This microprocessor is a fairly unique 8/16 bit design. It is uses a synchronous quadrature clock for timing, and only accesses memory during 1/4 of the clocks cycle, this leaves the bus free to be used by other parts of the computer the other 3/4 of the time. However the design is not fond of not being able to access the bus during this quarter cycle, and demands memory fast enough to be zero wait state so a 1MHz 6809 wants 250ns or faster memory and a 4Mhz 6309 wants 60ns or faster memory. The rated clock speeds on these chips is also notoriously conservative, it being possible to run them at 2-3 times the rated speed with no problem, and up to 7 times with proper cooling, without cooling a 4MHz H63D09E running at 25MHz lasts about 5 minutes before burning out, and requires 10ns or faster memory for proper operation. This is also the original microprocessor for the OS-9 operating system.