The Saturn CPU that was used in HP calculators back in the 90s had a hardware return stack, with only eight levels. It was only possible to use six levels safel

The Saturn CPU that was used in HP calculators back in the 90s had a hardware return stack, with only eight levels. It was only possible to use six levels safely, as the interrupt routine used the remaining two. The hardware stack enabled nice tricks back in the day, at the whole CPU was a fun little beast. 4-bits bus and addressable units (nibbles), plenty of 64bits registers, 20 bits addresses...