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kernel attributes: explain their location #57

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kernel attributes: explain their location #57

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8 changes: 8 additions & 0 deletions src/doc/kernel_attributes.md
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Expand Up @@ -5,6 +5,14 @@ allocated flash region. These attributes describe properties of the flashed
kernel on a particular hardware board. External tools can read these attributes
to learn about the kernel installed on the board.

The attributes are stored at the end of the kernel's allocated flash region to
help with discoverability. Tock tools generally assume that 1) userspace
applications are installed immediately after the end of the kernel region in
flash, and 2) that the address for the start of userspace applications is known.
By placing the kernel attributes at the end of the kernel flash region, Tock
tools can find the attributes by starting at the start of userspace applications
in flash and working backwards.

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in flash and working backwards.
in flash and working backwards.
Storing attributes at the end also has a secondary benefit of zero-filling the rest of
the flash region. While not essential for correct operation, as unused flash should
always be inaccessible, this provides an additional defense-in-depth step for the
Tock platform by default.

## Format

Kernel attributes are stored in a descending TLV (type-length-value) structure.
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