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Chapter 5 Memory Management Unit (MMU)
The R3900 Processor Core doesn't have TLB.
5.1 R3900 Processor Core Operating Modes
The R3900 Processor Core has two operating modes, user mode and kernel mode. Normally it operates in
user mode, but when an exception is detected it goes to kernel mode. Once in kernel mode, it remains until
an RFE (Restore From Exception) instruction is executed. The available virtual address space differs with
the mode, as shown in Figure 5-1.
(1) User mode
User mode makes available only one of the two 2 Gbyte virtual address spaces (kuseg). The most
significant bit of each kuseg address is 0. The virtual address range of kuseg is 0x0000 0000 to
0x7FFF FFFF. Attempting to access an address when the MSB is 1 while in user mode returns an
Address Error exception.
(2) Kernel mode
Kernel mode makes available a second 2 Gbyte virtual address space (kseg), in addition to the kuseg
accessible in user mode. The virtual address range of kseg is 0x8000 0000 to 0xFFFF FFFF.
User mode
0x7FFF FFFF
0x0000 0000
2GB
Kuseg
Kernel mode
0xFFFF FFFF
0x8000 0000
2GB
kseg
0x7FFF FFFF
0x0000 0000
2GB
Kuseg
Figure 5-1. Operating modes and virtual address spaces