Master of Magic

01 · The screen

One frame, rebuilt from memory

The play screen is three screens stacked. An interrupt at $2653 changes the video chip's mode twice on the way down every frame: coloured text for the view and the messages, the same text on an orange ground for the menu, and a multicolour bitmap in another memory bank for the pictures along the bottom.

The view window in the top left is an 11 × 11 opening onto the dungeon map, with your cell in the middle. The status line holds the mind (M) and body (B) bars and a clock. Messages scroll up the yellow window. The menu across the middle lists only what you can do where you stand. Along the bottom are pictures of everything you can see.

02 · Line of sight

28 rays, cast a pixel at a time

The view window shows only what you could see. Between choices the game keeps casting rays from your exact pixel, one pixel step at a time through the walls drawn in the map's characters, and a cell appears once a ray has reached it.

The 28 rays fan out clockwise from north. Each follows a seven-step pattern of single-pixel moves (table $3BF7), and the pauses written into the patterns stop every ray within about 40 pixels, five cells, so the lit area is round. A ray stops at the first wall pixel it meets. Shut doors need a second rule: a shut door's panel is drawn in the cell's own colour, and the caster stops at that colour only in cells whose colour byte is exactly $0F. Six rays are cast per pass and the window is redrawn after every 29.

Walls are found by the video chip

No routine checks the map to stop you walking into rock. You are a sprite, a 4 × 4 dot, and the video chip reports whenever a sprite's pixels overlap anything drawn in the foreground. The interrupt collects that report every frame into $038D; when it shows your dot touching a wall, the next step goes the other way ($0F95). The floor is drawn in a background colour, and water, rock and door panels in foreground colours, so all of them stop you.

Secret: the narrow corridors are exactly as wide as you

The dot is four pixels wide. The narrow corridors that run north and south, and the doors in them, have walls two pixels thick on each side of an eight-pixel cell, which leaves a gap of four. Only one of the eight pixel columns in a cell lets you through; at the other seven the dot touches a wall, bounces, and touches it again, and you stay where you are. Zzap!64's review warned that you can get stuck in a doorway unless you are exactly lined up with the passage, and this is why. In the emulator, pushing up level 3's long corridor from five neighbouring columns, only the middle one moved. East-west corridors are kinder: their walls are one pixel thick, and three rows get through. CLOSE puts you back in the middle of your cell, which lines you up again.

03 · The sound

Rob Hubbard's three tunes, from the stored bytes

The music driver at $C000 plays three tunes: the main theme on the title and in play, a short one when you die and a longer one when you win.

The main theme ends in a fade: the driver sets the volume from how far voice 3 has got through its list of patterns, so the theme fades from 4:28 to silence at 5:21 and loops at 5:35. M stops the music in play, and then the footsteps can be heard: every third pixel you move, and every third step of most monsters in sight, the interrupt plays a short burst of noise on voice 1. With the music on, the steps are silent.

04 · The dungeon

Four levels side by side in one map

The whole dungeon is one 4 KB block at $9000: 32 rows of 128 cells, each byte the character the view window shows. The 128 columns are four levels of 32, laid out left to right.

A staircase is two characters. Step onto the right-hand half of $B0 $B1 and $20CC adds 32 to your map address, which puts you on the same row of the next level to the east; step onto the left-hand half of $B2 $B3 and it subtracts 32. There are seven of each, and a single trapdoor, $A3, that drops you one way from level 1 to level 2. You start on level 4. The amulet lies at the end of a winding corridor on the east edge of level 1, a few cells from the minotaur, and the pedestal it has to go back to is beside the small pool on level 4.

Doors come in pairs of cells, one each side of the wall line. Bit 0 of the character says open or shut, and two more bits say where the other half is, so OPEN and CLOSE can flip both. There are 17 doors, nine on level 1, seven on level 3, one on level 4 and none on level 2, and a new game shuts them all. Until you open one, you can walk to 609 cells, all on levels 4 and 3.

05 · Choosing what to do

A menu of what applies, and a clock that stops while you choose

Every time you choose, $11A2 builds the menu again from what is around you, and the clock stops until you have chosen.

What happens between choices is measured in passes of $08CC: each pass casts six rays, gives every creature six ticks of its clock and advances the game clock by an eighth of a second. The menu waits for you with no passes at all, which is why the clock stands still there. A choice then buys a fixed number of passes, whether or not the action did anything: INVENTORY costs 12, CAST 20, SWAP 3. RUN runs passes until you press fire, and moves you about 1.2 cells a second.

There are 18 verbs in the table and the menu can only ever show 17. WALK, which moves you at half the speed of RUN, has a string and a handler at $08B0, and the builder never adds it.

06 · The cast

Thirty pictures for 64 things

Everything you can see appears in the strip at the bottom as a picture, cut from one multicolour bitmap sheet at $6000. Most kinds share one picture, and seven are drawn twice as tall.

A creature whose body strength goes below zero is marked dead by bit 7 of that byte, and the strip lays a sprite over its picture that spells DEAD, in light red, doubled in size. There are eight such sprites, all pointing at the same shape at $A500.

07 · Fighting

A roll of 0 to 31 against a defence

Every blow is a random number from 0 to 31 plus the attacker's to-hit score, against the defender's defence. Damage is the random number masked by the weapon plus an eighth of the attacker's strength.

Chances counted over the actual random numbers, computed from the three pages of the KERNAL ROM the generator at $2B9E reads: $E000, $E800 and $F000, the same in revisions 901227-01, -02 and -03. The sequence repeats every 256 numbers, and the numbers in one cycle are far from even: 160 of the 256 possible values turn up.

Bug: the second vampire's defence is your to-hit score

Strength, defence and to-hit are three tables of 64 entries, indexed by entry number. They were placed only 62 bytes apart, so the last two entries of each, the second vampire and the minotaur, are the first two of the next. The second vampire's defence is the player's to-hit score, 25, where the first vampire's is 60, and the minotaur's defence is to-hit entry 1, an unused object slot that holds 133.

With a defence of 133, no roll reaches the minotaur, and the Dagger of Death is the only way through: $2B93 replaces the roll with the minotaur's own defence, a certain hit, and $2B26 makes the damage 30. The first vampire, at 60, is out of reach of a roll that tops out at 56, unless you wear the Ring of Dexterity, which adds 8.

Bug: the mace and the dagger of wood never get their bonus

The routine that adjusts a roll for special weapons has two tests that cannot pass. Each asks whether one register equals two different values in turn:

The protection routine $24F1 has the same shape for the shield, three compares in a row against $1E, $1F and $20, so a shield never protects. Its other tests read the wrong table: the object's "can be worn" flag, which is always set for armour, helmet and rings, instead of its state in $4AC1, which says whether it is worn. So armour protects you while you only hold it in a hand, and the ring with the blue stone, the one the scrolls call cursed, makes monsters hit more often whenever you hold it.

08 · Magic

Secret: spells never roll the dice

The instructions promise that Magic Missile has a high chance of missing. The code fetches a random number for it and throws it away.

So a spell either always hits a creature or always misses it. MAGIC MISSILE always hits everything but the first vampire and the minotaur, which it always misses; FIREBALL and ENERGY DRAIN miss only the minotaur. The cost in mind is paid before you pick a target, so choosing -NOTHING- still spends it. The Magical Shield takes 8 off every creature's roll and a point of mind every 100 passes, and while it is up the menu offers UNCAST instead of CAST, so no other spell can be cast.

09 · The monsters

Bug: the monsters' homes are wiped, so they wander north

The instructions say most monsters patrol round their lair. The code gives every creature a home to walk back to when it is not hunting you, and the new-game routine wipes half of each address.

The home addresses live in two 31-byte tables at $4CAE and $4CCD. The set-up at $0924 clears two flag tables next to them with a loop that runs for 64 entries, so the first store runs on through every home's high byte. A home of $00ll is a page-0 address above the top of the map, and an idle creature heads for it: first towards its start column, then due north, and then back and forth under the northernmost wall it can reach. The file was saved after the game had already run once, so the loaded homes are already wiped. Seven creatures ignore their homes and get a random destination every 50 passes. In the game, 53 seconds in, 17 of the 24 that do not wander were north of their starting row; the port above reaches the same 17 at the same moment.

A creature that sees you re-arms a pursuit counter: bats give up at once, orcs after 6 decisions, skeletons 20, vampires 30, the wizard and the minotaur 60. Chasing, it walks to where it last saw you and carries straight on. Orcs, skeletons, vampires, the wizard and the minotaur open a shut door in their way and close it behind them; hellhounds, bats, spiders and snakes treat it as a wall. One snake is timid: it runs from you and never attacks.

10 · The demonstration

Secret: the demo recorder is still in the game

After one pass of the title's story the game plays itself. It does it by replaying a recording of joystick input from $A543, with the random numbers reset to where they were when it was recorded.

CTRL with R on the title screen starts a recording. $5BBA saves the random state and starts a game in which every input is written over the demonstration, and F ends it. The next time the title's story runs out, the demonstration plays the new recording: in the emulator, a short walk recorded this way was replayed through the same cells to the same pixel.

The recording is also the best test there is of a port. The Play tab runs the whole game in JavaScript; fed this recording poll by poll, it reproduces the real game's run through the demonstration, every one of its 2,371 world passes, byte for byte.

11 · The title picture

A picture hidden under the KERNAL

The title picture sits at $E000, in the RAM underneath the KERNAL ROM. The video chip always reads RAM, so it shows the picture; the processor at the same addresses reads the ROM.

The game uses both views of those addresses. The random number routine reads three 256-byte pages at $E000, $E800 and $F000. It gets KERNAL code, because the KERNAL is switched in, and uses it as noise. The picture's bytes at the same addresses are never read by the processor.

12 · Secrets, quirks and bugs

Things the manual never told you

Secret: the edge of a pool is a door live

The menu decides whether you stand on a door with a signed compare ($11E5: CMP #$F0, BMI), so any character below $70 passes too. On level 2 the top edge of the big pool is two such cells, floor except for a line of water, and standing on one offers OPEN. Choosing it prints "OPEN DOOR." and spends eight passes; the door routine checks for real door characters and changes nothing.

A reset leaves BASIC switched in live

The game catches RESTORE and the reset button with the CBM80 signature at $8004, and both restart it. After a reset, though, the processor's data direction register is zero, and the restart never sets it again, so switching BASIC out has no effect. The game's tables at $A000 read as BASIC ROM: the title comes up in the wrong colours and the view window stays empty.