Mercenary

01 · The city in lines

The city of Targ, one building at a time

The game keeps one building in memory, the one in the square you are in, and draws the rest of the city as its roads.

08-08, facing north, 3072 units from the edge. Bosher Stadium stands in the next square, dead ahead. Nothing is drawn.

6144 units further on, in 08-07. The same stadium, now drawn, far off on the horizon.

The square's model is loaded by $9575 only when the high byte of your X or Y changes ($94FE-$952F), from the table at $2600/$2700, and drawn only while your height is under 65536. From higher up you see the roads instead: they are drawn from height 2048 up, as lines between the centres of the squares they join, and from height $200000 a fixed picture of 13 roads replaces them ($902A). So a building is never in view from outside its square, and roads are never in view from the ground. The flying view above has a switch to draw every building at once; the game does 1/256 of that work.

02 · Bug

Bug: stand on one patch of grass at the start and E takes you into the Colony Craft

You crash-land at 08-08, and the Colony Craft hangs straight above you at ALT 64997. There is no lift on the ground here. But one invisible patch of grass, 1,024 units a side and about 14 seconds' walk from where you land, answers the E key as if it were the lift on the Colony Craft's deck. The ride goes down into the Colony Craft's hangar, and E again brings you up on its deck. The published solutions all fly there.

The Colony Craft sits over 08-08, so its deck and the grass underneath share one X and one Y. Only the height tells them apart. Its lift is entry 8 in the table of lift squares at $B4B6/$B4BE, and the E key's test checks the square and the middle bytes of X and Y. It never reads the height. So the patch under the deck's lift works exactly like the lift.

The ride down ends in room 8, the Colony Craft's hangar, whichever end you started from. The ride up from room 8 adds $40FF00 to your height, which puts you on the deck. The game does not know how you got in.

E on the grass: down into room 8, the Colony Craft's hangar.

E again: up on the deck, ALT 64997.

Walk off the pad: you tumble all the way down.

It does not break the game open. Hangar 8's three doors are all keyed to object 23 (room 8's record at $30A6), and you start without it, so the hangar and the deck are as far as you get. Take the lift back down, or walk off the deck: leaving its pad starts the tumbling fall ($A1DA), which never kills you, and you land back on the grass at 08-08. With the key, the patch would take you into the Colony Craft without a craft that can climb to 64997, which the Dart you start next to cannot do until it carries the Poweramp. That follows from the code; it was not played through.

Try it yourself

You need your own copy of the game and a C64 emulator. This was checked in VICE with the original Novagen disk.

1. Let the opening play to the end. Touch nothing until you stand at LOC 08-08 and Benson has finished "CRASH LANDED ON TARG". You face south.
2. Turn right three-eighths of a turn, to face north-west. The COMP dial turns red as you pass west; north-west is the middle of the red quarter.
3. Walk forward for about 14 seconds, about 100 steps, and stop.
4. Press E. If nothing happens you are off the patch, which is about 2.5 seconds of walking across. An emulator snapshot taken at step 1 makes retrying cheap.

The exact way is to put yourself there. X and Y are three bytes each, and the middle bytes are $73 and $79. In VICE's monitor, > 73 72 and > 79 72, then x to carry on, leave you in the middle of the patch at 08-08. Then press E.

03 · Secret

Secret: shoot a building from the next square and nobody notices

A building shot from outside its square is destroyed, but the game never counts it, never names it, and nobody comes after you for it.

Where the player is while the building falls: In 13-04, with the sign Next door in 12-04 Fire

Pass	$F4	$B7 = $F5?	What runs

The hit and the sixteen passes after it ($871B, $879B), for a missile that reaches 13-04's Commodore sign. The table follows the code's tests pass by pass; the pictures are the emulator's, from the two live runs: the shot from inside the square, and the shot from next door.

When a missile gets within 4096 units of a square's centre below height 2048, $871B marks the square destroyed in $2B00 at once and starts a countdown of 16 passes in $F4 for square $F5. Everything else waits for the countdown: sinking the building, adding it to the count of Palyar or Mechanoid sites, and starting its script, which names it and sends the owner's ship after you. The routine that does all that returns straight away unless you are standing in that square ($87A4). The countdown runs out anyway, and a later hit finds the square already destroyed and does nothing ($87C1). In the emulator, a missile fired east from 12-04, 1024 units up, destroyed 13-04's sign: $2B4D went from $51 to $D1, the Mechanoid count stayed 0, and Benson said nothing, then or later.

Firing from the ground does not work: level flight still points the nose down by a sine of −0.003, so a missile launched 63 units up hits the ground within five passes, before it reaches the next square's centre. From a little higher it does.

04 · Joke

Shooting the Commodore sign makes you a TRAITOR

Ten squares of the city hold billboards: five with the Commodore logo, five with Atari's. Destroying either kind gets a verdict.

13-04: model $C82E, the Commodore logo on two posts. Also at 03-13, 09-13, 04-15 and 09-15.

14-05: model $C7B2, the Atari logo. Also at 14-08, 12-11, 06-15 and 11-15.

TRAITOR!

Shot from inside its square in the Dart: the Commodore sign sinks into the ground and Benson prints TRAITOR! (live). Its square's byte in $2B00 says Mechanoid, script 17 + 32 = 49; the Atari squares run script 50, GOOD SHOW!

Each square's byte in the table at $2B00 holds its owner (bit 6 Mechanoid, bit 5 nobody's, neither Palyar) and, in its low five bits, a script number n. When the square is destroyed, script 32 + n runs once. Scripts 36 to 48 name the landmarks ("YOU HAVE JUST DESTROYED", then BOSHER STADIUM, say), and all but Novabill's then set off an attack by the square's owner; scripts 49 and 50 print their one word and leave you alone.

05 · The screen

One frame, rebuilt from memory

The view is a bitmap of 160 by 136 wide pixels, and there are two of them: the game draws into one while you look at the other.

Above the panel the screen is in multicolour bitmap mode: each byte holds four pixels of two bits, and each pixel is twice as wide as it is tall. Two bitmaps sit in the video chip's second bank, at $4000 and $6000. On every pass the main loop swaps them ($AF84): the counter $24 picks which one the game draws into ($25), and $C7 holds the value the view's raster interrupt writes into $D018 to show the other. At line 186 a second interrupt switches the screen to text for the panel ($BA1E).

Seen from above the whole city, a view takes 13 frames to draw, so the picture changes about four times a second. On the ground, with one building to draw, the main loop runs five passes a second.

06 · Flying

The flight model, pass by pass

Every craft but the interstellar ship runs the same flying step once a pass, with its handling read from a 16-byte record: take-off needs speed, the climb fades above the craft's ceiling, and a touchdown crashes only on a steep drop or a steep nose.

The throttle keys set a target, and the speed chases it: each pass the speed becomes (speed + throttle × thrust) × kept. On the ground a craft slower than 128 has its nose held level. Faster, it takes the level pitch's sine, -0.003, sinks below height 64 and lands again, every pass, until the stick lifts the nose. In the air the height changes by speed × sine of the pitch × the record's climb factor; climbing above the ceiling byte, that factor's power of two is replaced by the ceiling minus the height's top byte. A touchdown crashes when that pass's height step is 256 or more, or the nose is more than 45 degrees down.

Two things follow from the arithmetic. Every craft has a hard ceiling: above its ceiling byte the climb shrinks until the truncated step is zero. Flat out with the nose straight up, the Dart stops at ALT 24000, well short of the Colony Craft's 64997; with the Poweramp, as in the jet or the CHEESE, the top is ALT 105000, which the panel prints **000. And the sine table is sampled half a step off, so the level pitch's sine is −0.003, not 0. Level flight sinks slowly, a fast craft on the ground lands again every pass, and in reverse a craft climbs: SHIFT + 0 lifts the Dart off the ground with its nose level, 9 units a pass. The emulator shows the same heights, 67, 74, 82 and on.

07 · The craft

Eleven motion records

Each craft's handling is sixteen bytes at $9DAD, copied to $BEA0 when you board and after every airborne pass.

08 · Top speeds

Top speeds: Zzap!64's table, to the digit

With key 0 and then + held until the game refuses it, the port's top speeds are the figures Zzap!64 printed in May 1986.

09 · The underground

The underground: 174 rooms, each a box with doors drawn on its walls

Below the city every room is a record of a few bytes: three sizes, two colours and four bytes for each door, drawn as a wire box with the doors' outlines on its walls.

A room's record starts with its X size, its height and its Y size, in units of 256 (a city square is 65,536 across). Then come two colours: the view is filled with the first and the lines are drawn in the second. Each door is four bytes: its kind, which wall it is in and which door of the next room you come out of, then the room it leads to and its position on the wall. The list ends with a 0, or with $80 for a dark room.

10 · Benson's message window

Benson's message window

Benson, the craft's computer, speaks through a strip of 23 cells at the bottom of the panel, one character a frame.

Each character moves the window one cell left and enters at the right. After 25 characters the printer stops for 99 frames, so a line lasts 124 frames, about 2.5 seconds, and the next line pushes it out. The window is 23 cells wide, so the first two characters of every line scroll off before the pause: the author pads each line with blanks to centre it.

11 · How a message is stored

How a message is stored

A message is a list of one-byte tokens, most of them numbers of words in a dictionary.

A token of 8 or more is a word with a blank before it; 1 to 7 are endings such as S and ED, glued to the word before. $EF is a blank, $F8-$FE print a counter in decimal, $FF starts literal text that runs to the byte with bit 7 set, and 0 ends the message. Words 230 to 238 are nothing but spaces, nine down to one. They centre a line, and word 230 at the end of a message pushes it into a second, empty line that clears the window.

12 · The event scripts

The event scripts

What Benson says, and when, comes from 51 scripts in a small byte code, run one operation per pass of the main loop.

An operation byte holds the operation in its low six bits; bit 7 turns a test round. A test is followed by its operands and a two-byte target, where the script goes if the test holds. The interpreter waits while a message is printing, so every script moves at the pace of Benson's window.

13 · Money in BCD

Money in BCD, and a purchase without subtraction

The credits are eight decimal digits in four bytes, and the scripts have no way to subtract: a purchase adds the price's ten's complement.

Credits start at 9000 ($710C). The Dart's script adds 99995000 ($0A29), which is 5000 short of 100000000; the carry out of the top digit is thrown away, leaving 4000. The Hertz ship costs 999,999 (99000001 at $0E90), but the check before it ($0E6F) only asks whether the top byte is above zero, so it wants 1,000,000.

14 · Numbers

Two-byte numbers, multiplied by adding logarithms

The 3D view works in two-byte floats, a mantissa and a power of two, and multiplies two of them by adding their logarithms from a 256-byte table.

15 · Lines

Drawing a line: eight routines, one per octant

Each edge in the view is drawn by one of eight short routines that step along the longer axis, add a slope byte to an 8-bit counter (or subtract it), and step sideways whenever the counter wraps.

16 · Controls

The keys, as the game reads them

The game scans the keyboard itself and turns each key into a number, row × 8 + column, with 64 added for SHIFT and 128 for CTRL. The joystick goes in port 2.

17 · The opening

The opening: a countdown, then a crash

The game opens in space, runs its own event script through a failing Novadrive, and drops you in the middle of the city with 9000 credits.

The countdown, over a starfield of 21 stars.

MAXIMUM REVERSE THRUST, with Targ growing.

The descent: the city's roads from above.

CRASH IMMINENT! Then manual control.

From the emulator, about 75 seconds from the start of the game to standing at 08-08. The opening's messages were logged from the message window as they played.

The opening is written in the same byte code as the rest of the game's events, at $7001, and it lives in memory that the second bitmap overwrites once play begins: after the opening it is gone. Its starfield speeds up (−32 a frame, then × 4.05 once, then × 1.012 a frame) and slows down again, and the planet grows as a disc of radius 1024 over the distance, from 1023 to 0 ($74CA). The descent starts at height $7F0088, looking straight down. Script 0 then takes over: CRASH IMMINENT!, one of four landing remarks picked at random ("WHERE AM I", "YOU CRASHED", "GOOD LANDING!", "OUCH!"), the status report ("CRASH LANDED ON TARG", "STATE OF WAR BETWEEN PALYARS AND MECHANOIDS") and the offer of the Dominion Dart for 5000.

The first message on the original disk is NOVADRIVE COUNTDOWN. The copy this page was made from reads ABC UNLIMITED in its place, the cracking group's name, typed over the same bytes at $7009: it is the only change to the game's text.

18 · Protection

The reset button hangs the original

Novagen's disk makes the game look like a cartridge, so that pressing reset runs one of the game's own loops instead of handing the machine back.

$8000  $2170           ; cold start
$8004  C3 C2 CD 38 30  ; CBM80 (crack: 0s)

$2170  SEI
$2171  LDX #$00
$2173  STA $2180,X
$2176  DEX
$2177  BNE $2171       ; back to LDX
$2179  INC $2175       ; never reached
$217C  JMP $2170

Original: stuck in $2171-$2177.

Crack: BASIC's READY.

Both were reset in the emulator during play. The original is the contributor's own disk, booted to the same point as the crack.

On reset the KERNAL checks $8004 for the letters CBM80, and if they are there it jumps through $8000 as if a cartridge were plugged in. The original holds them, so reset runs $2170, inside the dictionary's pointer table. The code after the loop would wipe memory a page at a time: it steps the page byte of its own STA and goes round again. But the loop's branch goes back one instruction too far, to LDX #$00, so X is 0 at every store: the machine writes to $2180 for ever and never gets to the wipe. Either way, after a reset nothing runs but the loop. The cracked copy clears the signature and changes the branch to land on the $00 of LDX #$00, which is a BRK.

19 · Odds and ends

Things a pilot would not know

Small things found in the code and checked in the emulator.

• An Atari screen in a Commodore game. The bytes above are in the Atari's internal screen code, which is also how the game stores its panel labels (A is $21). They are a slice of an Atari's screen memory with the DOS menu on it, carried along in a block of the game's memory.
• Benson notices when you do nothing. When the offer of the Dart goes unanswered, he asks IS ANYBODY THERE, AM I TALKING TO MYSELF, ARE YOU STILL ALIVE and WHERE ARE YOU, 18 seconds apart; at the first key or stick movement he says AH! YOU ARE BACK, NOW YOU ARE HERE... I WILL START AGAIN, and starts the briefing over. Any key or stick movement sets bit 7 of $BEC0 ($B31B), and script 0 tests it.
• RESTORE does nothing. The NMI vector points at $8009, a bare RTI.
• It will not run on an American machine, and crashes trying. $5024 reads the KERNAL's PAL flag and, on NTSC, jumps back to the copy loop at $5000, whose self-modified addresses have already moved on: the second pass copies memory over zero page and then over itself.
  
• A save file holds the whole world. CTRL + S writes 1904 bytes gathered from 14 blocks ($8260): every square's state, every object's position, the scripts' state, the credits and the whole zero page. One of the blocks is $8000-$803F, so a position saved from the original disk carries its CBM80 signature into any copy that loads it.
• The escape waits for the author. With the billboard at 02-03 destroyed, the author's advert for his earlier game Encounter, your ship refuses to leave: THE AUTHOR WON'T LET YOU LEAVE UNTIL YOU FIX HIS ADVERT (script 7 tests $2B00 + $32). The Anti Time Bomb repairs it.
• GAME OVER, for ever. After the escape the script machine's busy flag (bit 6 of $BEC0) is never cleared, so no other event runs again, and GAME OVER comes round every 18 seconds or so: a 15-second wait, then the line.
• The attacking ship's missile is not homing. The ship itself closes in by 1/32 of the distance each pass, but its missile is aimed once, at launch, and flies straight.
• 00-00 has a building nobody has seen. Square number 0 also means "outside the city", so the model the table gives it ($E424) is never loaded.
• You can never have exactly 999,999 credits. Hertz's ship costs 999,999, but the test at $0E6F wants the top byte of the credits above zero: 1,000,000. It makes no difference: you start with 9000 and every price is a multiple of 5000, so your credits always end in 4000 or 9000.