foxBMS  1.2.0
The foxBMS Battery Management System API Documentation
fsystem.h File Reference

Function to switch between user mode and privilege mode. More...

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Macros

#define FSYS_SwitchToUserMode()    { __asm(" CPS #0x10"); }
 Switch back to user mode. More...
 

Functions

long FSYS_RaisePrivilege (void)
 raise privilege More...
 

Detailed Description

Function to switch between user mode and privilege mode.

SPDX-License-Identifier: BSD-3-Clause

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Author
foxBMS Team
Date
2020-07-21 (date of creation)
Updated
2020-07-21 (date of last update)
Prefix
FSYS

Definition in file fsystem.h.

Macro Definition Documentation

◆ FSYS_SwitchToUserMode

#define FSYS_SwitchToUserMode ( )     { __asm(" CPS #0x10"); }

Switch back to user mode.

This macro is used after raising the privileges with FSYS_RaisePrivilege(). Failure to call this macro may lead to unintended system behavior.

Definition at line 108 of file fsystem.h.

Function Documentation

◆ FSYS_RaisePrivilege()

long FSYS_RaisePrivilege ( void  )

raise privilege

This alias is mapped to an ASM function and raises to a privileged processor state if the system is currently in user mode. This is done by the following ASM code:

  • Copy the contents of a CP14 or CP15 coprocessor register (in this case SPSR – Saved Program Status Register) into local register r12:
    mrs     r12, spsr 
  • Perform bitwise AND on the contents of r12 and 0x0F
  • write to r0
  • update condition code flag on result of operation
    ands    r0, r12, #0x0F      ; return value 
  • Perform a logical OR on the contents of r12 and 0x1F, if equal to condition code flag from previous instruction
  • write to r12
    orreq   r12, r12, #0x1F 
  • Conditionally (equal on previous instruction condition code flag) load immediate result from r12 into control field mask byte, PSR[7:0] (privileged software execution) from SPSR
    msreq   spsr_c, r12 
  • Branch and exchange instruction set
  • Target address is contained in r14
    bx      r14 

It is important to issue FSYS_SwitchToUserMode() after the privileged mode is no longer needed. Otherwise the system would stay in privileged mode.

The datatype of the return value has to be reevaluated when changing platform and compiler.

Returns
return value is zero if caller was in user mode (type: long)