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I use the Eclipse IDE but compile with Keil and its RTX RTOS and TCPnet. Ended up porting over some examples of SGDMA and Ethernet TSE usage. Has not been an easy task.
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Not a glowing endorsement then, eh? :)
Why did you decide that a Cortex-M1 would be a better choice than a NIOS II processor?
If you've been building systems for a while, the processor architectures eventually all blur together. At that point, the best way to select the architecture is to base it on support; either from the vendor or the community.
I figure "when in Rome" (when in an FPGA), using the vendor's soft-core processing is the option least likely to cause pain.
Cheers,
Dave