HP 33s hp 33s_user's manual_English_E_HDPM20PIE56.pdf - Page 42

T 0.0000 "Oldest" number Z 0.0000 Y 0.0000 Displayed X 0.0000 Displayed The most "recent" number is in the X-register: this is the number you see in the second line of the display. 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. The X and Y-Registers are in the Display The X and Y-Registers are what you see except when a menu, a message, or a program line is being displayed. You might have noticed that several function names include an x or y. This is no coincidence: these letters refer to the X- and Y-registers. For example, {  raises ten to the power of the number in the X-register. Clearing the X-Register Pressing { c {º} always clears the X-register to zero; it is also used to program this instruction. The ‡ key, in contrast, is context-sensitive. It either clears or cancels the current display, depending on the situation: it acts like { c {º} only when the X-register is displayed. b also acts like { c {º} when the X-register is displayed and digit entry is terminated (no cursor present). It cancels other displays: menus, labeled numbers, messages, equation entry, and program entry. 2-2 RPN: The Automatic Memory Stack