Appraisal Reference

HP12c Basics

Two things confuse everyone who picks up an HP-12c for the first time: there is no equals key, and numbers have to be loaded before the operation that uses them. Once those land, the rest of the calculator follows. Step through the sequences below and watch what happens inside.

No equals key Step it through The three percent keys n and i Housekeeping

There is no equals key

And that is deliberate, not an omission.

An ordinary calculator waits for you to finish: you type the whole expression, press equals, and it works out what you meant. The HP-12c never waits. Every key acts the instant you press it, on numbers you have already given it.

Which means the order changes. Instead of 1000 + 250 =, you load the first number, tell the calculator you are done with it, load the second, and then press the operation:

NUMBER → ENTER → NUMBER → FUNCTION

1000 ENTER 250 + gives 1,250.00. The ENTER key is not equals — it is the opposite. It means I have finished typing this number, put it away.

Where does it get put away? Into a stack of four registers named X, Y, Z and T. X is what the display shows. Pressing ENTER copies X up into Y and leaves X free for the next number. When you then press an operation, it acts on Y and X together and drops the answer into X.

Why this is worth tolerating. Because nothing is ever pending. On an ordinary calculator a long chain leaves you unsure what has been applied and what has not. Here, every intermediate result is a real number sitting in the display, which is exactly what you want when a figure has to be defended line by line.

It also means a chain needs only one ENTER, at the very start. Every result after that pushes itself out of the way when you type the next number. Step through A chain of operations below and watch it happen.

Step it through

Pick a sequence, then press Next. The key lights as you reach it and the stack shows what moved.

The three percent keys

They answer three different questions, and using the wrong one gives a plausible number.

Key	Question it answers	Keystrokes	Shows
%	What is 8% of 240,000?	`240000 ENTER 8 %`	19,200.00
Δ%	By what percent did 300,000 change to 330,000?	`300000 ENTER 330000 Δ%`	10.00
%T	What percent of 450,000 is 90,000?	`450000 ENTER 90000 %T`	20.00

All three follow the same shape. The base or starting figure goes in first, then ENTER, then the second number, then the key. The order matters — reverse the two numbers on Δ% and you get the change measured from the wrong direction.

Two of these return a percentage and one returns a dollar amount, which is worth noticing before you write the answer down. % gives you dollars; Δ% and %T give you percentages. A 19,200 where you expected 8 is not a broken calculator.

The % key leaves the base in Y. After 240000 ENTER 8 % the display shows 19,200 but 240,000 is still sitting in Y. Press + and you get 259,200 — the base plus the 8%, in one more keystroke. Press − and you get 220,800. That is how a vacancy deduction or a percentage adjustment gets done without retyping anything.

n and i

Periods and the rate per period. Not years and not the annual rate — unless the periods happen to be years.

The five financial keys across the top — n, i, PV, PMT, FV — are five registers. Fill any four and press the fifth to solve for it. That is the whole design.

n is the number of periods and i is the rate per period. A thirty-year monthly mortgage at 6% is not 30 n and 6 i. It is 360 periods at 0.5 percent each.

Because that conversion is needed constantly, the calculator does it for you. The blue g key followed by n multiplies what you typed by twelve; g then i divides by twelve:

You press	Register becomes	Meaning
`30 g n`	n = 360	thirty years of monthly periods
`6 g i`	i = 0.5	six percent a year, monthly
`360 n`	n = 360	the same thing typed out

The sign convention catches everyone once. Money coming to you is positive; money leaving you is negative. Enter a loan as a positive PV and the payment comes back negative, because the payment leaves your pocket. That minus sign is the calculator agreeing with you, not objecting. If PV and PMT carry the same sign, the calculator will refuse the problem outright.

The fifth walkthrough above runs a full payment calculation, sign and all. Step through it and watch which registers fill before PMT is pressed.

Housekeeping

Four things worth knowing before you trust an answer.

Keys	What it does	When you need it
`f CLEAR FIN` f then x≷y	Clears the five financial registers only	Before every new time-value problem, without exception
`f CLEAR REG` f then CLx — spoken as f CLx	Clears the stack, the financial registers and the storage registers — but not stored programs	Starting fresh on the numbers, or when you are not sure what is left behind
`CLx`	Clears the display only	You mistyped the number you are on
`STO 1` / `RCL 1`	Stores and recalls a number	Parking a figure you will need three steps later
`x≷y`	Swaps X and Y	You loaded two numbers in the wrong order

Leftover registers are the single most common source of a wrong answer. The financial registers keep their contents until cleared — including after the calculator is switched off. A payment left in PMT from the last problem will quietly corrupt the next one, and nothing on the display warns you. f CLEAR FIN costs two keystrokes.

Find CLEAR on the keyboard above. The gold bracket in the third row spans three keys — FIN, REG and PREFIX. Those are not separate keys; they are what f reaches on x≷y, CLx and ENTER. Once you can see the bracket, f CLEAR FIN stops being a memorised incantation.

Two names for one keystroke. The gold legends above these keys are what f reaches, but the labels printed on the keys are what people say out loud. So f CLEAR REG in the manual is f CLx in a classroom, and f CLEAR FIN is f x≷y. Neither is wrong. Recognise both, because the written material and the spoken instruction will not match.

CLEAR REG does not touch a stored program. It empties the registers, and that is all. A program is a set of keystrokes entered in program mode, and it lives in its own memory — unaffected by any of the clearing above and unaffected by switching the calculator off. To clear one you have to go into program mode and clear it there — f PRGM on the R▽ key, which the Programming basics walkthrough above steps through. That is also where you will see why a program survives being switched off.

Set the display to two decimals once and leave it. f 2 shows two places, f 0 none, f 7 seven. The calculator always holds full precision internally regardless of what it shows, so this changes the display and never the arithmetic. The Decimal places walkthrough above shows one divided by three at four different settings with the held value visible beside it — worth stepping through once if you have ever doubted it.