# Process initialization

The initial state of a process is established by setting certain
Hexagon processor registers to well- defined initial values.

## Special registers

The GP register is set to the starting address of the process’s small
data area, as referenced by the program symbol, \_SDA\_BASE\_.

The UGP register is set to the highest-memory-address-plus-one of the
process’s thread-local storage area ([Thread-local storage](https://docs.qualcomm.com/doc/80-N3040-23/topic/thread-local-storage.html#thread-local-storage)).

The R28 register is set to the address of a function which the
process must call when it terminates. The process is responsible for
saving this address so it can be called later on. After saving the
address, the process can freely use the R28 register.

Note

All other special registers contain unspecified values at
process initialization, and operating systems are free to specify
their contents.

## General registers

Except for R28 and R29 (also known as SP, which points to the top of
the stack), the Hexagon general registers contain unspecified values
at process initialization.

Note

The contents of stack memory below the top of stack is
always undefined.

Last Published: Oct 06, 2026

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