If the 21 came after the 45...?

[Matt Agajanian]

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?

[Thomas Radtke]

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

[Matt Agajanian]

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.

[Eric Smith]

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.

[Andrés C. Rodríguez (Argentina)]

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.

[Matt Agajanian]

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