The machine mode privileged spec defines how each CPU/HART starts execution. The MTVEC register defines the entry point on interrupts.

These are grouped into an startup.S assembler file, along with a basic reset handler to setup an environment that can execute compiled C code.

• Reset
  • The location of the reset vector is implementation defined.
  • The reset vector is the default value of the PC of each CPU/HART - That is the instruction is fetched from the reset vector address, rather than used to load a PC (and SP) as in some architectures.
  • The address could be the start of a routine, or jump to the reset handler.
• NMI
  • The location of the NMI vector is also implementation defined.
  • A logical location may be near the reset vector.
  • The NMI is used only for hardware conditions, unlike some architectures where it can be externally triggered.
  • As for reset, on the error condition the NMI vector is loaded directly to the PC.
• Interrupt Vectors
  • Again this is implementation specific, but there are two main modes, vectored and direct.
• The reset handler function.
• The vector table.
  • This can be a single entry point (MTVEC),
  • or a table that takes between 128 bytes.
  • Again the location in memory is implementation specific.

One possible layout is:

Address	Code
0x000000	Reset Vector
0x000004	NMI Vector
0x000008	Undefined Region
…..	…
0x00007C	…
0x000080
0x000080