HP21 uses same RAM chip as HP25?

[Didier Lachieze]

(05-21-2019 11:14 AM)calc-calcs Wrote:  I suppose the 21 uses a different chip, but it is not even listed as using any other RAM chip at all, or maybe that part is just missing from the list.

The ACT includes 8 memory registers, so the 21 doesn't need a dedicated RAM chip as there is only 1 user memory register in addition to the stack. The RAM chip is used on the 25 for the additional memory registers and the program memory.
From Jacques Laporte description on this page:

Quote:There are 3 chips in the HP-21 (ACT , Anode Driver + ROM, Cathode Driver) and 5 in the HP-25 (one extra ROM and the RAM chip).

Note: the 22 needs the RAM chip because while being not programmable, it includes 10 user memory registers in addition to the TVM registers.