RPN Power 48 manual · Chapter 3 of 19

The stack

How the RPL stack works: numbered levels, ENTER, SWAP, DROP, ROT, PICK, ROLL, DEPTH, LASTARG, UNDO and how commands use the levels.

3.1Stack levels and what commands do with them#

The stack is a tower of objects. Level 1 is the newest and sits at the bottom of the display; level 2 is just above it, and so on. Unlike a four-register stack, this one is unlimited, so you never lose a number off the top. The screen shows as many levels as fit above the command line.

A command removes the objects it needs from the top levels and pushes its answer. + removes levels 1 and 2 and pushes the sum. A function such as √x takes one object; a command such as XROOT takes two. If the stack has too few objects the command fails with Too Few Arguments and the stack is restored.

The numbers 1 to 4 on the stack. 4 is the newest, on level 1. The display shows: RAD { HOME } / 6: / 5: / 4: 1 / 3: 2 / 2: 3 / 1: 4. — The numbers 1 to 4 on the stack. 4 is the newest, on level 1.

TZYX

1 ENTER

2 ENTER

3 ENTER

123

4 ENTER

1234

Pushing numbers. In each column the first value is level 1 (the newest object).

3.2ENTER, space and copying with ENTER#

ENTER takes what you typed on the command line, compiles it into objects and pushes them. A space does the same for separating several objects on one line. There is no hidden stack lift to worry about: a new number never replaces an old one.

ENTER on an empty command line copies level 1 (the DUP command), which is a quick way to square a number or use it twice.

Example 3.1

Square by copying

Type 7, ENTER to push it, ENTER again to copy it, then multiply.
7ENTERENTER×

Example 3.2

Enter several numbers on one line

Type 1 2 3 with spaces, then ENTER: three objects arrive.
1SPC2SPC3ENTER

TZYX

7 ENTER

ENTER

×

ENTER on an empty line copies level 1.

3.3The STACK menu#

Left shift TOOL opens the STACK menu. It has three pages of six commands.

STACK page 1: DUP, SWAP, DROP, OVER, ROT, UNROT. The display shows: RAD { HOME } / 6: / 5: / 4: / 3: / 2: / 1: / DUP SWAP DROP OVER ROT UNROT. — STACK page 1: DUP, SWAP, DROP, OVER, ROT, UNROT.

STACK page 2: PICK, ROLL, ROLLD, DEPTH, DUP2, DROP2. The display shows: RAD { HOME } / 6: / 5: / 4: / 3: / 2: / 1: / PICK ROLL ROLLD DEPTH DUP2 DROP2. — STACK page 2: PICK, ROLL, ROLLD, DEPTH, DUP2, DROP2.

STACK page 3: DUPN, DROPN, CLEAR, LASTARG, PICK3, NIP. The display shows: RAD { HOME } / 6: / 5: / 4: / 3: / 2: / 1: / DUPN DROPNCLEAR LASTAPICK3 NIP . — STACK page 3: DUPN, DROPN, CLEAR, LASTARG, PICK3, NIP.

Command	What it does	Stack before and after (level 1 last)
DUP	Copies level 1.	1 2 → 1 2 2
SWAP	Exchanges levels 1 and 2.	1 2 → 2 1
DROP	Removes level 1.	1 2 → 1
OVER	Copies level 2 to level 1.	1 2 → 1 2 1
ROT	Moves level 3 to level 1.	1 2 3 → 2 3 1
UNROT	Moves level 1 to level 3.	1 2 3 → 3 1 2
PICK	Copies level n to level 1 (n is typed first).	1 2 3 2 → 1 2 3 2
ROLL	Moves level n to level 1.	1 2 3 4 3 → 1 3 4 2
ROLLD	Moves level 1 to level n.	1 2 3 4 3 → 1 4 2 3
DEPTH	Pushes the number of objects.	1 2 3 → 1 2 3 3
DUP2	Copies levels 1 and 2.	1 2 → 1 2 1 2
DROP2	Removes levels 1 and 2.	1 2 3 → 1
DUPN	Copies the top n objects.	1 2 3 2 → 1 2 3 2 3
DROPN	Removes the top n objects.	1 2 3 4 2 → 1 2
CLEAR	Empties the stack.	1 2 3 → (empty)
LASTARG	Puts back the arguments of the last command.	—
PICK3	Copies level 3 to level 1.	1 2 3 → 1 2 3 1
NIP	Removes level 2.	1 2 3 → 1 3

3.4Swap, drop, over and rotate#

Example 3.3

Swap levels 1 and 2

Put 1, 2 and 3 on the stack.
1ENTER2ENTER3ENTER
Open STACK and press SWAP (second key).
◀STACKTOOLF2

The shortcut for SWAP is the right shift on the 0 key.

Example 3.4

Drop the top object

Put 1, 2 and 3 on the stack and press DROP (third key).
1ENTER2ENTER3ENTER◀STACKTOOLF3

Example 3.5

Copy level 2 with OVER

Put 1, 2 on the stack and press OVER (fourth key).
1ENTER2ENTER◀STACKTOOLF4

Example 3.6

Rotate three objects

Put 1, 2, 3 and press ROT (fifth key). Level 3 moves to level 1.
1ENTER2ENTER3ENTER◀STACKTOOLF5

Example 3.7

Rotate the other way

UNROT (sixth key) moves level 1 down to level 3.
1ENTER2ENTER3ENTER◀STACKTOOLF6

TZYX

start

123

SWAP

132

ROT

321

UNROT

132

Swap, then rotate. First value is level 1.

3.5Pick, roll, depth and the n commands#

Commands that need a count take it from level 1: type the count and then press the command key.

Example 3.8

Copy an object from deep in the stack

Put 1, 2, 3 on the stack, type 2 and press PICK (STACK page 2, first key).
1ENTER2ENTER3ENTER2◀STACKTOOLNXTF1

Example 3.9

Roll the third object to the top

Put 1 to 4 on the stack, type 3, press ROLL (page 2, second key).
1ENTER2ENTER3ENTER4ENTER3◀STACKTOOLNXTF2

Example 3.10

Roll the top object down

Same stack, type 3, press ROLLD (page 2, third key).
1ENTER2ENTER3ENTER4ENTER3◀STACKTOOLNXTF3

Example 3.11

Count the objects with DEPTH

Left shift 0 is DEPTH.
1ENTER2ENTER3ENTER◀DEPTH0

Example 3.12

Copy two objects

DUP2 (page 2, fifth key).
1ENTER2ENTER◀STACKTOOLNXTF5

Example 3.13

Remove two objects

DROP2 (page 2, sixth key).
1ENTER2ENTER3ENTER◀STACKTOOLNXTF6

Example 3.14

Copy the top three objects with DUPN

Type 2 and press DUPN (page 3, first key).
1ENTER2ENTER3ENTER2◀STACKTOOLNXTNXTF1

The count is consumed. DUPN 2 copies the top two objects.

Example 3.15

Remove the second object with NIP

NIP is the sixth key on page 3.
1ENTER2ENTER3ENTER◀STACKTOOLNXTNXTF6

Example 3.16

Copy level 3 with PICK3

PICK3 is the fifth key on page 3.
1ENTER2ENTER3ENTER◀STACKTOOLNXTNXTF5

3.6Recover from mistakes: LASTARG and UNDO#

Right shift HIST is LASTARG: it pushes back the arguments that the last command used, so you can try another command on them. Left shift HIST is UNDO: it restores the stack as it was before the last action. Press UNDO again to redo.

Example 3.17

Get back the numbers you added

Add 3 and 4, then press LASTARG.
3ENTER4+▶LASTHIST

Example 3.18

Undo an operation

Add 3 and 4, enter them first so both are on the stack, then undo.
3ENTER4ENTER+◀UNDOHIST

Example 3.19

Undo straight after typing

Type 4 and press + without ENTER, then undo. The 4 you typed stays on the stack.
3ENTER4+◀UNDOHIST

The plain HIST key brings back your last command lines (up to eight) into the command line so you can edit and run them again.

3.7Plan a longer calculation#

Work from the inside out, and use the unlimited stack to keep every intermediate result. Compare with brackets: to find $\frac{12}{2+4}$ you add 2 and 4 first, then divide.

\frac{12}{2+4}=2

Example 3.20

Calculate 12 divided by (2 + 4)

Push 12 and then 2 and 4.
12ENTER2ENTER4ENTER
Add the top two, then divide.
+÷

TZYX

12 2 4 ENTER

1224

+

126

÷

Evaluating 12 / (2 + 4). First value is level 1.

Example 3.21

Calculate (2 × 3) + (4 × 5)

2\times3+4\times5=26

Multiply each pair, then add.
2ENTER3×4ENTER5×+

Result of the example "Calculate (2 × 3) + (4 × 5)" The display shows: RAD { HOME } / 6: / 5: / 4: / 3: / 2: / 1: 26. — You should see`1: 26`

3.8Edit the object on level 1#

Press ▼ when there is no command line, or right shift ENTER (EDIT), to copy level 1 into the command line. Change it with the cursor keys and press ENTER: the edited object replaces the old one.

Example 3.22

Edit a number

Push 5, open it for editing, add a digit and press ENTER.
5ENTER▶EDITENTER6ENTER

HP 48 and HP 50g is a trademark of HP Inc.. Trident Calculator is an independent product by South View Studios and is not affiliated with, sponsored by or endorsed by HP. The name is used only to describe the kind of calculator this model resembles; Trident's names, keys, display, fonts and software are its own clean-room design.