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I would like to define a C variable in RAM which can survive a CPU reset in NIOS II IDE. Something like __persistent in PIC world or __no_init in Renesas world.

Is there a similar keyword for NIOS compiler, which is based on gcc?

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    \$\begingroup\$ In order to have a complete question first you need to identify the compiler, is it gcc? You may need to do this manually creating a section in the linker script. \$\endgroup\$ – Chris Stratton Jun 7 at 13:14
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NIOS uses a GCC variant, so you can use the GCC style attributes to place a variable in the .noinit section which basically forgoes any initialisation of the variable. e.g.

char someVar __attribute__ ((section (".noinit")));

The RAM is not cleared during a NIOS processor reset, and GCC wont initialise the value of it, so the variable should survive.

It wouldn't however survive a full FPGA reset/reconfigure as that would reinitialise the RAM entirely. To cope with that you would need external SRAM.

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Putting the variable in .noinit section seems to be the right way to do it. However, .noinit section is missing in auto-generated NIOS linker file, so I had to add it manually:

.noinit (NOLOAD):
{
    PROVIDE (_alt_partition_noinit_start = ABSOLUTE(.));
    *(.noinit .noinit.*)
    . = ALIGN(4);
    PROVIDE (_alt_partition_noinit_end = ABSOLUTE(.));
} > onchip_memory2_0

The important part is the (NOLOAD) directive which tells the linker that it shouldn't include any initialization code for this section.

Of course, the content of the on-chip RAM will be lost during reconfiguration or when its reset signal is asserted.

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