Devmem2 is a simple utility to read/write to any memory mapped locations. It is not available in debian etch but I find the source code (free GNU license) , compiled and run it perfectly on my gesbc debian eth environment.
$ gcc devmem2.c -o devmem2
To use devmem2 to toggle RTS register of the Serial Port 4 to high (RS485 mode).
./devmem2 0xfffd0004 w 0xc00008c1