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> Cortex R4 soft processorsHmm, Cortex R4 isn't a soft processor AFAIK - it's designed for ASIC silicon implementation.Cortex-M1 is designed for low gate count - it only supports the ARMv6M variant of the Thumb instruction set, has a very simple single issue pipeline, and no cache interface, and perhaps can run at 100MHz in a fast FPGA (not entirely sure, never used one).Cortex-R4 is designed for higher performance implementations in silicon - it supports ARM and Thumb instruction sets (more suitable for algorithmic work), has a dual-issue pipeline (two instructions per cycle peak), and targets around 400MHz. A very different beast indeed. As ttfn says, if you want more details it's best too look at the two architecture documents - there are lots of differences!