[docs] add section "crt0 early boot trap handler"

docs/datasheet/software.adoc

@@ -499,7 +499,7 @@ The `crt0.S` start-up performs the following operations:
 [start=1]
 . Clear <<_mstatus>>.
 . Clear <<_mie>> disabling all interrupt sources.
-. Install an early-boot dummy trap handler to <<_mtvec>>.
+. Install an <<_early_trap_handler>> to <<_mtvec>>.
 . Initialize the global pointer `gp` and the stack pointer `sp` according to the <<_ram_layout>> provided by the linker script.
 . Initialize all integer register `x1 - x31` (only `x1 - x15` if the `E` CPU extension is enabled).
 . Setup `.data` section to configure initialized variables.
@@ -519,6 +519,22 @@ The bootloader uses the same start-up code as any "usual" application. However,
 `crt0` for the bootloader (like calling constructors and destructors). See the `crt0` source code for more information.
 
 
+:sectnums:
+===== Early Trap Handler
+
+The start-up code provides a very basic trap handler for the early boot stage. This handler does nothing but trying to move
+on to the next linear instruction whenever an interrupt or synchronous exception is encountered.
+
+This simple trap handler does not interact with the stack at all as it just uses a single register that is backup-ed
+using the <<_mscratch>> CSR. Furthermore, the information if the trap-causing instruction is compressed or uncompressed
+is **not** determined by loading the instruction from memory. Instead, the transformed instruction word is read from the
+<<_mtinst>> CSRs. These two features allow the trap handler to execute with minimal latency and high robustness.
+
+[NOTE]
+The early-trap handler should be replaced by a more capable / informative one as soon as the application software is started
+(for example by using the <<_neorv32_runtime_environment>>).
+
+
 :sectnums:
 ===== After-Main Handler