The Dallas DS89C420 was, IIRC, the first one with all the features to make the dual data pointer really useful. In this version, you could access the data pointer such that it used both data pointers alternately, or not, and it could auto increment or decrement either one, or not, and a few other things that several of the other ones that I only encountered later couldn't do.

This allowed me to point, via one DPTR, to a fixed external port, to move data to or from memory at an increasing or decreasing location with a minimum of time and of instructions.

When this MCU was first introduced, it was claimed to operate at 50 MHz with a single clock cycle per instruction. It did, however, turn out that, while some of the parts worked OK at that rate, the family members later developed couldn't execute instructions from internal memory at that rate, except from the internal 1kB SRAM that could be mapped pretty creatively, the later family members did work satisfactorily at 33 MHz. This was plenty fast for those operations that used the dual DPTR as described above.

RE