Welcome

Free85 is a scientific graphing calculator that runs in your browser. It is a complete, independently written firmware of original Z80 code, running on a faithful emulation of a TI-85-compatible pocket calculator.

Free85 is clean-room software. Everything in it (the firmware, the font, the screen artwork, the tests, and this manual) was written from scratch for this project. It contains no Texas Instruments ROM code, disassembly, fonts, artwork, or binary tables. The TI-85 is referenced only to describe the hardware profile the firmware runs on; the project is not affiliated with or endorsed by Texas Instruments. Because the internals are original, Free85 makes no promise of compatibility with TI programs, files, tokens, ROM calls, or internal data structures. What it does promise is that every physical key, every shifted function printed above a key, and every reachable menu leads to a real, working feature.

Free85 is open source under the MIT License. See the LICENSE file for the licence text and NOTICE.md for the project notices.

This manual gets you from a blank screen to confident everyday use: running the calculator, understanding the keyboard and display, doing your first calculations, adjusting modes, and finding functions in the catalog. When you want depth (graphing, statistics, programming, and everything else), the companion Guidebook covers each subject in its own chapter. The final section of this manual maps out where to go.

The hardware at a glance

The machine Free85 targets is a Z80 processor at 6 MHz driving a 128 by 64 pixel monochrome LCD, with eight 16 KiB ROM pages (131,072 bytes in all), 32 KiB of RAM, and a keyboard of 49 matrix keys plus a separate ON key.

Running Free85

The easiest way to use Free85 is the browser calculator published from this repository by GitHub Pages. It lives at https://chriswilson2020.github.io/Free85/ and is also linked from the repository README in case the address changes. Open the page and the calculator is ready to use, no installation at all.

To run it on your own machine you need Node.js 24 or newer. From the repository root:

npm run dev

Then open http://localhost:3000/ in a browser. The bundled open-source FREE85.ROM loads automatically, and the page also lets you select another compatible 128 KiB ROM if you have one.

There is also a quick terminal preview that boots the machine, presses any keys you list, and prints an ASCII rendering of the LCD:

npm run run:free85 -- GRAPH

This is handy for scripting and for checking behaviour without opening a browser. The key names it accepts are the physical key names used throughout this manual, such as ENTER, 2ND, GRAPH, and F1.

The keyboard

The keyboard has fifty keys arranged in a few zones. Once you know the zones, your fingers find things quickly.

• Soft keys. F1 F2 F3 F4 F5 sit directly under the screen. They have no fixed meaning of their own: they select whatever five labels are currently shown along the bottom row of the display. When the labels change, the keys change with them.
• Navigation and modifiers. The next two rows share the cursor pad on their right-hand side: the first row holds 2nd, EXIT, and MORE beside the ▲ and ▼ keys, and the second holds ALPHA, x-VAR, and DEL beside ◀ and ▶. EXIT backs out one level; 2nd EXIT is QUIT, which jumps straight back to the home screen.
• Application keys. GRAPH, STAT, PRGM, CUSTOM, and CLEAR open the major applications: graphing, statistics, programming, your custom menu, and clearing the entry line.
• Function rows. LOG, SIN, COS, TAN, ^, then LN, EE, (, ), ÷, then x² beside the digit pad. Each carries a shifted function printed above it and, on most keys, a letter for alpha entry.
• Digit pad. The digits, decimal point, (-) for negative numbers, the arithmetic operators down the right-hand side, STO▶ for storing variables, and ENTER in the bottom corner.
• The ON key. A separate key that wakes the calculator or interrupts a running operation; 2nd ON turns it off.

How 2nd works

2nd is a one-shot modifier. Press it once, and the next key you press performs the shifted function printed above that key instead of its normal one. For example, ^ normally types the power operator, but 2nd ^ types PI. Try it: press 2, then 2nd ^, then ENTER, and the calculator evaluates 2PI to 6.2831853071796. The modifier applies to exactly one keypress and then switches itself off.

How ALPHA works

ALPHA is the letter modifier. Most keys have a letter assigned to them: LOG is A, SIN is B, and so on through the keyboard to . for Z, with x-VAR providing x. Press ALPHA once and the next key types its letter: ALPHA LOG types A.

Press ALPHA twice and letter entry locks: the status line shows LOCK, and every key types its letter until you press ALPHA again to unlock. For example, ALPHA ALPHA SIN ALPHA 1 types B1 (the lock typed B, the third ALPHA released it, and 1 went back to being a digit).

Lowercase letters exist too. 2nd ALPHA selects lowercase letter entry; then ALPHA followed by a letter key types the lowercase letter. For example, 2nd ALPHA ALPHA SIN types b.

What MORE does

Menus in Free85 show at most five choices at a time, one per soft key, and MORE pages to the next set. In the MATH menu, for example, the first page offers ABS(, SQRT(, FACT(, NPR(, and NCR(; pressing MORE brings the next five, starting with SINH( and COSH(. Keep pressing MORE to cycle through all the pages. The same paging works on any row of soft labels, including the home screen’s own menu row.

Keys worth memorising

Every key also has a second function and often a letter, far more than fits in one table. Guidebook appendix B lists the complete map of all fifty keys with their normal, shifted, and alpha meanings.

The screen

Switch the calculator on and you see the home screen:

Reading from the top:

• Status region. The top line shows the calculator’s current state at a glance: the angle mode (RAD or DEG) and display format (AUTO, SCI, ENG, or FIX) on the left, and the editor state on the right (INS for insert mode, OVR for overwrite, and LOCK when alpha lock is on).
• Title banner. FREE85 HOME tells you which screen you are on.
• Version banner. VERSION 3.0.0 names the firmware release and disappears as soon as you type.
• Entry line. The underscore is the cursor. Whatever you type appears here.
• Result area. The middle of the screen. After you press ENTER the result appears here, introduced by =.
• Soft-menu labels. The bottom row names what F1 through F5 currently do. On the home screen these open the main menus: MATH, GRF, VAR, MEM, and SYS, with a second page under MORE.

Menus and editors take over the screen when you open them. Press F1 on the home screen and the MATH menu appears, listing five functions with a hint line at the bottom:

Here F1-F5 INSERT MORE means: press the soft key matching a listed item to insert it into your entry, or press MORE for the next page. EXIT takes you back to the home screen with your entry untouched.

Other screens work similarly. Press GRAPH and the calculator draws the graph screen: axes and grid dots. Press MORE on the graph screen and the table of values opens, with columns for Y1, Y2, and Y3 and soft labels UP DN GRAPH EXIT. Full-screen dialogs, such as error messages and the mode screen, tell you on-screen which keys they respond to. Equations themselves are typed on the home entry line; the Guidebook’s graphing chapters explain the workflow.

First calculations

Time to calculate something. From the home screen, press:

2 + 3 ENTER

The entry line shows 2+3 and the result area answers = 5:

Notice that the expression stays on the entry line after evaluation. That is deliberate: you can keep typing to extend the expression and press ENTER again, or press CLEAR to start fresh.

The previous answer

The (-) key’s shifted function is ANS, the previous answer. After the calculation above, press CLEAR to empty the entry line, then:

2nd (-) + 1 0 ENTER

The entry line reads ANS+10 and the result is = 15: the 5 from before, plus ten. ANS can appear anywhere in an expression, as many times as you like, and always means the most recent numeric result.

Recalling the previous entry

The ENTER key’s shifted function is ENTRY. Press CLEAR to empty the line, then 2nd ENTER, and the previous entry, 2+3, reappears with the cursor at the end, ready to edit and re-evaluate.

Editing

The cursor keys move along the entry line, and DEL deletes the character just before the cursor position. For example, type 1 2 3, press ◀ once, and press DEL: the 2 disappears, leaving 13.

By default the editor is in insert mode (the status line shows INS), and typing pushes existing characters to the right. With 13 on the line, press ◀ and type 2: the line becomes 123. Press 2nd DEL to toggle overwrite mode instead; the status line changes to OVR, and typing replaces the character under the cursor, so the same ◀ 2 on 13 then produces 12. Press 2nd DEL again to return to insert mode.

When something goes wrong

Errors are full-screen and polite. Type 1 ÷ 0 ENTER and a message screen replaces the home screen. It shows three lines: the error name DIVIDE BY ZERO, the hint CLEAR OR EXIT beneath it, and EXIT BACK at the bottom, telling you that EXIT returns you to your entry. The name is worth reading rather than glancing past: SYNTAX ERROR means the machine could not read what you typed, while DIVIDE BY ZERO, DOMAIN ERROR, NUMERIC OVERFLOW and PRECISION LOST all mean it read you perfectly well and the answer does not exist or cannot be trusted. Appendix C of the Guidebook lists them all. Press CLEAR or EXIT and you are back on the home screen with 1/0 intact and the cursor at the end, so you can fix the mistake instead of retyping it.

Modes

Press 2nd MORE to open the SYSTEM MODE screen:

Three settings are shown, and the soft keys ANG FMT - + MEM adjust them:

• Angle. ANGLE RAD or ANGLE DEG. Press F1 (ANG) to toggle between radians and degrees. The choice is echoed in the home-screen status line and affects all trigonometric functions.
• Display format. Press F2 (FMT) to cycle FORMAT through four settings:
  • AUTO: ordinary decimal output, switching to an exponent for values outside the compact display range;
  • SCI: one digit before the decimal point and an explicit exponent;
  • ENG: one to three digits before the point and an exponent divisible by three;
  • FIX: a fixed number of decimal places with half-up rounding.
  While FIX is selected, ▲ and ▼ change the number of decimal places, from FIX 0 up to FIX 11. The format changes how results are displayed, not the precision the calculator stores.
• Contrast. CONTRAST 16 by default. F3 (-) lowers the setting one step at a time and F4 (+) raises it; the number updates as you press.

F5 (MEM) opens the memory browser from here, showing the object count and letting you inspect and delete stored objects. Press EXIT to leave the mode screen and return home.

Number-base display

Free85 can show integer results in other bases. After calculating an integer, say 42, press 2nd 1 to open the NUMBER BASE screen. Its soft keys DEC HEX OCT BIN display the previous answer in the chosen base; press F2 (HEX) and the screen shows 0x002A. Base entry, two’s-complement behaviour, and the Boolean word operations are covered in Guidebook chapter 10, Number Bases and Boolean Operations.

The catalog and custom menu

Every function you can call lives in one alphabetical list: the catalog. Press 2nd CUSTOM to open it:

The header shows the page number and the current item (the list starts at ABS), with the hint ARROWS SELECT. Use ▲ and ▼ to move through the list, and press ENTER to paste the highlighted item into your entry line. For example, 2nd CUSTOM ▼ ▼ ENTER pastes ACOSH( at the home screen. Press EXIT to leave the catalog without choosing anything.

The catalog and the custom menu work as a pair. While an item is highlighted in the catalog, pressing one of F1 through F5 assigns it to that custom-menu slot; the screen confirms with a message such as ASSIGNED F2.

Press CUSTOM (unshifted) to open your custom menu. Its five slots come preloaded with ABS, EXP, LIS, SQR, and STA, shown as soft labels, and the screen explains itself: F1-F5 RUN SLOT and MORE: CATALOG. Pressing a soft key inserts that slot’s function (CUSTOM F1 pastes ABS(), and pressing MORE jumps to the catalog so you can reassign slots. Once you have arranged your five most-used functions here, they are always two keypresses away.

Where next

This manual has walked you through the front door; the Free85 Guidebook is the full tour, one chapter per subject:

1. Operating the Calculator: modes, the editor, and previous entries.
2. Variables and Stored Data: storing, recalling, and managing named data.
3. Mathematics, Calculus, and Comparisons: the full function set and the fourteen-digit arithmetic behind every result, with worked examples throughout.
4. Cartesian Graphing, Drawing, Formats, and Persistence: plotting, drawing, graph formats, and what the calculator remembers of a plot.
5. Polar Graphing: curves in polar coordinates.
6. Parametric Graphing: curves driven by a parameter.
7. Differential-Equation Graphing: graphing solutions of differential equations.
8. Physical and User Constants and Conversions: built-in values and unit conversions.
9. Strings and Characters: text values and the character palette.
10. Number Bases and Boolean Operations: binary, octal, and hexadecimal display, entry, and word logic.
11. Complex Numbers: arithmetic and functions beyond the reals.
12. Lists: ordered collections and list arithmetic.
13. Matrices and Vectors: linear-algebra objects and their operations.
14. Equation, Polynomial, and Simultaneous Solving: root finding for any expression, polynomials up to degree four, and linear systems of up to four unknowns.
15. Statistics and Statistical Plots: column summaries, least-squares regression in linear, logarithmic, exponential, power, and polynomial forms, and the scatter, connected xy-line, histogram, and box plots.
16. Calculator Programming: the on-calculator programming environment.
17. Worked Application Examples: complete applications, step by step.
18. Memory Management: browsing, inspecting, and deleting stored objects.
19. Calculator Linking: transferring stored objects between calculators and backing up a whole machine over the link port.

Four appendices round it out: appendix A is the command and function catalog, appendix B the complete key map of all fifty keys, appendix C the system variables and error messages, and appendix D records feature status across the release. Pick the chapter that matches what you want to do next, and enjoy the calculator.