Nrf52840-Dongle Memory Map Explained

[jibba]

Hi

Can you help me by explaining the default memory map on the dongle?
What i have is this (see below)
I suspect the following:

- ? 3 ? is maybe some shared ram between bootloader and user-app

is this true? is there some doc on this?
also what i dont know, the ? 2 ?, where does it end? where does ? 3 ? end?

thanks

# Notes

## Nrf52840-Dongle

- Flash: 1M
- Ram: 256k

### Memory Map (Example ot/cli/ftd/usb/pca10059/mbr)

#### Flash

| Name                | Region                    | Region Dec        | Size        | Size Dec |
|---------------------|---------------------------|-------------------|-------------|----------|
| MBR                 | 0x0000'0000 - 0x0000'1000 |       0 -   4'096 | 0x0000'1000 |    4'096 |
| USER Application    | 0x0000'1000 - 0x000D'C000 |   4'096 - 901'120 | 0x000D'B000 |  897'024 |
| USER OpenThread NVM | 0x000D'C000 - 0x000E'0000 | 901'120 - 917'504 | 0x0000'4000 |   16'384 |
| BOOTLOADER          | 0x000E'0000 - 0x    '     | 917'504 -         | 0x    '     |     '    |

#### RAM

| Name                | Region                    | Region Dec (Rel.) | Size        | Size Dec |
|---------------------|---------------------------|-------------------|-------------|----------|
| ? 3 ?               | 0x2000'0000 - 0x2000'0008 |       0 -       8 | 0x0000'0008 |        8 |
| USER RAM            | 0x2000'0008 - 0x2004'0000 |       8 - 262'144 | 0x0003'FFF8 |  262'136 |

[jibba]

Yes im using the dongle.

Could you elaborate a little bit more please?
I see both mutliple flash regions and some shared-ram.

Meanwhile i know that the first section in flash is MBR, then the last section at the top of flash is the BL.
Why do i need to have those 8 bytes shared-ram within my application, as i imagine, the jumps go as follows: MBR->BL->APP. where from APP i will never jump back in either one of them, just target reset. So why do i need to reserve shared ram?

[Amanda Hsieh]

Hi, 

The first 8 bytes in ram are used for vector forwarding and must be reserved for the MBR.  Letting the app overwrite this part of the RAM will lead to undefined behavior. We don't have a separate document for the MBR, but the Softdevice use the exact same version, so you may refer to the Softdevice specification for details (Master boot record and bootloader). And the RAM memory requirements are the same as when the Softdevice is disabled: Memory resource requirements.

-Amanda H.