The trick you describe was being used by Silicon Valley Computer ADP50L IDE cont...

The trick you describe was being used by Silicon Valley Computer ADP50L IDE controller from early nineties (1991). Memory mapped I/O instead of traditional x86 port access lets you skip doing manual loop for 'rep movsb', result can be 50% speed bump

https://forum.vcfed.org/index.php?threads/performance-of-lo-...

Port IO Read Speed : 219.39 KB/s

MMIO Read Speed : 310.77 KB/s

Some variants of XTIDE hardware also implement this, as does the free bios.