A drawing program for a calculator. Move the crosshairs, press INS to mark a point, press ON to commit. Mark two points instead of one and it draws the line between them. Draw over something that is already there and it disappears.
That last behaviour is not a separate erase mode. The commit is one line —
CLEAR LCD→ n RCL XOR DUP n STO
— which takes what is now on the screen, XORs it against the stored picture, and stores the result. Drawing a pixel that is already lit turns it off. The manual page says it plainly: “Erasing a point: same as drawing a point; just align the cross-hairs with the point to be erased.” One operation, both directions, no state to track.
DOODL is two programs. DOODL runs the loop and owns the picture; LINE does the interpolation, reading the plot parameters to work out the step size and walking from one endpoint to the other. Together they are about two hundred characters of RPL.
The tool, remaking the thing it originally made.
What it was for
This is where the Space Chase ships came from. That game stores each ship as a list of twelve bytes — 255 24 24 36 102 90 90 102 36 24 24 255 — and read the right way, one byte per column of eight vertical pixels, it is a small winged fighter with a hexagonal cockpit. Someone sat with the crosshairs and placed those pixels.
The two halves came back separately and neither knew about the other. The ships arrived as a byte list in a photocopied program listing. The editor arrived as source printed on page 285 of a conference paper. Running DOODL again and feeding it those same pixels closes a loop that was open for thirty-seven years.
The paper
Both programs were published in The HP-28S, Calculus, and Programming — John Selden, Annie Selden, Michael Morrison, Randy Weems and Shawn Yeager, Addison-Wesley, 1990. John Selden taught the calculus course that put these calculators in our hands and built the follow-on course the programs came out of. The full account is on the catalog entry.
Annie Selden is professor emeritus at Tennessee Tech and a founding member of the Association for Women in Mathematics. John Selden died in 2024.
The version here
You can draw with it above. As with Space Chase, it is an interpreter running the published listing rather than an emulation — there is no distributable HP-28S ROM. The digitizer is live, so the crosshairs are genuinely yours, and the XOR commit behaves exactly as the manual describes. The keys shown are only the ones this program listens for: the menu row under the display, and ON.
The calculator itself is not simulated. Only the parts that this program used.