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Hello there.

Since the 45 came before the 21 and it was the first with LASTx, why doesn't the 21 have a LASTx operation?

The 45, as well as the 35, is high end. The 21 is low end.

O I C. Thanks. Well. I've got a 25 and 29. To have the next gen 35 will be amazing in and of itself.

I was thinking that the microcode, RAM, or other limitation was the reason. I figured, with the improved function set, memory arithmetic and Radians mode, there was little to no room left for LASTx.

Thanks for the clarification.

Edited: 24 Mar 2012, 3:02 p.m.

The -21 has only CPU and ROM, it has no RAM chip, so all memory in the CPU is devoted to the stack and one memory register. The -25, for instance, has a RAM chip with 16 registers; 8 for memories R0 thru R7, seven for 49 program steps (7 bytes per register, one byte per function) and one register was still available for Last X.

This explains so much! Thanks! Now, the program/register allocation structure that goes all the way back to the HP-34C makes sense.

Yes, it is a matter of ROM size.

The HP-35 has 7680 bits of ROM, and the HP-45 has 14336 (not counting an additional 2560 occupied by the undocumented timer mode).

The HP-21 has 10240 bits of ROM. That combined with the increased efficiency of the second-generation "Woodstock" processor architecture allows it to have more functionality than the HP-35, but not as much as the HP-45.

The HP-25 and HP-25C have 20480 bits of ROM, the same amount as the HP-45 (including the latter's timer mode), yet it packs in a lot more functionality, including programmability, thanks to the efficiency of Woodstock.

On the other hand, the HP-29C, which superficially seems like it has only relatively minor feature enhancements over the HP-25/25C, uses twice as much ROM, at 40960 bits.

Edited: 24 Mar 2012, 7:20 p.m.