RPN-65 Question.

[Steve Simpkin]

I don't think so. The Arithmetic and Register (A&R) chip used on the Classic series contained 7 56-bit (14 BCD digits) registers. This was not quite enough for calculating trig functions. Three of these registers were the X, Y, and Z registers of the four-level stack. Two more were working registers and one was the user-visible memory register (STO/RCL.) The final register did double duty as the topmost register in the stack (T) and as a working register for trigonometric functions. Thus, when a trig function was used, the topmost value in the stack was lost. It appears the HP-65 used register 9 instead of stack register T. But the result was the same. 7 registers were just not enough for the Stack and working registers during trig calculations.

Among the improvements on the ACT chip used in the Woodstock series was the addition of one more internal register.

From the "20 Series Technology and Packaging" page:

"The heart of the 20 series calculators, as well as many pocket calculators that followed, was the Arithmetic, Control and Timing chip (ACT). This chip integrated the earlier generation's register, arithmetic, control, and clock driver circuit into a single package reducing size and cost.

The ACT chip had 8 56-bit data registers, an address register, two return address registers, 16 flags, and a 4-bit display format register. The chip could directly address 4096 instructions with its 12-bit address space. Because there was now one more register on the chip, trig calculations no longer overwrote the top of the stack as they did on the HP-35. "

https://www.hpmuseum.org/tech20.htm