11.3.1 Interrupt Vector Table Base Address (IVTBASE)

The start address of the vector table is user-programmable through the IVTBASE. The user must ensure the start address is such that it can encompass the entire vector table inside the program memory.

Each vector address is a 16-bit word (or two address locations on PIC18 devices). For ‘n’ interrupt sources, there are ‘2n’ address locations necessary to hold the table, starting from IVTBASE as the first location. Thus, the starting address needs to be chosen such that the address range from IVTBASE to “IVTBASE+2n-1” can be encompassed within the program Flash memory.

For example, if the highest vector number was 81, IVTBASE needs to be chosen such that “IVTBASE+0xA1” is less than the last memory location in program Flash memory.

A programmable vector table base address is useful in situations to switch between different sets of vector tables, depending on the application. It can also be used when the application program needs to update the existing vector table (vector address values).