- Supervisor Mode Operations

4.2 Address Spaces

Supervisor Mode Operations

Supervisor mode is designed for layered operating systems in which a true kernel runs in R4000 Kernel mode, and the rest of the operating system runs in Supervisor mode.

The processor operates in Supervisor mode when the Status register contains the following bit-values:

KSU = 012
EXL = 0
ERL = 0

In conjunction with these bits, the SX bit in the Status register selects between 32- or 64-bit Supervisor mode addressing:

when SX = 0, 32-bit supervisor space is selected and TLB misses are handled by the 32-bit TLB refill exception handler
when SX = 1, 64-bit supervisor space is selected and TLB misses are handled by the 64-bit XTLB refill exception handler

Figure 4-5 shows Supervisor mode address mapping. Table 4-2 lists the characteristics of the supervisor mode segments; descriptions of the address spaces follow.

Figure 4-5 Supervisor Mode Address Space

*NOTE: The R4000 uses 64-bit addresses internally. In 32-bit mode, a valid address must be a 32-bit signed number, where bits 63:32 = bit 31. In normal operation it is not possible for a 32-bit Supervisor-mode program to create an invalid address through arithmetic operations. However 32-bit-mode Supervisor programs must not create addresses using base register+offset calculations that produce a 32-bit 2's-complement overflow; in specific, there are two prohibited cases:

offset with bit 15 = 0 and base register with bit 31 = 0, but (base register+offset) bit 31 = 1
offset with bit 15 = 1 and base register with bit 31 = 1, but (base register+offset) bit 31 = 0

Using this invalid address produces an undefined result.

Table 4-2 32-bit and 64-bit Supervisor Mode Segments

32-bit Supervisor Mode, User Space (suseg)

In Supervisor mode, when SX = 0 in the Status register and the most-significant bit of the 32-bit virtual address is set to 0, the suseg virtual address space is selected; it covers the full 231 bytes (2 Gbytes) of the current user address space. The virtual address is extended with the contents of the 8-bit ASID field to form a unique virtual address.

This mapped space starts at virtual address 0x0000 0000 and runs through 0x7FFF FFFF.

32-bit Supervisor Mode, Supervisor Space (sseg)

In Supervisor mode, when SX = 0 in the Status register and the three most-significant bits of the 32-bit virtual address are 1102, the sseg virtual address space is selected; it covers 229-bytes (512 Mbytes) of the current supervisor address space. The virtual address is extended with the contents of the 8-bit ASID field to form a unique virtual address.

This mapped space begins at virtual address 0xC000 0000 and runs through 0xDFFF FFFF.

64-bit Supervisor Mode, User Space (xsuseg)

In Supervisor mode, when SX = 1 in the Status register and bits 63:62 of the virtual address are set to 002, the xsuseg virtual address space is selected; it covers the full 240 bytes (1 Tbyte) of the current user address space. The virtual address is extended with the contents of the 8-bit ASID field to form a unique virtual address.

This mapped space starts at virtual address 0x0000 0000 0000 0000 and runs through 0x0000 00FF FFFF FFFF.

64-bit Supervisor Mode, Current Supervisor Space (xsseg)

In Supervisor mode, when SX = 1 in the Status register and bits 63:62 of the virtual address are set to 012, the xsseg current supervisor virtual address space is selected. The virtual address is extended with the contents of the 8-bit ASID field to form a unique virtual address.

This mapped space begins at virtual address 0x4000 0000 0000 0000 and runs through 0x4000 00FF FFFF FFFF.

64-bit Supervisor Mode, Separate Supervisor Space (csseg)

In Supervisor mode, when SX = 1 in the Status register and bits 63:62 of the virtual address are set to 112, the csseg separate supervisor virtual address space is selected. Addressing of the csseg is compatible with addressing sseg in 32-bit mode. The virtual address is extended with the contents of the 8-bit ASID field to form a unique virtual address.

This mapped space begins at virtual address 0xFFFF FFFF C000 0000 and runs through 0xFFFF FFFF DFFF FFFF.

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