Add initial notes on trap handlers

Signed-off-by: Daniel Maslowski <[email protected]>

trap-handlers.adoc

@@ -0,0 +1,59 @@
+[[trap-handlers]]
+== Machine-Level Interrupts and Exceptions
+
+=== Delegation
+
+A RISC-V Machine mode execution environment may delegate handling interrupts and exceptions to the Supervisor or Hypervisor mode execution environment.
+
+For reference, see https://github.com/riscv/riscv-isa-manual/blob/main/src/machine.adoc#machine-cause-register-mcause[the definitions in the RISC-V ISA spec].
+
+==== Interrupts
+[%autowidth,float="center",align="center",cols=">,>,<",options="header",]
+|===
+| Code / Description |OpenSBI |oreboot SBI
+|0 _Reserved_ +
+1 Supervisor software interrupt +
+2 _Reserved_ +
+3 Machine software interrupt +
+4 _Reserved_ +
+5 Supervisor timer interrupt +
+6 _Reserved_ +
+7 Machine timer interrupt +
+8 _Reserved_ +
+9 Supervisor external interrupt +
+10 _Reserved_ +
+11 Machine external interrupt 
+12-15 _Reserved_ +
+&#8805;16 _Designated for platform use_ 
+| O +
+O +
+O +
+O +
+O +
+O +
+O +
+O +
+O +
+O +
+O +
+O +
+O
+| O +
+O +
+O +
+O +
+O +
+O +
+O +
+O +
+O +
+O +
+O +
+O +
+O
+
+X = explicitly set
+O = explicitly unset
+I = initial value unchanged
+
+==== Exceptions