In FastBus mode GCLK is sourced from BCLK. The FCLK input is ignored. This means that BCLK is used to control the AMBA ASB interface and the internal ARM920T processor core.
On reset, the ARM920T is put into FastBus mode and operates using BCLK. A typical use for FastBus mode is to execute startup code while configuring a PLL under software control to produce FCLK at a higher frequency. When the PLL has stabilized and locked, you can switch the ARM920T to synchronous or asynchronous clocking using FCLK for normal operation.