CHAPTER 2 VR4120A
106 Preliminary User’s Manual S15543EJ1V0UM
2.4.2.4 Operating modes
The processor has three operating modes that function in both 32- and 64-bit operations:
User mode
Supervisor mode
Kernel mode
User and Kernel modes are common to all VR-Series processors. Generally, Kernel mode is used to execute the
operating system, while User mode is used to run application programs. The VR4000 Series processors have a third
mode, which is called Supervisor mode and categorized in between User and Kernel modes. This mode is used to
configure a high-security system.
When an exception occurs, the CPU enters Kernel mode, and remains in this mode until an exception return
instruction (ERET) is executed. The ERET instruction brings back the processor to the mode in which it was just
before the exception occurs.
2.4.2.5 User mode virtual addressing
In user mode, a single virtual address space labeled User segment is available; its size is
2-Gbyte (231 bytes) in 32-bit mode (useg)
1-Tbyte (240 bytes) in 64-bit mode (xuseg)
Figure 2-28. User Mode Address Space
64-bit mode
32-bit mode Note
8000_0000H
FFFF_FFFFH
Address error
0000_0100_0000_ 0000H
FFFF_FFFF_FFFF_FFFFH
Address error
xuseguseg
0000_0000H
7FFF_FFFFH
2 Gbytes with
TLB mapping
0000_0000_0000_ 0000H
0000_00FF_FFFF_FFFFH
1 Tbyte with
TLB mapping
Note The VR4120A uses 64-bit addresses within it. When the processor is running in Kernel mode, it saves the
contents of each register or restores their previous contents to initialize them before switching the context.
For 32-bit mode addressing, bit 31 is sign-extended to bits 32 to 63, and the resulting 32 bits are used for
addressing. Usually, it is impossible for 32-bit mode programs to generate invalid addresses. If context
switching occurs and the processor enters Kernel mode, however, an attempt may be made to save an
address other than the sign-extended 32-bit address mentioned above to a 64-bit register. In this case,
user-mode programs are likely to generate an invalid address.