Instruction Clock vs Oscillator

The 8-bit PIC MCU system clock is always designated as Fosc in data sheets. The instruction clock is derived from the Fosc through a divider to deliver an instruction clock of Fosc/4 (1/4 the oscillator speed). This is done to allow the fetch and execute pipeline in the architecture to execute one assembly instruction per instruction clock cycle (except for branches and GOTOs which execute in two).

If a higher speed than a divide by four instruction clock is needed, then many devices have a 4x PLL option to boost the Fosc up by four which is later divided back down by four for the instruction clock. This will yield an instruction clock equal to the Fosc speed.