Xilinx MicroBlaze Reference Manual page 57

It is strongly discouraged to do this, unless absolutely necessary for performance reasons,
CAUTION!
because it allows application processes to interfere with each other.
When setting the parameter
instructions
LBUEA
will bypass the MMU translation, treating the extended address as a physical address. This
is useful to run software in virtual mode while still having direct access to the full physical
address space, but is discouraged in all cases where protection between application
processes is necessary.
There are six ways to leave user mode and virtual mode:
1. Hardware generated reset (including debug reset)
2. Hardware exception
3. Non-maskable break or hardware break
4. Interrupt
5. Executing "
6. Executing the software break instructions "
C_BASE_VECTORS
In all of these cases, except hardware generated reset, the user mode and virtual mode
status is saved in the MSR UMS and VMS bits.
Application (user-mode) programs transfer control to system-service routines (privileged
mode programs) using the
C_BASE_VECTORS + 0x8
The exception handler determines which system-service routine to call and whether the
calling application has permission to call that service. If permission is granted, the
exception handler performs the actual procedure call to the system-service routine on
behalf of the application program.
The execution environment expected by the system-service routine requires the execution
of prologue instructions to set up that environment. Those instructions usually create the
block of storage that holds procedural information (the activation record), update and
initialize pointers, and save volatile registers (the registers that the system-service routine
uses). Prologue code can be inserted by the linker when creating an executable module, or
it can be included as stub code in either the system-call interrupt handler or the system-
library routines.
Returns from the system-service routine reverse the process described above. Epilogue
code is executed to unwind and deallocate the activation record, restore pointers, and
restore volatile registers. The interrupt handler executes a return from exception instruction
( ) to return to the application.
RTED
MicroBlaze Processor Reference Guide
UG984 (v2018.2) June 21, 2018
C_MMU_PRIVILEGED_INSTR
, , ,
LHUEA LWEA SBEA
BRALID Re,C_BASE_VECTORS
+ 0x8 or
C_BASE_VECTORS
or
BRALID
. Executing this instruction causes a system-call exception to occur.
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Chapter 2: MicroBlaze Architecture
to 2 or 3, the extended address
, , and are not considered privileged, and
SHEA SWEA
+ " to perform a user vector exception
0x8
" jumping to physical address
BRKI
+ 0x18
instruction, jumping to physical address
BRKI
57
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