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I am under the impression that the origin of RAM on a Cortex M is 0x20000000. However, in the linker file for it, the ORIGIN of RAM is set to 0x1FFFF800. Is that an error, or is there a reason for the discrepancy?

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The Cortex M does not "have RAM at 0x2000000". The Cortex M is just a CPU. It is down to the chip manufacturer where they choose to place RAM in the memory map.

In the case of the MKL26Z64VFT4 used on the Teensy LC:

The on-chip SRAM is a single contiguous block split into two ranges: 1/4 is allocated to SRAM_L, and 3/4 is allocated to SRAM_U. Regardless of the block size, the SRAM is implemented such that:

  • SRAM_L is anchored to 0x1FFF_FFFF and occupies the space before this ending address.
  • SRAM_U is anchored to 0x2000_0000 and occupies the space after this beginning address.

Valid address ranges for SRAM_L and SRAM_U are then defined as:

  • SRAM_L = [0x2000_0000–(SRAM_size/4)] to 0x1FFF_FFFF
  • SRAM_U = 0x2000_0000 to [0x2000_0000+(SRAM_size*(3/4))-1]

Since there is 8kB of SRAM, 2kB is below 0x2000000 and 6kB is above 0x20000000. 2kB is 0x800. 0x2000000 - 0x800 = 0x1FFFF800.

  • Thanks for that, good to know. One question, though: why does "the cortex memory map" exist, if the chip manufacturers are free to modify it? – Michael Stachowsky Jan 23 '17 at 18:48
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    For the M0 there is a "default" memory map, which is only a suggestion. It's up to the manufacturer what they do. – Majenko Jan 23 '17 at 18:50

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