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# Executables

Sentinel 65X's firmware is designed to load and execute binaries in the `PGZ` format. This is a multi-segment binary file format which can load data into any number of areas of memory from a compact binary.

## PGZ Format

The first byte of a PGZ file is always an ASCII "Z", hexadecimal value `0x5A`, decimal value `90`.

This is followed by one or more segments, packed with no padding or separator. Each such segment has the following format:

| Offset | Length  |  Description   |
| :----: | :----:  | :------------: |
| 0      | 3 bytes | Target address |
| 3      | 3 bytes | Length of data |
| 6      | ...     | Data           |

This enables the binary loader to be extremely simple, as the data to be read into memory is always read from a contiguous region of memory, and the information provided is sufficient to make use of the C standard library `memcpy()` function.

The final segment is special: its target address represents the entry point to the executable, and its length of data value is always `0x000000`. The loader can easily detect this to identify when loading is done and where to enter the program.