Epson ARM.POWERED ARM720T - Manual (Page 55)

2: Programmer’s Model
ARM720T CORE CPU MANUAL EPSON 2-15
2.9 Relocation of low virtual addresses by the FCSE PID
The ARM720T processor provides a mechanism, Fast Context Switch Extension (FCSE), to
translate virtual addresses to physical addresses based on the current value of the FCSE
Process IDentifier (PID).
The virtual address produced by the processor core going to the IDC and MMU can be
relocated if it lies in the bottom 32MB of the virtual address. That is, virtual address bits
[31:25] = b0000000 by the substitution of the seven bits [31:25] of the FCSE PID register in
the CP15 coprocessor.
A change to the FCSE PID exhibits similar behavior to a delayed branch if:
• the two instructions fetched immediately following an instruction to change the
FCSE PID are fetched with a relocation to the previous FCSE PID
• the addresses of the instructions being fetched lie within the range of addresses to
be relocated.
On reset, the FCSE PID register bits [31:25] are set to b0000000, disabling all relocation. For
this reason, the low address reset exception vector is effectively never relocated by this
mechanism.
Note:
All addresses produced by the processor core undergo this translation if they lie in
the appropriate address range. This includes the exception vectors if they are
configured to lie in the bottom of the virtual memory map. This configuration is
determined by the V bit in the CP15 Control Register c1.