HP 33s - Manual (Page 307)

User Memory and the Stack B–3
Clearing Memory
The usual way to clear user memory is to press { c {}. However,
there is also a more powerful clearing procedure that resets additional information
and is useful if the keyboard is not functioning properly.
If the calculator fails to respond to keystrokes, and you are unable to restore
operation by resetting it or changing the batteries, try the following MEMORY
CLEAR procedure. These keystrokes clear all of memory, reset the calculator, and
restore all format and modes to their original, default settings (shown below):
1. Press and hold down the ‡ key.
2. Press and hold down  .
3. Press  . (You will be pressing three keys simultaneously). When you release
all three keys, the display shows  &  if the operation is
successful.
Category CLEAR ALL MEMORY CLEAR
(Default)
Angular mode Unchanged Degrees
Base mode Unchanged Decimal
Contrast setting Unchanged Medium
Decimal point Unchanged ")"
Denominator (/c value) Unchanged 4095
Display format Unchanged FIX 4
Flags Unchanged Cleared
Fraction–display mode Unchanged Off
Random–number seed Unchanged Zero
Equation pointer EQN LIST TOP EQN LIST TOP
Equation list Cleared Cleared
FN = label Null Null
Program pointer PRGM TOP PRGM TOP
Program memory Cleared Cleared
Stack lift Enabled Enabled
Stack registers Cleared to zero Cleared to zero
Variables Cleared to zero Cleared to zero