| 2-1 CPU Operation Modes and Memory Space |   |
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There are 2 operating modes in the SH-3’s CPU; the privileged mode and the user mode.
In privileged mode all instructions can be executed and all memory areas can be accessed. It is in this privileged mode immediately following a reset.
In the user mode however instructions changing important system registers including control registers within the CPU cannot be executed and there are memory areas that cannot be accessed.
The privileged mode is for running an OS and the user mode is for running applications.
The functions that can be used vary depending on the operating mode.
In privileged mode all 4 GB of memory area is accessible.
In user mode the upper 2 GB is not accessible. If it is accessed an address error occurs, and exception handling is executed.
The on-chip peripheral functions are mapped in the P4 area which can only be used in privileged mode.
If cache is turned on, P0 or U0 area, which can be used regardless of whether it is user or privileged mode, and P1 and P3 areas can be used for cache.
Cache on-off control cannot be performed for individual areas.
If MMU is turned on then the P0 or U0 area and the P3 area are subject to MMU address translation and memory protection.
If the cache is turned on at the same time, on-off control can be performed for MMU management units of 1 KB or 4 KB page units.
In this manner the functions that can be used vary with the operating mode.