Hello, I seen the C251 Development Tools, as in large programs RTOS is required; in C251, which RTOS we have to use; Same as RTX51 (Full) or not available.
still we are not using any 251 device Then don't. Intel just discontined them and who knows when Atmel will join. If you want a 16 bitter use the far superior Philips XA. In the latest poll OF 16 bit use the XA registerd 16%, the 251 ended up in 'other'. Anyhow, were I to go beyond 16 bits today, I would go straight to the ARM. Erik