The Legend of Zelda: A Link to the Past screen and sound effects (kan's US disassembly)

On the game over screen and on the exits that use it, a circle closes on Link: an indirect HDMA channel (mode $41, register $2126, window 1's left edge) feeds the window a 224-line table of left and right edges, and the three window-select registers get $33, $03, and $33, which enable window 1 inverted for BG1, BG2, BG3, sprites, and the color window, and it copies the main and subscreen layer choices into the window-enable registers, so every layer shown is masked outside it. Each frame the radius changes by 7 lines, from 126 to 0 as the circle closes and from 0 to 126 as it opens, so either takes 18 frames (computed). For each line within the radius of the point 12 lines below Link's position, the code divides the line's distance by the radius to index a 129-entry quarter-circle table (255 at the center falling to 0 at the edge), multiplies the entry by the radius, keeps the high byte, and doubles it to get the half-width, so the window is about twice as wide as it is tall (computed); lines outside get the word $00FF, an empty window. Outdoors the routine also sets the fixed color to black. It builds each table elsewhere and copies it into the HDMA buffer only once the beam has passed line 192. On the game over screen the window-select registers then change to $33, 0, and $30, so BG3 and the sprites stay visible outside the circle. alttp-av/iris-spotlight

About: The Legend of Zelda: A Link to the Past HDMA

{
 "step": 7,
 "radius": 126,
 "frames": 18,
 "lines": 224,
 "copyAfterLine": 192,
 "quarterCircle": [
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  255,
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  31,
  0
 ]
}

The radius counts in screen lines. The table builder walks from the outermost line pair inward, one line above and one below the center at a time, and the counter it hands the division starts at the radius and drops by 1 a line, so the index is about 128 × distance / radius. A 1 bit in a window-select pair's high position enables window 1 for the layer and the low one inverts it.

The world map is a Mode 7 picture tilted into perspective by HDMA: two indirect HDMA channels (mode $42) write the same table into the matrix registers A ($211B) and D ($211E), one value a line for 224 lines, while B and C stay 0. Zoomed out, the value falls from $0177 on the top line to $0108 on the bottom; zoomed in, from $0088 to $0060. In the registers' 8.8 fixed point that is 1.46 to 1.03 map pixels per screen pixel zoomed out and 0.53 to 0.38 zoomed in (computed), so the top of the map is drawn smaller, as if farther away, and zooming in magnifies it about 2.75 times (computed). The map switches between the two tables at once, by repointing both channels; it opens zoomed in, but zoomed out when the flute calls it up. Each frame the vblank code sets the rotation center from two words: X is $0100, and Y is set to the map's vertical scroll plus $0100 whenever the code sets that scroll. alttp-av/world-map-perspective

About: The Legend of Zelda: A Link to the Past Mode 7

{
 "lines": 224,
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}

The factors divide each value by 256: $0177 is 375/256 and $0108 is 264/256; $0088 is 136/256 and $0060 is 96/256, and 375/136 and 264/96 are both about 2.75. A larger matrix value steps farther through the map per screen pixel, so it draws the map smaller. The table pointers address the A register ($1B) for one channel and D ($1E) for the other. The opening sequence's map view uses a third table built in RAM.

Zoomed in, the world map scrolls with the d-pad one line a frame vertically and two pixels a frame horizontally, and stops at fixed limits: from −72 to 224 vertically and from −224 to 480 horizontally. On opening, or on zooming in, the code centers the view on Link: its vertical scroll is Y/16 − 72 and its horizontal scroll 128 + 2.5 × (X/16 − 128), from his overworld Y and X, both made even; the code scales the distance from 128 without its sign and then restores it. Zoomed out, the scroll is fixed at 128 across and 200 down. Each zoom change plays a sound effect (bank 3, number $24) and locks out the next change for 8 frames. alttp-av/world-map-pan

About: The Legend of Zelda: A Link to the Past Mode 7

{
 "panX": 2,
 "panY": 1,
 "top": -72,
 "bottom": 224,
 "left": -224,
 "right": 480,
 "lockout": 8,
 "zoomedOutScroll": {
  "x": 128,
  "y": 200
 }
}

The pan index takes the vertical d-pad bits ($0C of the held byte) for the vertical entry and the horizontal bits ($03), doubled and offset by one entry, for the horizontal one; the disassembly's comments name the table's entries down, right, up, and left. A scroll moves only while it isn't exactly at the limit for that direction. The limits are the table's words read as signed: $FFB8 is −72, $00E0 is 224, $FF20 is −224, and $01E0 is 480.

The opening's spinning Triforce is drawn in software: a second thread, with its own stack, direct page, and registers, clears a 2,048-byte bitmap, builds a rotation from sine and cosine tables, rotates and projects each point of the shape, calls the shape's drawing routine, and flags the bitmap ready; at the next vblank the interrupt code copies all 2,048 bytes by DMA to VRAM word address $5800. The game's own code and the drawing thread share the processor by interrupts: while the polyhedron is on, the vblank interrupt ends by switching stacks to the drawing thread and sets a line interrupt, at line $90 (144) in the opening, whose handler switches back. The drawing thread starts a new picture only when the game has asked for one and the last one has been copied. alttp-av/triforce-polyhedron-thread

Value: bitmap bytes 2048 · vram word 22528 · irq line 144

The thread's saved stack pointer lives at $1F0A; each switch stores the current one there and loads the other, so the vblank handler returns into the drawing thread and the line interrupt returns into the game. The game's frame code therefore runs from line 144 until vblank, and the drawing from vblank until line 144, in the opening. The VRAM word address $5800 is byte address $B000, as the disassembly's comment says.

A dark room is darkened by subtractive color math, not by darker palettes. Every room's setup selects the subscreen as the addend ($02) and subtracts it from BG1, BG2, sprites in palettes 4 to 7, and the backdrop ($B3); when bit 0 of the room's first header byte is set, it also sets the fixed color to a gray of level 31, the brightest of 0 to 31, and a lit room gets a gray of 0. With the lantern and no torch lit, the code turns BG1 on the subscreen, so the subtraction takes that layer's colors instead, and each frame scrolls BG1 to set its cone picture in front of Link: one of four positions in the layer, chosen by his facing, offset from his position and clamped. Each torch he lights turns the subscreen off and lowers the gray: 8 with one torch, 4 with two, 0 with three or more, which leaves the room at full brightness. alttp-av/dark-room-lantern

About: The Legend of Zelda: A Link to the Past Color math

Value: Fixed gray by torches lit 31, 8, 4, 0 · cgwsel 2 · cgadsub 179

$B3 sets subtract (bit 7), the backdrop (bit 5), sprites (bit 4), and BG2 and BG1 (bits 1 and 0); $02 sets the subscreen as the addend. The gray is written as the same 5-bit level to the red, green, and blue fixed-color channels. The lantern code skips the subscreen when the room's effect value is 2, and a lit room (bit 0 clear) gets index 3, a gray of 0. The four cone positions are the layer's four 256-pixel quadrants. Which torches burn out, and when, isn't read here.

Before the story counter reaches 2 (Zelda's rescue), and on screen $70 until bit $20 of its saved flags is set, the overworld's rain is BG1 alone on the subscreen, halved and added to BG2, the sprites, and the backdrop (color math $72). Every 4 frames the code steps the layer's scroll through a four-step cycle that alternately adds 4,352 (17 × 256) to its vertical scroll and 256 to its horizontal scroll. Lightning is a change of color math on fixed values of the game's 8-bit frame counter: on 3, 36, and 88 it becomes $32, the same add without halving, which brightens the screen, and on 5, 44, and 90 it goes back to $72; on 36 the thunder sound plays (bank 2, number $36). In each 256 frames the screen therefore flashes for 2, 8, and 2 frames (computed). alttp-av/rain-and-lightning

About: The Legend of Zelda: A Link to the Past Color math

Value: every 4 · Vertical shift (× 256) 0, 17, 0, 17 · Horizontal shift (× 256) 1, 0, 1, 0 · flash on 3, 36, 88 · flash off 5, 44, 90 · thunder at 36 · flash frames 2, 8, 2

$72 sets half (bit 6), the backdrop (bit 5), sprites (bit 4), and BG2 (bit 1); $32 is the same without half. The scroll steps add to the high bytes of BG1's scroll words. The flash counts are the hexadecimal compares $03, $24, $58 and $05, $2C, $5A. The frame counter is the main loop's, so a lag frame doesn't advance it.

Each overworld area chooses its moving overlay, a BG1 picture alone on the subscreen: on screens $00 and $40 a fog (overlay $9D), except that screen $00 gets overlay $9E once bit $40 of special screen $80's saved flags is set; on screens $03, $05, and $07 overlay $95; on $43, $45, and $47 overlay $9C; on $70 the rain ($9F) until bit $20 of its saved flags is set, then overlay $9C; elsewhere the rain before the story counter reaches 2 and overlay $96 after. The fog, the rain, overlay $9E, and three special areas' overlays ($93, $94, and $97) are halved and added to BG2, the sprites, and the backdrop ($72); overlays $95 and $9C are added to the backdrop alone ($20). The fogs $97 and $9D drift on their own by 1/8 pixel a frame right and down (computed). Other overlays follow the camera's horizontal steps: at half the step, or a quarter for $95 and $9E. Overlay $9C instead rises by 1/8 pixel a frame (computed) and keeps BG2's horizontal scroll, except on screen $47, where neither drift runs. alttp-av/overworld-overlays

About: The Legend of Zelda: A Link to the Past Color math

{
 "fogDrift": 0.125,
 "parallax": {
  "default": 0.5,
  "0x95": 0.25,
  "0x9E": 0.25
 }
}

$1C = $16 puts BG2, BG3, and sprites on the main screen and $1D = $01 puts BG1 alone on the subscreen; $82 in the color-select register picks the subscreen as the addend. Overlay $97 is special screen $80's own, and the code drops it once the same bit $40 is set; $94 and $93 belong to special screens $81 and $89. The half and quarter come from one or two right shifts of the camera's step into a fraction.

The warp between the worlds bends the picture with a wave that runs down the screen: the routine sets two indirect HDMA channels to mode $42 on the horizontal scroll registers of BG2 ($210F) and BG1 ($210D) and fills a 224-word buffer with BG2's current horizontal scroll. Every second frame it moves the buffer's values down by one band of 8 lines, copying the band at line 208 into the one at 216 and so on up to the band at line 0 into the one at 8, and writes a new offset into the top band. The offset swings like a spring: a speed changes by $40 (1/4 pixel) each update toward a limit of −$200 (2 pixels), then back toward $200, which sets the offset swinging between −9 and 9 pixels, a swing every 33 updates, or 66 frames (computed by stepping the code). Each band's write covers its 1st, 3rd, 5th, and 7th words; if the channels read the buffer through the window effect's table, one word a line (assumed, not confirmed), the lines between keep the unbent scroll. Once a color counter of the warp reaches $30 and the offset lies between 0 and 7, the top band goes back to the unbent scroll, and when the bottom bands have all returned to it, HDMA turns off. alttp-av/mirror-warp-wave

About: The Legend of Zelda: A Link to the Past HDMA

Value: band lines 8 · every frames 2 · speed step 64 · speed limit 512 · swing -9, 9 · period updates 33 · period frames 66

The speed is in 1/256 pixel: its low byte carries in a fraction and its high byte, sign-extended, moves the offset. When the speed passes its limit the code also sets the offset to 0, which after the first swing lands within 2 pixels of where it was. The first swing, from a speed of 0, reaches −9 and jumps back to −2. The copy loop moves 16 bytes a band and writes 4 words of it (offsets 0, 4, 8, and 12). Both channels read the same buffer, so BG1 and BG2 bend together. The setup writes only the channels' modes and targets, not their table address; the address the window effect leaves is the one quoted here, and the warp's band layout assumes it.

A palette fade moves each color component one level at a time, on steps a table spreads through the fade rather than in proportion: for each filtered color (colors 32 to 215 and 224 to 239, and the backdrop) and each of its red, green, and blue components, the component's value in the saved palette, v from 0 to 31, picks a pair of 16-bit masks, and on step n the component moves by 1 toward black (or back up, for a fade-in) unless bit 15 − (n mod 16) of the first mask (n below 16) or the second (n of 16 or more) is set. Each pair leaves exactly v of the steps 0 to 30 clear (computed from the table), so after a fade-out's final step, step 30, every component of every filtered color is at 0; a component of 16 moves on every even step, and one of 1 only on step 15. Colors that are black in the saved palette are skipped. alttp-av/palette-fade

About: The Legend of Zelda: A Link to the Past Screen fade

{
 "steps": 31,
 "maskWords": [
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 ]
}

The saved palette is at $7EC300 and the working one at $7EC500, 2 bytes a color, so byte offsets $40 to $1AF and $1C0 to $1DF are colors 32 to 215 and 224 to 239. The add table's three words step red ($0001), green ($0020), and blue ($0400) down or up. The counter runs from 0 to the target the caller sets; the transitions that reset it set 31, so the steps are 0 to 30, and no pair's step 31 is clear. BitMasks is a shared table of single bits, $8000 shifted right by its index, whose comments name dungeons for its other use.

Three of the overworld's transitions (its submodules $0D, $17, and $24) darken and coarsen the picture together: they reset the fade counter to 0 with a target of 31, then each frame step the palette fade once and, on frames that find the counter odd, grow the mosaic size by 1, written with BG1, BG2, and BG3 enabled ($07) and BG mode value $09. Over the fade's 31 frames the size reaches 15, blocks 16 pixels square (computed), and the screen is then blanked. The transition back in, submodules $16 and $29, shrinks the size by 1 on odd counts in the same way. alttp-av/mosaic-transition

About: The Legend of Zelda: A Link to the Past Mosaic

Value: frames 31 · max size 15 · block pixels 16

The mosaic check runs before the fade's step in each frame, so it sees the counter at 0 to 30 and grows the size on its 15 odd values; on the frame the counter reaches 31 the fade swaps direction, resets the counter to 0, and moves on. The size is the register's high nibble, so adding $10 adds 1. Where in the overworld these submodules run isn't read here.

The Ether medallion flashes the screen by brightening the palette: on a flash frame the code adds 14 to the red, green, and blue of each of colors 32 to 127 in the saved palette, capping each at 31, and writes the results to the working palette; the backdrop takes the brightened color 32 unless it is black in the saved palette. A restore frame copies the saved colors 32 to 127 back and puts color 32 in the backdrop. Before the lightning's step counter leaves 0, a per-spell counter, raised by 1 each frame, picks flash or restore by its bit 2, so the screen alternates every 4 frames; after, only step $0B flashes. The flashing stops when the spell's state byte is 0. alttp-av/ether-flash

About: The Legend of Zelda: A Link to the Past Palette swap

Value: add 14 · cap 31 · colors 32, 127 · alternate frames 4

The add is 14 in each channel's own position: $000E to red, $01C0 (14 × 32) to green, and $3800 (14 × 1,024) to blue, each capped at the channel's maximum. The saved palette's byte offsets $40 to $FF are colors 32 to 127, the background palettes 2 to 7. A cap that the color is already under adds the full 14, so a dark color brightens as much as a light one, up to the cap.

The Quake medallion shakes the screen vertically by ±3 pixels: on each frame of its active phase the code takes a stored offset, starting at 3, as the vertical scroll offset added to BG1's and BG2's vertical scroll, then negates the stored value, so the picture moves 3 pixels one way and 3 the other on alternate frames. Its finish clears both scroll offsets. alttp-av/quake-shake

About: The Legend of Zelda: A Link to the Past Screen shake

Value: amplitude 3 · period 2 (pixels)

The offset is added to BG1's and BG2's vertical scroll in the main loop's merge; BG3's scroll doesn't take it there. The routine also adds the offset to the byte at $30, which keeps Link still against the shaking background. The horizontal offset ($011A) stays 0 during the shake.

Each overworld screen's song comes from a 160-byte table, one byte per screen, whose low nibble is the song and high nibble an ambient sound. The game builds the table from one of four Light World versions, followed by the Dark World and special screens: the fourth if the story counter is 3 or more, else the third if the sword byte is 2 or more, else the second if the story counter is 2, else the first. Scrolling onto a screen whose song differs from the one playing sends the fade command $F1 at once, and the game keeps $F1 as the song playing; when the new screen is entered, a fade in that place is replaced by the screen's song. An ambient nibble of 0 sends 5 instead. In the Dark World without the Moon Pearl the crossing sends neither, so the song carries on; the mirror warp's arrival there plays song 4. alttp-av/area-music

{
 "lightWorldTables": [
  {
   "when": "story counter under 2, sword under 2",
   "default": "$03",
   "others": {
    "$05": "$00 $01 $08 $09",
    "$13": "$12-$14 $1A-$1C $22-$24 $2A-$2C"
   }
  },
  {
   "when": "story counter 2, sword under 2",
   "default": "$02",
   "others": {
    "$55": "$00 $01 $08 $09",
    "$52": "$03-$07 $0B-$0E",
    "$07": "$18 $19 $20-$22 $28 $29"
   }
  },
  {
   "when": "story counter under 3, sword 2 or more",
   "default": "$02",
   "others": {
    "$52": "$00 $01 $03-$09 $0B-$0E",
    "$07": "$18 $19 $20-$22 $28 $29"
   }
  },
  {
   "when": "story counter 3 or more",
   "default": "$02",
   "others": {
    "$52": "$00 $01 $03-$09 $0B-$0E $30 $31 $38 $39"
   }
  }
 ],
 "darkWorld": {
  "default": "$09",
  "others": {
   "$9D": "$40 $41 $43-$49 $4B-$4E"
  }
 },
 "special": {
  "default": "$02",
  "others": {
   "$05": "$80",
   "$12": "$9F"
  }
 }
}

The builder copies 64 Light World bytes and then 96 more (the Dark World's 64 and the special screens' 32), so the table covers screens $00 to $9F. The world byte is ORed into the screen number, so the Dark World screens are $40 to $7F and read the Dark World part. Before Zelda's rescue every Light World screen plays song 3, the rain; the Light World Overture (2) replaces it after, and the Lost Woods screens lose their own song (5) once the sword byte reaches 2, the Master Sword's value in the item table. Kakariko's song (7) goes at story counter 3. The song and sound data are binary includes in the disassembly, so what ambient values 1, 5, and 9 sound like isn't read here. The special areas' mosaic transitions send the song after the mosaic instead, skipping screens $80 and $2A.

The game sends sound to the sound CPU through four ports: songs on the first, ambient sounds on the second, and two sets of 63 effects on the third and fourth, which it writes every frame, zero when nothing is queued. The ambient set has 32 slots, two of them empty, with one sound in two slots and six slots that repeat entries $0B to $10. The effect sets share entries: the second set's slot $38 plays its slot $34, and the third set's slots 5 and $1D play the second set's effects 7 and $2D. Each set has an accompaniment table: a nonzero entry starts a second sound with the first, which the ambient set uses for every odd slot, pairing it with the next slot on the seventh voice. Each effect set also has an echo table, read when an effect starts; a nonzero entry adds the effect's voice to a set of voices the sound program keeps. alttp-av/sfx-tables

About: The Legend of Zelda: A Link to the Past Sound effects

{
 "ports": {
  "song": "APUIO0",
  "ambient": "APUIO1",
  "effectsA": "APUIO2",
  "effectsB": "APUIO3"
 },
 "slots": {
  "ambient": 32,
  "effectsA": 63,
  "effectsB": 63
 },
 "accompaniment": {
  "ambient": "every odd slot n starts n+1",
  "effectsA": {
   "$0D": "$3F",
   "$13": "$3E",
   "$24": "$3D",
   "$29": "$3B",
   "$2C": "$3A",
   "$2E": "$39",
   "$2F": "$38",
   "$34": "$33",
   "$35": "$36"
  },
  "effectsB": {
   "$0F": "$3C",
   "$10": "$3B",
   "$1A": "$38",
   "$1B": "$3A",
   "$23": "$39",
   "$2D": "$37",
   "$2E": "$35",
   "$2F": "$33",
   "$35": "$34",
   "$3C": "$3D",
   "$3D": "$3E",
   "$3E": "$3F"
  }
 }
}

The pointer tables' 63 entries leave ID 0 for nothing queued, and the effect IDs' top two bits are masked off before the echo lookup (the sound program ANDs the ID with $3F). The sounds themselves are binary includes in the disassembly, so which effect is which isn't read here. The echo tables feed a step that may clear a voice's echo when an effect takes it, under a flag in the sound program's memory that this claim doesn't trace.

The songs set the echo themselves, through four commands in their track data. $F5 sets which voices echo and the left and right echo volumes; $F6 sets the flag that disables echo writes; $F7 sets the echo delay, the feedback, and one of four 8-tap filters; $F8 slides the two echo volumes to new values over a given number of ticks. A change of delay first zeroes both echo volumes, the feedback, and the echo voices, disables echo writes, and then writes the new delay. The echo buffer starts at $D000 − $800 × the delay and is $800 bytes long per step of delay, so it ends at $D000, where the songs' pointer table begins (computed). The sound program starts with a delay of 1, 512 samples, and a main volume of $60 on both sides. The first filter is the identity filter, which leaves the echo unchanged. alttp-av/echo-commands

About: The Legend of Zelda: A Link to the Past Echo

{
 "startDelay": 1,
 "masterVolume": 96,
 "filters": [
  [
   127,
   0,
   0,
   0,
   0,
   0,
   0,
   0
  ],
  [
   88,
   191,
   219,
   240,
   254,
   7,
   12,
   12
  ],
  [
   12,
   33,
   43,
   43,
   19,
   254,
   243,
   249
  ],
  [
   52,
   51,
   0,
   217,
   229,
   1,
   252,
   235
  ]
 ]
}

The start address is computed from ConfigureEcho: the delay times 8, inverted and added to $D0 with the carry set, is $D0 − 8 × delay as a page number, and 8 pages are $800 bytes, the size the delay register gives each step. The filter writes start at FIR0 and add $10 to the register number each time, through FIR7. A delay step of 512 samples is 16 ms at the chip's 32,000 Hz. Which songs use echo, and with what values, is in the song data, which the disassembly includes as binary files, so no song's settings are read here.