Ti 34 circa 1987

[jebem]

I didn't spend too much time with this nice machine, but I believe that the A to F labels are used to type in hexadecimal numbers.
I think it has only a single memory register available for arithmetic operations, hence we just use the STO key without arguments. Of course it also have extra dedicated registers for Statistics.

More detailed info can be found in the Toshiba 7988 SoC processor, as this Texas uses just this chip with a textbook implementation (no extra features added):
http://pdf.dzsc.com/JT7/JT7988Y-AS.pdf