title: ARM modes: User and System
body: <p>Can you explain how the ARM mode get changed in case of a system call handling?
  I heard ARM mode change can happen only in privileged mode, but in case of a system call handling while the ARM is in user mode (which is a non-privileged mode), how does the ARM mode change?</p>
  
  <p>Can anybody explain the whole action flow for the user mode case, and also more generally the system call handling (especially how the ARM mode change)?</p>
  
  <p>Thanks in advance.</p>