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tcy = 666us @ 18 MHz oscillator frequency (24 MHz was discontinued ?!)
A 16*16 multiplication takes 4 cycles = 2,66 us
tcy = 100ns @ 40MHz oscillator frequency
A 16*16 multiplication takes 6 cycles = 0,6 us
This is another stupid example of useless advertising.
If I would like to use intensive math operation on 16 bits, definitely I wouldn't choose an 8 bit micro.
Because even if the multiplication takes 4 or 6 cycles you have to load previous the special SFR registers
(MA and MB for 80C390) and (MD0, MD1, MD4, MD5 for 80C517) and then read back the results from almost the same registers. And these times are not counted when they advertise this features on front page data sheet.
It's true that due to the determined execution time of multiplication and the parallel execution during the phase two, it's needless a busy flag (though 80C390 has one, MST)
and the fact that the results can be read any time once process started (after 4 or 6 cycles).
Thus, few microseconds gained are not big deal when using seldom 16 bits math routines overall program.
Like Atmel said in AVR ad campaign: An 8-bit MCU that can help you beat the pants off your competition.
Or printed on back car bumper sticker: My Microcontoller CAN BEAT THE HELL out of your microcontroller.