ArmPlatformPkg/PrePi: Fix comment (minor change)

[mirror_edk2.git] / ArmPlatformPkg / Sec / Helper.asm

//\r
//  Copyright (c) 2011-2012, ARM Limited. All rights reserved.\r
//  \r
//  This program and the accompanying materials                          \r
//  are licensed and made available under the terms and conditions of the BSD License         \r
//  which accompanies this distribution.  The full text of the license may be found at        \r
//  http://opensource.org/licenses/bsd-license.php                                            \r
//\r
//  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,                     \r
//  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.             \r
//\r
//\r
\r
    EXPORT  return_from_exception\r
    EXPORT  enter_monitor_mode\r
    EXPORT  copy_cpsr_into_spsr\r
    EXPORT  set_non_secure_mode\r
    \r
    AREA   Helper, CODE, READONLY\r
\r
// r0: Monitor World EntryPoint\r
// r1: MpId\r
// r2: Secure Monitor mode stack\r
enter_monitor_mode\r
    mrs     r4, cpsr                    // Save current mode (SVC) in r1\r
    bic     r3, r4, #0x1f               // Clear all mode bits\r
    orr     r3, r3, #0x16               // Set bits for Monitor mode\r
    msr     cpsr_cxsf, r3               // We are now in Monitor Mode\r
\r
    cmp     r2, #0                      // If a Secure Monitor stack base has been passed, used it\r
    movne   sp, r2                      // Use the passed sp\r
\r
    mov     lr, r0                      // Use the pass entrypoint as lr\r
    \r
    msr     spsr_cxsf, r4               // Use saved mode for the MOVS jump to the kernel\r
\r
    mov     r4, r0                      // Swap EntryPoint and MpId registers\r
    mov     r0, r1\r
\r
    bx      r4\r
\r
// We cannot use the instruction 'movs pc, lr' because the caller can be written either in ARM or Thumb2 assembler.\r
// When we will jump into this function, we will set the CPSR flag to ARM assembler. By copying directly 'lr' into\r
// 'pc'; we will not change the CPSR flag and it will crash.\r
// The way to fix this limitation is to do the movs into the ARM assmbler code and then do a 'bx'.\r
return_from_exception\r
    adr     lr, returned_exception\r
    movs    pc, lr\r
returned_exception                           // We are now in non-secure state\r
    bx      r0\r
\r
// Save the current Program Status Register (PSR) into the Saved PSR\r
copy_cpsr_into_spsr\r
    mrs     r0, cpsr\r
    msr     spsr_cxsf, r0\r
    bx      lr\r
\r
// Set the Non Secure Mode\r
set_non_secure_mode\r
    push    { r1 }\r
    and     r0, r0, #0x1f     // Keep only the mode bits\r
    mrs     r1, spsr          // Read the spsr\r
    bic     r1, r1, #0x1f     // Clear all mode bits\r
    orr	    r1, r1, r0\r
    msr     spsr_cxsf, r1     // write back spsr (may have caused a mode switch)\r
    isb\r
    pop     { r1 }\r
    bx      lr                // return (hopefully thumb-safe!)\r
\r
dead\r
    B       dead\r
    \r
    END\r