytes, requiring that a bank (the upper 4 bits of the

Bank 0 and in Bank 15 can be accessed directly without concern for banking. These

special data areas are called Access RAM. The high Access RAM area is where most

of the Special Function Registers are located.

When using MPLAB C Compiler, this banking is usually transparent, but the use of the

#pragma varlocate directive tells the compiler where variables are stored, resulting

in more efficient code.

Uninitialized data memory variables, arrays and structures are usually stored in

memory with the section attribute, udata. Initialized data can be defined in MPLAB C

Compiler so that variables will have correct values when the compiler initialization

executes. This means that the values are stored in program memory, then moved to

data memory on start-up. Depending upon how much initialized memory is required for

the application, the use of initialized data (rather than simply setting the data values at

run time) may adversely affect the efficient use of program memory. Since file registers

are 8 bits, when using variables consideration should be made on what is the best

datatype to define them as. For example, when a variable value is not expected to

exceed 255, defining it as a char instead of an int will result in smaller, faster code.

2.2.2 Special Function Registers

Special Function Registers (SFRs) are CPU core registers (such as the Stack Pointer,

STATUS register and Program Counter) and include the registers for the peripheral

modules on the microprocessor. The peripherals include such things as input and

output pins, timers, USARTs and registers to read and write the EEDATA areas of the

device. MPLAB C Compiler can access these registers by name, and they can be read

and written like a variable defined in the application. Use caution, though, because

some of the Special Function Registers have characteristics different from variables.

Some have only certain bits available, some are read-only and some may affect other

registers or device operation when accessed. These registers are mapped to

addresses in Bank 15 of the data memory.

2.2.3 Return Address Stack

CALL and RETURN instructions push and pop the Program Counter on the return

address stack. The return stack is a separate area of memory, allowing 31 levels of

subroutines.

The CALL/RETURN stack is distinct from the software stack maintained by MPLAB C

Compiler. The software stack is used for automatic parameters and local variables and

resides in file register memory as defined in the Linker Script.