HP 35s User Manual page 48

The most "recent" number is in the X–register: this is the number you see in the
second line of the display.
Every register is separated into three parts:
A real number or a 1-D vector will occupy part 1; part 2 and part 3 will be
null in this case.
A complex number or a 2-D vector will occupy part 1 and part 2; part 3 will
be null in this case.
A 3-D vector will occupy part 1, part 2, and part 3.
In programming, the stack is used to perform calculations, to temporarily store
intermediate results, to pass stored data (variables) among programs and
subroutines, to accept input, and to deliver output.
2-2
RPN: The Automatic Memory Stack
P a r t 3
P a r t 2
T
P a r t 1 0
P a r t 3
P a r t 2
Z
P a r t 1 0
P a r t 3
P a r t 2
Y
P a r t 1 0
P a r t 3
X
P a r t 2
P a r t 1 0
" O l d e s t " n u m b e r
. 0 0 0 0
. 0 0 0 0
. 0 0 0 0
. 0 0 0 0
D i s p l a y e d
D i s p l a y e d