9.4 Data Memory Organization

Important: The operation of some aspects of data memory are changed when the PIC18 extended instruction set is enabled. See the PIC18 Instruction Execution and the Extended Instruction Set section for more information.

The data memory in PIC18 devices is implemented as static RAM. The memory space is divided into as many as 64 banks with 256 bytes each. The Data Memory Map table below shows the data memory organization for all devices in the device family.

The data memory contains Special Function Registers (SFRs) and General Purpose Registers (GPRs). The SFRs are used for control and status of the controller and peripheral functions, while GPRs are used for data storage and scratchpad operations in the user’s application. Any read of an unimplemented location will read as ‘0’.

The value in the Bank Select Register (BSR) determines which bank is being accessed. The instruction set and architecture allow operations across all banks. The entire data memory may be accessed by Direct, Indirect or Indexed Addressing modes. Addressing modes are discussed later in this subsection.

To ensure that commonly used registers (SFRs and select GPRs) can be accessed in a single cycle, PIC18 devices implement an Access Bank. This is a virtual 256-byte memory space that provides fast access to SFRs and the top half of GPR Bank 5 without using the Bank Select Register. The Access Bank section provides a detailed description of the Access RAM.

Figure 9-3. Data Memory Map