Commits on Sep 2, 2020

1. [ARMv6-M] initialize the LR register …

   the ARMv6-M Architecture Reference Manual (ARM DDI 0419D) indicates in section
   B1.5.5 "Reset behavior" that the LR (Link Register) starts in an unknown state
   when the Reset handler is taken and that its "Value must be initialised by
   software"
   
   So this PR does that: it initializes the LR register to 0xFFFF_FFFF (-1) first
   thing in the Reset handler (only for v6). The manual doesn't say which value to
   use so I decided to use the value used by the ARMv7-M (v7 sets LR to 0xFFFF_FFFF
   before invoking the Reset handler; see its Architecture Manual for details).
   
   The values of LR (these are pushed onto the stack in function preludes) are used
   to unwind the stack (e.g. GDB's `backtrace` or a future `cortex_m_panic_unwind`
   handler). Having the initial stack frame use a known value on all Cortex-M
   variants makes it easier to implement `panic_unwind` and avoids virtual
   unwinders like GDB `backtrace` trying to unwind beyond the `Reset` handler
   
   Note that this implementation uses a trampoline that runs before `Reset` to set
   LR on v6. This is required because the prelude of the `Reset` routine will push
   LR onto the stack; we want that LR value to be -1. Calling `register::lr::write`
   from `Reset` would perform the write after LR has been pushed onto the stack and
   that's too late

   

   japaric committed Sep 2, 2020
   Configuration menu

   • View commit details
   • Copy full SHA for 6020e4f
   • Browse repository at this point

   Copy the full SHA

   6020e4f View commit details

   Browse the repository at this point in the history