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IDT MIPS32 4Kc Processor Core
Memory Management
79RC32438 User Reference Manual
2 - 21
November 4, 2002
Notes
Figure 2.19 Address Translation During a Cache Access
Modes of Operation
The 4Kc processor core supports three modes of operation:
User mode
Kernel mode
Debug mode
User mode is most often used for application programs. Kernel mode is typically used for handling
exceptions and privileged operating system functions, including CP0 management and I/O device
accesses. Debug mode is used for software debugging and most likely occurs within a software develop-
ment tool. The address translation performed by the MMU depends on the mode in which the processor is
operating.
Virtual Memory Segments
The Virtual memory segments are different depending on the mode of operation. Figure 2.20 shows the
segmentation for the 4 GByte (232 bytes) virtual memory space addressed by a 32-bit virtual address, for
the three modes of operation.
The core enters Kernel mode both at reset and when an exception is recognized. While in Kernel mode,
software has access to the entire address space, as well as all CP0 registers. User mode accesses are
limited to a subset of the virtual address space (0x0000_0000 to 0x7FFF_FFFF) and can be inhibited from
accessing CP0 functions. In User mode, virtual addresses 0x8000_0000 to 0xFFFF_FFFF are invalid and
cause an exception if accessed.
Debug mode is entered on a debug exception. While in Debug mode, the debug software has access to
the same address space and CP0 registers as for Kernel mode. In addition, while in Debug mode the core
has access to the debug segment dseg. This area overlays part of the kernel segment kseg3. dseg access
in Debug mode can be turned on or off, allowing full access to the entire kseg3 in Debug mode, if so
desired.
Instruction Virtual
Address
(IVA)
Data
Virtual Address
(DVA)
JTLB
ITLB
Instruction
Cache
RAM
DTLB
Data Cache
RAM
IVA
Entry
Entry
Data
Physical Address
(DPA)
Instruction
Physical Address
(IPA)
Tag (IPA)
Tag (DPA)
Comparator
Comparator
Data Hit/Miss
Instruction Hit/
Miss