ARM Q&A

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  •  what are the different modes in ARM architecture?
    • User Mode:
      • is the usual ARM program execution state, and is used for executing most application programs.
    • Supervisor Mode:
      • is the protected mode for the operation system. 
    • IRQ mode:
      • is used for General purpose interrupt handling. 
    • Fast IRQ mode:
      • support for Data transfer or channel process. 
    • Abort Mode
      • is entered after a data or instruction prefetch Abort.
    • Undefined Mode
      • is entered when an undefined instruction is executed.
    • System Mode
      • is privileged user mode for the operation system.
  • Difference between the return from handler and return from function in assembly language implementation. 
    • interrupt function the return type is RETI, which clear the source of the interrupt. Otherwise, it will be called over and over. 
    •  function return is just RET.
  • What is the difference between FIQ and IRQ?
    • FIQ has higher priority than IRQ: FIQ is masked by fewer exceptions and if FIQ & IRQ happen together FIQ goes first
    • FIQ has more banked registers(r8 – r14) available than IRQ has only fewer registers (r13 & r14) to save/restore
    • FIQ has a more convenient vector location: no initial branch is needed.
  • what are CPSR and SPSR?
    • current program status register.
    • saved program status register. 
      • The SPSR is used to store the current value of the CPSR when an exception is taken so that it can be restored after handling the exception. Each exception handling mode can access its own SPSR. User mode and System mode do not have an SPSR because they are not exception handling modes.
  • How fast interrupt (FIQ) works?

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