HP 35s Scientific Calculator - Manual (Page 307)

User Memory and the Stack B-7
Accessing Stack Register Contents
The values held in the four stack registers, X, Y, Z and T, are accessible in RPN
mode in an equation or program using the REGX, REGY, REGZ and REGT
commands.
To use these instructions, press d first. Then, pressing < produces a menu in
the display showing the X–, Y–, Z–, T–registers. Pressing Õ or Ö will move the
underline symbol, indicating which register is presently selected. Pressing 
will place an instruction into a program or equation that recalls the value of the
chosen stack register for further use. These are displayed as REGX, REGY, REGZ,
and REGT.
For example, a program line entered by first pressing d and then entering the
instructions REGX x REGY x REGZ x REGT will compute the product of the values in
the 4 stack registers and place the result into the X-register. It will leave the previous
values of X, Y, and Z in the stack registers Y, Z, and T.
Many such efficient uses of values in the stack are possible in this manner that
would not otherwise be available on the HP35s.