Alfred,

For the std '51 ALE is 1/6 oscillator frequency (2 strobes per machine cycle).

This allows the use of a "flip-flop" to generate a 50-50 duty cycle frequency toggling at the "run rate" of the '51.

As noted any "MOVX" omits the second strobe this can be an advantage when MOVX is used to "read" an ADC. Often you read the "last conversion" you start a new conversion.

Using the MOVX and the ALE (post FLIP FLOP) you allow the LSB to convert without being near a "clock" edge (which some convertors prefer i.e. MAX172).

regards,
p