Four?! Why not more?

[Steve Simpkin]

(06-14-2021 06:56 PM)Matt Agajanian Wrote:  

(06-14-2021 01:21 AM)Steve Simpkin Wrote:  <Large Snip>

This last factor (and tradition) probably kept HP's later calculator models at 4-levels. The addition of a Last X register starting with the HP-45 also helped.

<End Snip>

Well, since including a Last X register began with the 45 in the Classic series, the tech was already there. So,
why not include the Last X register beginning with the 21?

The Last X feature required one of the 56-bit registers from an external RAM chip to store the Last X value. The HP-21 did not have an external RAM chip so this feature could not be offered on that model.

http://www.brouhaha.com/~eric/hpcalc/memory_size.html