MIPS R10000 Microprocessor User's Manual - SGI TechPubs Library

246 Chapter 14.
14.10 Status Register (12)
The Status register (SR) is a read/write register that contains the operating mode,
interrupt enabling, and the diagnostic states of the processor. The following list
describes the more important Status register fields; Figure 14-11 shows the format
of the entire register, and Table 14-10 describes the Status register fields.
Some of the important fields include:
• The 4-bit Coprocessor Usability (CU) field controls the usability of 4
possible coprocessors. Regardless of the CU0 bit setting, CP0 is always
usable in Kernel mode. The XX bit enables the MIPS IV ISA in User
mode.
• By default, the R10000 processor implements the same user instruction
set as the R4400 processor. To enable execution of the MIPS IV
instructions in User mode, the MIPS IV User Mode bit, (XX) of the CP0
Status register must be set.
The MIPS IV instruction extension uses COP1X as the opcode; this designation
was COP3 in the R4400 processor. For this reason the CU3 bit is omitted in the
R10000 processor, and is used as the XX bit. In Kernel and Supervisor modes,
the state of the XX bit is ignored, and MIPS IV instructions are always
available.
User
Mode bit settings are shown in Table 14-9; dashes in the table represent don’t
cares.
Table 14-9 ISA and Status Register Settings for User, Supervisor and
Kernel Mode Operations
Mode UX SX KX XX MIPS II MIPS III MIPS IV
0 - - 0 Yes No No
0 - - 1 Yes No Yes
1 - - 0 Yes Yes No
1 - - 1 Yes Yes Yes
Supervisor
-
-
0
1
-
-
-
-
Yes
Yes
No
Yes
Yes
Yes
Kernel - - - - Yes Yes Yes
NOTE: Operation with the MIPS IV ISA does not assume or require that the
MIPS III instruction set or 64-bit addressing be enabled — KX, SX and UX may
all be set to zero.
Version 2.0 of January 29, 1997 MIPS R10000 Microprocessor User's Manual