ACE 2

01 · The game

Two pilots, one screen

ACE 2 is a head-to-head air combat game. Two players, or one player and the computer, fly against each other with the screen split across the middle.

Plane one flies from an aircraft carrier at the west edge of the map, plane two from an airbase at the east edge. Each pilot sees the view from their own cockpit, with an instrument panel under it: radar, altitude, speed, power, fuel, a pitch ladder, a bank indicator, a compass and a two-line message panel. The strip down the left of the screen shows each pilot's planes left and score.

The options choose between two games. The close-range dogfight starts both planes in the air near each other, armed with a cannon and heat-seeking missiles. The full-scale game starts them at their bases, lets each pilot choose a load of missiles, and gives each side a target on the ground: a spy ship at sea and a radar station on land, defended by surface-to-air missiles. A pilot who is shot down takes a new plane from the hangar until the planes run out.

02 · Music

Music: Rob Hubbard's title tune, five minutes before it repeats

The title pages play one tune, by Rob Hubbard. It runs for 5,880 note steps, 5 minutes 14 seconds, before it starts again. Press Title music to hear it.

The SID has three voices, and the driver gives each its own part. Each voice follows its own track list ($E6CE, $E712 and $E7CB), a string of pattern numbers, and the 38 patterns hold the notes (pointers at $E682 and $E6A8). The driver is called 50 times a second, and a note step falls on three calls in every eight ($E570-$E572): 18.75 steps a second. A note's first byte gives its length in steps and whether it is a rest; an instrument byte can come before it.

Eleven instruments fill two tables of eight bytes each ($E5CB and $E624): the waveform, the envelope, a pulse width that sweeps between two limits, vibrato, and effects for a note's first frames, such as a burst of noise before a pulse tone. Four instruments are drums, which step through a table of waveforms and pitches a frame at a time ($E5A2, $E9A9 and $EB0A). The SID's filter takes the high frequencies out of a sound above a cutoff that the music can move. Instrument 0, the bass on voice 3, lowers the cutoff by 3 on every call from the start of each note. Its resonance, a ringing at the cutoff, is at its highest ($D417 = $F4).

The same driver silences the chip when a game starts. F5, held during play, switches between the music and the engine noises, which share the chip ($11E9).

03 · One frame

One frame of play, rebuilt from memory

The C64's screen is made of characters, tiles of 8 by 8 pixels whose shapes come from a character set in memory. Four times a frame the game tells the video chip which set to use: once for each pilot's view and once for each panel. Press a band to see the screen lines it covers.

Both views are multicolour character graphics, four colours to a cell. Each pilot has two character sets for the view. Each pass of the game's main loop draws the horizon, the enemy and the ground targets into the set that is not on show. Then the interrupt swaps them ($6225, $4C5C). The interrupt is a routine the video chip starts at set lines of every frame, whatever the main loop is doing. Plane one uses the sets at $A000 and $B000, plane two those at $A800 and $B800. The panels are drawn into the same sets, character by character: the bars, the dials and the message text are bitmaps written into the characters the panel's screen codes point at.

The sky is the chip's first shared colour ($D022, light blue). The ground is the colour in colour RAM, the memory that holds a colour for each character on the screen. The game sets it to blue while the plane is over the sea and yellow while it is over land ($46C8). The cockpit frame round each view is made of sprites, small pictures the video chip draws over the screen, set again for each band.

A pass of the main loop does plane one's half, plane two's half and the work they share, and takes several frames. In a recorded one-player flight the game made 156 passes in 500 frames, about 16 a second. The music or the engine sounds, the fuel and the turn run in the interrupt instead, on a clock of 50 ticks a second ($43FC).

04 · The map

The map tells the plane whether it is over sea or land

X or V shows a pilot the map, with both planes, the missiles and the ground targets on it. The same picture is what the game reads to decide what is under each plane. Move over the map to read it as the game does.

Once a pass, $14AF takes the plane's position, finds the character of the map picture under it and reads the pair of pixels inside that character's shape. Light blue pixels (binary %01) are sea and yellow ones (%11) are land. When the answer changes, the cockpit's ground changes colour, and each ground target appears in the view only over its own kind of ground ($61E4): the spy ship over the sea, the radar station over land. Black and white pixels, the map's north arrow, its scale bar and its border, change nothing.

Each position is a 24-bit number, three bytes, and the map shows only the top byte. The map runs from 4 to 235 across and from 4 to 91 down. A plane is held at the edge, and at its own edge it may land: the west edge for plane one, the east edge for plane two ($3D57). Below 1,024 feet with the nose near level the message panel says OK FOR LANDING, and with the wings level the plane lands and is refuelled, rearmed and repaired ($1FAD, $208E).

05 · Messages

Two of the cockpit messages are never shown

The message panel shows two lines of seven letters. Each pilot has a list of 32 messages, and the panel moves to the next live one about every 14 passes. Pick a message to see it in the game's own font.

A message lives for as long as its timer runs ($0350 for pilot one, $0370 for pilot two), and every pass counts every timer down ($11E9). The panel cycles through the live ones ($2E57). Some messages carry figures that the game writes into the text before it is shown: the range of the enemy plane or missile, the altitude, and the flares and chaff left. Pilot two has its own copies of those, and its message 26 reads ENEMY SPYSHIP where pilot one's reads RADAR STATION.

Message 12, YOU TRAITOR, and message 19, a first copy of TARGET LOCKED, are in both lists, but no instruction in the game raises either (Discoveries).

06 · Flight

Thin air gives more thrust

Each pass, the power keys move the throttle, and the throttle, the bank, the altitude and the weight decide whether the plane speeds up or slows down.

The manual says the afterburners light above 75 % power. In the code that is throttle 192 of 255: below it the thrust is halved, and so is the fuel burnt. The altitude term is why the manual calls the plane faster in thin air: at 60,000 feet the thrust is nearly twice what it is at sea level. Both planes fly by the same routine and the same numbers, though the manual describes them as different fighters.

07 · Weapons

A missile needs ten passes of lock

A missile fires only at a locked target, and a lock takes ten passes with the enemy inside the sight's box and in range.

A plane's damage counts up to 255, and at 255 the plane is lost. A missile does 255, 128 or 96 by the option, so one, two or three hits kill. A hit also takes the stick away for a moment: the game shakes it left and right for 8 ticks after a cannon hit and 25 to 32 after a missile, and counts that down every fourth tick of the 50 a second clock ($5A6F), so a missile leaves the pilot without control for 2 to 2.6 seconds.

Each plane carries six flares and six chaff pods ($208E). The decoy key drops whichever fits the missile in the air: chaff against a radar missile, flares against a heat seeker ($2531). Chaff turns a radar missile away with a chance of 180 in 256, and a flare turns a heat seeker within range 2 with a chance of 220 in 256. Against the computer the odds go up: 200 in 256 for chaff, and 230 plus the skill level for flares ($265B).

08 · The computer pilot

The skill level is how often the computer pilot acts

In a one-player game the computer flies plane two. The skill level, 1 to 20 on the options screen, sets how often it takes a decision.

Acting, the computer meets an enemy missile first: it drops decoys when the missile is close, or levels its wings and dives or climbs away from the missile's height ($5B47). With fuel low it goes home at 800 feet. In the full-scale game it may go low for the spy ship with air-to-ship missiles ($5C43). Close to the enemy it picks a move at random and keeps it for a while: a hard turn away, throttling back so the enemy overshoots, a roll and a loop, or a hard turn at 3,000 feet ($5D30). In the hangar it arms its own plane by chance with one of two loads ($109D).

09 · The colour code

The colour code's answers are in the game

Before each game the colour code security system asks for the colour of one square of a grid printed on the back of the inlay card. Three wrong answers wipe the game from memory. The game holds its own copy of the 40 squares it asks about. Press a square, or ask a question the way the game does.

The question screen lists 14 colours, and the cursor runs down them with the stick while the border shows the colour under it. Fire answers. Light green is taken for green and cyan for blue ($37E1). A right answer overwrites the checking code itself with random bytes, and the question is never asked again. A third wrong answer fills memory from $CFFF down to $0100 with one random byte and jumps into the wreckage.

10 · Controls

Plane two's power keys are the other way round from the manual

Each pilot has a joystick and five keys on their own side of the keyboard. The game reads every key it uses from the keyboard's matrix itself ($366B).

Joystick port 1 shares its wires with the keyboard. A key pulls one row of the keyboard low, while the stick pulls all eight. So the game reads all eight rows and keeps only what is low in every one of them. That is the stick, because a key shows in one row only. Plane two's weapon selector slides down the panel to the weapon chosen, and plane two cannot fire until it arrives ($4C1A). Outside a game, in a one-player game, or when neither pilot has planes left, the stick in port 2 also drives plane one, so a lone player can use either port.