Wednesday, November 30, 2011

Odd/even value for Cortex M3 reset vector

After reset the CM3 loads it's startup address from address 0x00000004.
The value at that location should be odd (LSB set) denoting the thumb destination. All exception and interrupt vectors following reset vector should be odd.
The reset vector might not have the LSB set by the linker. This is when reset handler code is written in assembler.
It's label is usually used in exception handlers table, but the linker doesn't know that this is a THUMB function address, so it doesn't change the value in exception table. It actually shouldn't since it could be a label of some data table used elsewhere - linker doesn't know anything about exception table either!
We need to tell the linker that some label is a THUMB function. GCC provides '--thumb-entry' command line switch to tell the linker about this one function but I couldn't get it to work. What helped was marking the label as THUMB function with '.type' and/or '.thumb_func' directive - depends on the GCC build/version you are using.

.type Reset, %function
  <code here>

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