Explorer

01 · Fragments

Secret: the right name puts you one step from a fragment

Eight of the nine fragments lie in the same places in every game, and a doorway sends a name to the same place in every game. So a name can be a shortcut.

Fragment	East, north	Name	Then

how close does it land?

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How it was checked. In the emulator, a doorway trip to CSBBII arrived at 57,921 east and 7,081 north, and one step north found fragment 9. RMOYFV arrived at 12,352 east and 56,806 north, and one step south found fragment 2. The rest of the table comes from the page's copy of the game's sums, which gives the game's own answer for 20,007 names (test_names.js). The fragment positions are the table fragments at $2213, the same in three new games started in the emulator.

Each fragment's position is two 16-bit numbers in the table fragments at $2213. Fragments 2 to 9 are written into the program, and only a drone moves them. Fragment 1 is the only one whose place changes from game to game. The game puts it 160 units east and 144 north of a place it picks at random, then flies the opening pass north from there. If you leave the keys alone during the pass, you land 1,296 units north of fragment 1 and 144 or 160 west of it, so it is a little east of due south, just beyond the sonar's range.

A doorway turns any name into a place with two sums over its letters (How it works shows them). There is no name of one to six letters that lands on a fixed fragment's own cell. For each of the eight, though, one name lands in the cell next to it, and one step finishes the trip. A doorway is not the only way in: a crash and running out of energy both ask for a name too. You still need the drones and a beacon to bring the fragments together, but you no longer need the sonar to find them.

02 · Doorways and energy

Bug: the doorway works out an energy refill and never gives it

The instructions warn that energy runs low and say "There's a way of restoring it". The doorway's code works out a refill, and its last step jumps past the instructions that would store it.

After a doorway has moved you, the game works out a change to your energy (name_destination at $0E23). It takes the number of letters in the name, less 8, times 512, and from that it takes a quarter of the distance travelled, east plus north. A long name for a nearby place gives energy, and a short name for a place further off takes it. The game halves the distance twice with an instruction that treats a number over 32,767 as negative, so a trip longer than that counts as a negative distance and gives energy too. HOME, from the landing place, would give 5,814. Then the game adds the change to your energy and branches on the carry, the 6502's flag for a sum that overflowed.

Both ways out of that branch go back to the walk at ground_loop. With the carry set, the branch goes to $0F4A, which is JMP ground_loop. Without it, the processor runs on to $0F3F, which is also JMP ground_loop. The four instructions after that jump, at $0F42, would store the new energy, and the processor never reaches them. So doorway trips are free, and the only energy the game ever gives back is the 2,000 you get when you run out, with a doorway's prompt (How it works).

03 · Bearings

Bug: the sonar points the wrong way to anything south-west of you

The sonar and "Fix 3 nearest" give the bearing of a fragment or a beacon in degrees. In one quarter of the compass, the number is wrong by up to 90 degrees.

The game works out a bearing in two steps (bearing_of at $383F). First it finds the angle between the line to the target and the nearest north-south line, from 0 to 90, using a table of tangents at $39B3. Then it turns that angle into a compass bearing for the quarter the target is in, using three numbers at $3A37. North-east is the angle itself, south-east is 180 less it, and north-west is 360 less it.

South-west should be 180 plus the angle. The game uses 270 less it, which mirrors the quarter about the line to the south-west. A target exactly south-west reads right, at 225. Everywhere else in that quarter the reading moves away from the truth: a fragment just west of due south reads almost due west. A player who follows the sonar there walks off at the wrong angle. As they walk, the fragment's direction swings round towards the south-east, where the reading is right again, so they still get there by a longer road.

04 · Unused pictures

Secret: pictures the jungle never shows

Objects 20, 41 and 42 have records in object_records at $363F, but the scenery lists never name them, so the game never draws them. Object 20 is the edge of some water. Object 41 is fragment 1's own picture, and object 42 is part of the beacon's mast. Two more pictures, at $986A and $9AFE, have no record at all: a post and a rail over water, one wider than the other.

05 · Open questions

What is not settled

The game's credits say "program: Graham Relf", and Relf writes that he programmed Explorer, on the Spectrum. GameBase64 names Brian O'Shaughnessy as the programmer of the Commodore 64 version. The code keeps the Spectrum's colour bytes everywhere, which says it is a conversion, but not who converted it.

Whether the Spectrum version stores the doorway's refill is not known. If it does, the jump at $0F3F was a slip in the conversion.