Four register classes, per the manual's diagrams. + − × ÷: X gets the result, Y takes Z, Z clears. yx: Y takes the old X, Z clears. sin cos tan ln log ex and inverses: Y holds, Z clears. 1/x √ x2 rad deg: Y and Z both hold. Nothing is ever replicated downward.

C↓ rolls the stack down — X discarded, Y→X, Z→Y, 0 into Z. Three presses therefore clear everything but memory. F then C↓ cancels the shift. Lift is suppressed only right after ENT↑. π and MR lift unconditionally.

Error limits are Appendix C of the manual, not approximations: tan above 90°, sin and cos at 401°, sin-1 cos-1 above 1, √ of a negative, ln and log of zero or less, ex outside −28 to 18.42068, yx with a non-positive base, and ÷ or 1/x at or below 10^-8. An error zeroes X and Z and leaves Y and memory intact. C↓ clears it; so does any other key except 1/x ÷ log ln, which leave it standing. Memory overflow is reported without disturbing memory.

Details from the manual. π is the truncated 3.1415926. yx is computed as e^x·ln y, as the original did. Trig is degrees only. No scientific notation. The display shuts off after 35 seconds idle, showing only decimal points — nothing is lost, and any key wakes it.

Eight digits, no guard digits. Registers are truncated to eight positions after every operation, not just at display — the manual's sphere program proves it. Every numeric result printed in the manual reproduces exactly, with one exception: yx goes through the ROM's own log and antilog, so 1.085¹⁰ reads 2.26098 on real hardware and 2.2609834 here.

Keyboard: digits, . + - * /, Enter, Backspace (C↓), f, c (CHS), x (x↔y), Esc.