9.4.2 Access Bank

While the use of the BSR with an embedded 8-bit address allows users to address the entire range of data memory, it also means that the user must ensure that the correct bank is selected. Otherwise, data may be read from or written to the wrong location. Verifying and/or changing the BSR for each read or write to data memory can become inefficient.

To streamline access for the most commonly used data memory locations, the data memory is configured with a virtual Access Bank, which allows users to access a mapped block of memory without specifying a BSR. The Access Bank consists of the first 96 bytes of memory in Bank 5 (0500h-055Fh) and the last 160 bytes of memory in Bank 4 (0460h-04FFh). The upper half is known as the “Access RAM” and is composed of GPRs. The lower half is where the device’s SFRs are mapped. These two areas are mapped contiguously as the virtual Access Bank and can be addressed in a linear fashion by an 8-bit address (see the Data Memory Map section).

The Access Bank is used by core PIC18 instructions that include the Access RAM bit (the ‘a’ parameter in the instruction). When ‘a’ is equal to ‘1’, the instruction uses the BSR and the 8-bit address included in the opcode for the data memory address. When ‘a’ is ‘0’, the instruction ignores the BSR and uses the Access Bank address map.

Using this “forced” addressing allows the instruction to operate on a data address in a single cycle without updating the BSR first. Access RAM also allows for faster and more code efficient context saving and switching of variables.

The mapping of the Access Bank is slightly different when the extended instruction set is enabled (XINST Configuration bit = 1). This is discussed in more detail in the Mapping the Access Bank in Indexed Liberal Offset Mode section.