trezor-firmware/embed/trezorhal/util.s

  .syntax unified

  .text

  .global memset_reg
  .type memset_reg, STT_FUNC
memset_reg:
  // call with the following (note that the arguments are not validated prior to use):
  // r0 - address of first word to write (inclusive)
  // r1 - address of first word following the address in r0 to NOT write (exclusive)
  // r2 - word value to be written
  // both addresses in r0 and r1 needs to be divisible by 4!
  .L_loop_begin:
    str r2, [r0], 4 // store the word in r2 to the address in r0, post-indexed
    cmp r0, r1
  bne .L_loop_begin
  bx lr

  .global jump_to
  .type jump_to, STT_FUNC
jump_to:
  mov r4, r0            // save input argument r0
  // this subroutine re-points the exception handlers before the C code
  // that comprises them has been given a good environment to run.
  // therefore, this code needs to disable interrupts before the VTOR
  // update. then, the reset_handler of the next stage needs to re-enable interrupts.
  // the following prevents activation of all exceptions except Non-Maskable Interrupt (NMI).
  // according to "ARM Cortex-M Programming Guide to Memory Barrier Instructions" Application Note 321, section 4.8:
  // "there is no requirement to insert memory barrier instructions after CPSID".
  cpsid f
  // wipe memory at the end of the current stage of code
  ldr r0, =ccmram_start // r0 - point to beginning of CCMRAM
  ldr r1, =ccmram_end   // r1 - point to byte after the end of CCMRAM
  ldr r2, =0            // r2 - the word-sized value to be written
  bl memset_reg
  ldr r0, =sram_start   // r0 - point to beginning of SRAM
  ldr r1, =sram_end     // r1 - point to byte after the end of SRAM
  ldr r2, =0            // r2 - the word-sized value to be written
  bl memset_reg
  // give the next stage a fresh main stack pointer
  ldr r0, [r4]
  msr msp, r0
  // point to the next stage's exception handlers
  // AN321, section 4.11: "a memory barrier is not required after a VTOR update"
  .set SCB_VTOR, 0xE000ED08 // reference "Cortex-M4 Devices Generic User Guide" section 4.3
  ldr r0, =SCB_VTOR
  str r4, [r0]
  // go on to the next stage
  ldr lr, =0xffffffff   // set the link register to reset value. there is no reason to return here.
  ldr r0, [r4, 4]
  ldr r1, =0
  ldr r2, =0
  ldr r3, =0
  ldr r4, =0
  ldr r5, =0
  ldr r6, =0
  ldr r7, =0
  ldr r8, =0
  ldr r9, =0
  ldr r10, =0
  ldr r11, =0
  ldr r12, =0
  bx r0

  .global shutdown
  .type shutdown, STT_FUNC
shutdown:
  cpsid f
  ldr r0, =0
  mov r1, r0
  mov r2, r0
  mov r3, r0
  mov r4, r0
  mov r5, r0
  mov r6, r0
  mov r7, r0
  mov r8, r0
  mov r9, r0
  mov r10, r0
  mov r11, r0
  mov r12, r0
  ldr lr, =0xffffffff
  ldr r0, =ccmram_start
  ldr r1, =ccmram_end
  // set to value in r2
  bl memset_reg
  ldr r0, =sram_start
  ldr r1, =sram_end
  // set to value in r2
  bl memset_reg
  bl clear_otg_hs_memory
  b . // loop forever

  .end