Add a sprite example

This is way better documented. Moreover, the `input.s` example has been
further simplified.

Signed-off-by: Miquel Sabaté Solà <msabate@suse.com>

1 files changed, 577 insertions, 0 deletions

diff --git a/examples/sprite.s b/examples/sprite.s
new file mode 100644
index 0000000..f429bb1
--- /dev/null
+++ b/examples/sprite.s
@@ -0,0 +1,577 @@
+;;;
+;; The iNES is the de facto standard for the distribution of NES binary programs
+;; and it's compatible with the format used by NES cartridges themselves (used,
+;; even, by the Wii Virtual Console). The layout is composed by segments in
+;; memory, which are defined with the `.segment` macro and known by the compiler
+;; through a linker configuration. You can provide a configuration of your own,
+;; but bear in mind that compilers like `cc65` (the one used here, which is the
+;; most common) already provide a default configuration for the linker that
+;; glues a set of pretty common defined named segments. You can read about this
+;; in `cfg/nes.cfg` from inside your cc65 installation.
+;;;
+
+;;;
+;; The "HEADER" is the first segment of any iNES binary and it contains basic
+;; information about what this "cartridge" requires in order to work. Besides
+;; basic identification, it defines some relevant things like PRG and CHR sizes,
+;; plus mapping if desired. See https://www.nesdev.org/wiki/NES_2.0#Header for
+;; documentation on this, or also: https://www.nesdev.org/neshdr20.txt.
+;;;
+.segment "HEADER"
+    ;; The first thing to do is to define the magic "NES\0" string identifier
+    ;; ($1A is the ASCII that MS-DOS wanted as end of string). Some people write
+    ;; it in hexadecimal through and through, but cc65 already compiles
+    ;; characters into their proper hexadecimal values, so there's no need to be
+    ;; extra cryptic here.
+    .byte 'N', 'E', 'S', $1A
+
+    ;; The next two bytes define the size of the PRG and CHR ROMs in this order.
+    ;; More specifically, they define how many 8KB banks are available for PRG
+    ;; and CHR. Hence, the next two bytes define a 16KB (2x 8KB) of PRG-ROM, and
+    ;; 8KB of CHR-ROM.
+    .byte $02
+    .byte $01
+
+    ;; Next we have two bytes for selecting a mapper. This is a huge topic (see
+    ;; the NesDev wiki for this), but it basically refers to the fact that some
+    ;; cartridges had specific requirements on how to place their data, or the
+    ;; amount of it they required, or how they expected their data to be
+    ;; mirrored in the memory, etc. During the life-time of the NES, and as
+    ;; developers pushed the boundaries of the NES hardware, more intricate
+    ;; mappers were used on the hardware of cartridges themselves. Thus, in
+    ;; these two bytes we are telling the emulator: "hey, act as if this was a
+    ;; cartridge that used this kind of mapper".
+    .byte $00                   ; Horizontal mirroring (good for vertical scrollers)
+    .byte $00                   ; No mapper nor special-case flags.
+
+    ;; The previous are the mandatory bytes in order to get a "cartridge" going.
+    ;; After this there are some other bytes you can put into the header, like
+    ;; if the "cartridge" counts on a battery-backed RAM-mapped section to save
+    ;; states (e.g. The Legend of Zelda) or rare flags like specifying the
+    ;; region (NTSC vs PAL), but for now this falls out of my radar :-)
+
+;;;
+;; This is the segment where we tell the processor where to find the code for
+;; three important topics: the Non-Maskable Interrupts handler, the Reset
+;; handler, and the IRQ handler. If you look at the configuration from `cc65`
+;; that was provided on your installation (or if you are pesky enough to create
+;; one yourself), you will notice that these vector addresses are placed at the
+;; very end of memory ($fffa-ffff). The NES (and emulators) will look at these
+;; three last positions in memory to know where to jump for each case.
+;;;
+.segment "VECTORS"
+    .addr nmi
+    .addr reset
+    .addr irq
+
+;;;
+;; Required by the default linker configuration. Theoretically there should be a
+;; semantical difference between this section and "CODE", but as for the linker
+;; goes, there is no difference and everything will be put sequentially on the
+;; resulting binary. Hence, if you want (and as I do here), you can leave this
+;; empty (so to make the default configuration of the linker happy), and put
+;; everything into the "CODE" segment. In fact, according to the Famicom Party
+;; Book (https://famicom.party/book/04-hardwareoverview/), the "STARTUP" section
+;; is only used by C programs compiled down into 6502 assembly, so it might not
+;; be even relevant for us (and in fact said book actually removes this segment
+;; in its linker configuration down the road).
+;;;
+.segment "STARTUP"
+
+.segment "CODE"
+
+;;;
+;; The Reset handler is executed when the whole thing starts (i.e. the user has
+;; pressed either the reset or the power on buttons on the NES). Thus, this
+;; piece of code is pretty standard and an implementation is even given in the
+;; NesDev wiki which is pretty much followed by everyone as I could see. The
+;; main idea is to leave the hardware in a known state and then jump into the
+;; main game subroutine.
+;;;
+reset:
+    ;; We first instruct the NES to disable everything. That is, we don't want
+    ;; any pesky interrupt to make us jump into the `nmi` section, for example,
+    ;; before we have configured everything.
+
+    ;; Ignore IRQs and disable decimal mode (the NES 6502 chip, for copyright
+    ;; issues or legal reasons against MOS that I don't fully know nor care,
+    ;; does not have decimal mode anyway, but it's considered good practice).
+    sei
+    cld
+
+    ;; Disable APU frame IRQ. This is the first instance we see of Memory-Mapped
+    ;; I/O. This is a core concept in NES programming and, to sum things up, the
+    ;; memory range $2000-$6000 is reserved to I/O operations, and each address
+    ;; is reserved to a specific hardware operation. This is because the NES CPU
+    ;; doesn't directly control the PPU nor other chips. In this case, ranges
+    ;; $4000-$4017 control the APU (Audio Processing Unit). More precisely, the
+    ;; $4017 address controls what is called the "Frame counter" from the APU
+    ;; (https://www.nesdev.org/wiki/APU#Frame_Counter_($4017)). Setting #$40 to
+    ;; it disables it completely, so we are in a known state. If we were to use
+    ;; sound, at the end of the reset code we should enable it back. We do *not*
+    ;; do it here because we don't need it.
+    ldx #$40
+    stx $4017                   ; APU Frame Counter
+
+    ;; Set up the stack register with the proper value (the stack will grow in
+    ;; decreasing order from $01FF -> $0100).
+    ldx #$ff
+    txs
+
+    ;; And now disable, in this order, NMI, rendering and DMC IRQs. Note that
+    ;; `x` was set to $ff, so increasing it by one results in a zero, which is
+    ;; the value then stored in the aforementioned memory locations.
+    ;;
+    ;; The one on $4010 refers again to the APU (as described before), and it
+    ;; directly controls the DMC. Again, if you wanted sound, you should enable
+    ;; this back after the whole reset block.
+    ;;
+    ;; On the other side, the low addresses of $2000 control the PPU. In
+    ;; particular, we disable NMIs from the PPU by setting the PPUCTRL address
+    ;; ($2000) to zero, and we do the same for the PPUMASK ($2001). Don't worry
+    ;; about them just now, we will go deeper down below.
+    inx
+    stx $2000                   ; PPUCTRL
+    stx $2001                   ; PPUMASK
+    stx $4010                   ; APU DMC
+
+    ;;;
+    ;; NOTE: If you are using a mapper which needs some special configuration,
+    ;; now it would be a good time set it up. I am not using a special mapper,
+    ;; so there's nothing from me to do here.
+    ;;;
+
+    ;; At this point, we have to wait for the PPU to stabilize. This is
+    ;; typically done by checking a flag from the PPUSTATUS address ($2002) and
+    ;; waiting until the proper value is set by the PPU. Since this wait can
+    ;; take a while, programmers typically put other initialization code here,
+    ;; like sprite resetting and such.
+    ;;
+    ;; The PPUSTATUS memory address contains general information on the status
+    ;; of the PPU, and is read-only. Moreover, reading from the PPUSTATUS has a
+    ;; convenient side-effect: it resets the "address latch" for PPUADDR. We
+    ;; will see how we can take advantage of this when we use the PPU properly
+    ;; down the road.
+
+    ;; First of the two waits.
+@vblankwait1:
+    bit $2002                   ; PPUSTATUS
+    bpl @vblankwait1
+
+    ;; The PPU has at least started, now we have a bunch of cycles for it to
+    ;; stabilize, which will be properly announced through the PPUSTATUS memory
+    ;; address. Instead of just waiting for it again, we will take the chance to
+    ;; initialize more stuff.
+
+    ;; One typical thing to do is to leave the RAM in a known state. That is, we
+    ;; will set to 0 addresses $0000 - $07FF. Apparently there are some people
+    ;; who say that doing this is bad because it will hide programming mistakes
+    ;; (e.g. bad initialization code). So, if you are one of these people, you
+    ;; can safely remove this loop. Otherwise let's get the RAM clean. That
+    ;; being said, notice that we are skipping $200-$2ff. This is no mistake as
+    ;; you will see below.
+    ldx #0
+    lda #0
+@ram_reset_loop:
+    sta $000, x
+    sta $100, x
+    sta $300, x
+    sta $400, x
+    sta $500, x
+    sta $600, x
+    sta $700, x
+    inx
+    bne @ram_reset_loop         ; if x overflows back to #00, then we are done.
+
+    ;; And now another thing we can do is to reset the sprites. In NES programs
+    ;; sprites live in a special range of memory addresses from the RAM called
+    ;; OAM (Object Attribute Memory). This range is located in $0200-02ff (the
+    ;; range we did not initialize when cleaning up RAM).
+    ;;
+    ;; "Resetting sprites" is just a matter of giving them a value which will
+    ;; not bother us in the future. One way to do this is to set each value to
+    ;; $ef, which will give each "sprite" off-screen Y-coordinates. How any of
+    ;; this is the case will be shown whenever we deal with loading proper
+    ;; sprites below.
+    lda #$ef
+@sprite_reset_loop:
+    sta $200, x
+    inx
+    bne @sprite_reset_loop
+
+    ;; And write these resetted "sprites" into the PPU. How does this work?
+    ;; Well, address $2003 has the OAMADDR. That is, from which position the PPU
+    ;; should start the DMA process. In our case we set it to the very
+    ;; beginning. Then, if you write to the OAMDMA memory address ($4014), you
+    ;; will instruct the PPU to start a DMA process starting at $1XX*N. XX is
+    ;; the byte given at the OAMADDR, thus XX = #$00; and N = 2 (see `lda
+    ;; #$02`). Therefore, we are telling the PPU to start the DMA process from
+    ;; $200. The PPU will assume that the following 256 bytes of memory are the
+    ;; ones to be copied, resulting in a DMA copy of $200-$2ff, right where the
+    ;; sprite data is located in memory.
+    lda #$00
+    sta $2003                   ; OAMADDR
+    lda #$02
+    sta $4014                   ; OAMDMA
+
+    ;; As advertised, the other wait. Once we are out of this loop, we are 100%
+    ;; guaranteed to have a properly working PPU which is ready to render stuff
+    ;; into the screen.
+@vblankwait2:
+    bit $2002                   ; PPUSTATUS
+    bpl @vblankwait2
+
+    ;; And we also reset the palettes. How is this done? Well, the $2006 memory
+    ;; address is the PPU address (PPUADDR). This address is given byte by byte,
+    ;; the most significat byte first. Thus, the four lines below store into
+    ;; $2006 the value $3F00. This is the first address where palettes are
+    ;; stored. Palettes are the answer from old systems like the NES to: how can
+    ;; you display this amount of colors on screen without taking too much
+    ;; memory? The NES allows developers to store eight palettes (four
+    ;; background, four foreground), and each palette group four colors. This
+    ;; way, whenever we want to draw a sprite or a piece of background, we don't
+    ;; specify which colors to pick for each pixels, but we rather apply a
+    ;; palette to a srite or background tile definition.
+    lda #$3F
+    sta $2006                   ; PPUADDR
+    lda #$00
+    sta $2006                   ; PPUADDR
+
+    ;; After setting the address for the first palette, now we loop #$20 times:
+    ;; 8 palettes * 4 bytes per palette (1 byte per color) = 32 (#$20
+    ;; hexadecimal). At each iteration we will write into $2007 the value #$0F.
+    ;; This memory address is the PPUDATA, and #$0F corresponds to a black
+    ;; color. In each iteration, therefore, we are telling the PPU that we want
+    ;; a black color for all the colors from all palettes. Moreover, whenever
+    ;; you write into PPUDATA, the memory address for the next store is
+    ;; incremented by one automatically. This is why we don't have to index the
+    ;; write or anything like that, but we can simply write to the same memory
+    ;; address 32 times and it will be performed onto 32 consecutive positions.
+    lda #$0F
+    ldx #$20
+@palettes_reset_loop:
+    sta $2007                   ; PPUDATA
+    dex
+    bne @palettes_reset_loop
+
+
+    ;; At this point everything is clear and with a state we know, now we can
+    ;; jump into our main subroutine and start loading sprites, palettes, etc.;
+    ;; and start the game proper.
+    jmp main
+
+;;;
+;; This is our main subroutine. At this point we can assume that the hardware
+;; has already been set to a proper and defined state. So now we can load all
+;; the data we need for our game and enter the main game loop.
+;;;
+.proc main
+    ;; Before starting the game loop proper we initialize all our assets: load
+    ;; the palettes, nametables and sprites for this game.
+    jsr init_palettes
+    jsr init_nametable
+    jsr init_sprites
+
+    ;; Reset scroll. This could have been done in the reset process, but
+    ;; scrolling feels like something that should be in control entirely by the
+    ;; game itself. Any ways, the scroll is handled through the $2005
+    ;; (PPUSCROLL) memory address. We first need to write the X coordinate and
+    ;; then the Y coordinate for the camera. Since we are not planning on doing
+    ;; anything fancy with scrolling, we just initialize it to a zero position.
+    bit $2002                   ; PPUSTATUS
+    lda #$00
+    sta $2005                   ; PPUSCROLL
+    sta $2005                   ; PPUSCROLL
+
+    ;; Remember when we disabled rendering and NMI on the reset code? Now it's
+    ;; time to enable them back. Things to note:
+    ;;   - PPUCTRL: we enable three bits:
+    ;;     - 4: background pattern table starts at $1000. The CHR is divided by
+    ;;          two pattern tables, and each table contains 256 8x8 tiles. In my
+    ;;          case, my sprite data is located at the first pattern table (thus
+    ;;          $0000, and that's why the 3rd bit from PPUCTRL is set to zero),
+    ;;          and the background data is on the second pattern table (that
+    ;;          starting at $1000). In the end this all depends on how you want
+    ;;          to structure your CHR file.
+    ;;     - 5: sprite size is 8x16. Set this to 0 if your sprites in the
+    ;;          character file are 8x8.
+    ;;     - 7: allow a NMI at the start of the vertical blanking interval.
+    ;;          That is, whenever the screen has displayed one frame, send us
+    ;;          an interrupt so we can generate the next one (see the code on
+    ;;          `nmi`).
+    ;;   - PPUMASK: we enable 4 bits:
+    ;;     - 1: Show background in leftmost pixels on the screen.
+    ;;     - 2: Show sprites in leftmost pixels on the screen.
+    ;;     - 3: Show background.
+    ;;     - 4: Show sprites.
+    ;;
+    ;; As for showing on the leftmost pixels, this might not be a good idea if
+    ;; the game is a scroller, since the game might flicker depending on how you
+    ;; manage the camera. This is out of scope.
+    cli
+    lda #%10110000
+    sta $2000                   ; PPUCTRL
+    lda #%00011110
+    sta $2001                   ; PPUMASK
+
+@main_game_loop:
+    ;;;
+    ;; NOTE: enter here your game loop logic.
+    ;;;
+
+    ;; This is a hand-shake between the code on `main` and the code on the
+    ;; `nmi`. That is, whenever we are done with the game loop logic, we set
+    ;; this flag to indicate that the rendering is doable. After this, the code
+    ;; on `main` will be blocked until this flag is unset by the `nmi` code,
+    ;; which will indicate that it can proceed with another iteration of the
+    ;; game loop. On the contrary, the `nmi` code will be blocked until this
+    ;; flag is set, at which point it will start rendering and unset the flag
+    ;; whenever that is done.
+    lda #%10000000
+    ora $20
+    sta $20
+@wait_for_render:
+    bit $20
+    bmi @wait_for_render
+
+    ;; Rendering is done, we can perform another iteration of the loop!
+    jmp @main_game_loop
+.endproc
+
+;; init_palettes copies all the palettes for our game into the proper PPU
+;; address.
+.proc init_palettes
+    ;; Remember these four instructions? That's what we also did when resetting
+    ;; palettes on the reset code. That is, we are preparing the PPU to write
+    ;; data starting from $3f00.
+    lda #$3F
+    sta $2006                   ; PPUADDR
+    lda #$00
+    sta $2006                   ; PPUADDR
+
+    ;; We are going to copy the eight palettes stored in the `palettes` tag into
+    ;; PPU. This is done by indexing with the `x` register, which is incremented
+    ;; until it reaches #$20 (32 in hexadecimal), which corresponds to 8
+    ;; palettes * 4 bytes each (just like we did in the reset code).
+    ldx #0
+@load_palettes_loop:
+    lda palettes, x
+    sta $2007                   ; PPUDATA
+    inx
+    cpx #$20
+    bne @load_palettes_loop
+    rts
+palettes:
+    ;; Let there be palettes! This is the data for all the palettes we have for
+    ;; our game. Each hexadecimal value is a byte that represents a color (you
+    ;; can use YY-CHR to quickly take a look at which color represents each
+    ;; hexadecimal value, or use any of the tables that can be found online, or
+    ;; even fceux has this with its "Palette editor" under "Tools"). There is,
+    ;; though, one big catch: the first color from all palettes *must* be the
+    ;; same. To be more precise, it's not like the NES will explode if you pick
+    ;; other colors, but the NES *will* assume that the first color from all
+    ;; palettes is the one found in $3F00 (that is, the first color from the
+    ;; first palette). In this case, $0F will be this "default color". Moreover,
+    ;; for this simple game we are not using all palettes, and that's why for
+    ;; some of them all four bytes are simply zero'ed.
+
+    ;; Background
+    .byte $0F, $12, $22, $32
+    .byte $0F, $00, $28, $30
+    .byte $0F, $28, $16, $2D
+    .byte $0F, $28, $16, $2D
+
+    ;; Foreground
+    .byte $0F, $00, $05, $30
+    .byte $0F, $00, $00, $00
+    .byte $0F, $00, $00, $00
+    .byte $0F, $00, $00, $00
+.endproc
+
+;; WRITE_PPU_DATA is a macro that will write into PPUADDR the given address and
+;; into PPUDATA the given byte value.
+.macro WRITE_PPU_DATA address, value
+    bit $2002                   ; PPUSTATUS
+    lda #.HIBYTE(address)
+    sta $2006                   ; PPUADDR
+    lda #.LOBYTE(address)
+    sta $2006                   ; PPUADDR
+    lda #value
+    sta $2007                   ; PPUDATA
+.endmacro
+
+;; init_nametable loads the relevant data to the nametable that is then going to
+;; be used in order to build up the background.
+.proc init_nametable
+    ;; The general idea here is that the background defaults to the
+    ;; "transparent" color, which is the first color on the palette. Then for
+    ;; the background we only need to store into the PPU those elements which
+    ;; are not the default thing. Hence, if we want to draw a couple of
+    ;; background elements in our game in some positions, we have to instruct
+    ;; the PPU where to put each item and where to find it on the pattern table.
+    ;; After that, we can change/toggle the palette for a background tile if we
+    ;; so want it, and we are done with it.
+
+    ;; Anyways, before loading data into the PPU, we always have to check the
+    ;; PPUSTATUS, that's why the first instruction of each block is a `bit
+    ;; $2002`. This is a safe-guard so to reset the "address latch". That is, if
+    ;; instead of writing two consecutive bytes we only wrote one by mistake (or
+    ;; because we only wanted to write the high byte), it resets back its
+    ;; internal index.
+
+    bit $2002                   ; PPUSTATUS
+
+    ;; Load the first item for the background! Let's make sense of the values.
+    ;; We first need to set the address that the PPU will use (that is, which
+    ;; couple of bytes we need to pass to PPUADDR). This can be tricky, but
+    ;; luckily some tools makes things easier for us. For example, the NEXXT
+    ;; tool allows users to draw a NES screen and then it gives the offset
+    ;; address that you can use for each drawn element. This way I got that the
+    ;; offset for the first element was $00C8, which added to the base address
+    ;; of $2000 (start of the first nametable), gives us the address $20C8.
+    ;; Therefore, if I want this background element to be rendered in the
+    ;; position I envisioned on this tool, I need to write $20C8 into PPUADDR.
+    ;; As for the PPUDATA address, I need to pass #$02 because that's the index
+    ;; inside of the CHR file of the star in the second pattern table (check the
+    ;; PPUCTRL setting at the end of the `main` function on why it's the second
+    ;; pattern table).
+    lda #$20
+    sta $2006                   ; PPUADDR
+    lda #$C8
+    sta $2006                   ; PPUADDR
+    lda #$02
+    sta $2007                   ; PPUDATA
+
+    ;; Now we have to do the same for the rest of the elements. I have gone
+    ;; through the same process of fetching the offset from a canvas I drew on
+    ;; NEXXT and here's the result. Instead of repeating te same code over and
+    ;; over but with slightly different values, I am using this macro I created
+    ;; to do the same.
+    WRITE_PPU_DATA $20B9, $04
+    WRITE_PPU_DATA $21CE, $04
+    WRITE_PPU_DATA $21BA, $04
+    WRITE_PPU_DATA $22B8, $04
+    WRITE_PPU_DATA $22E7, $04
+    WRITE_PPU_DATA $227A, $02
+
+    ;; Now everything we wanted to write for the background is there, but we
+    ;; might want to toggle the colors for some of the elements. You can do this
+    ;; with palettes, which for background tiles is done via attribute tables.
+    ;; Each attribute table lives right after each nametable, and it consists of
+    ;; 1KB of memory in which you can establish the palettes to be used for the
+    ;; tiles appearing on the background screen as defined on that specific
+    ;; nametable. Information on this is also given to us by tools like NEXXT.
+    ;; Shortly, for one of the elements I want to change their palette I was
+    ;; told that the "attribute offset" was $03CE. Again, added to the base
+    ;; address, it means that we need to pass $23CE to the PPUADDR. The value
+    ;; for PPUDATA is a bit more tricky: on the attribute table each 8x8 tile is
+    ;; broken down into 4 2-sized squares. Each of these 4 squares of the tile
+    ;; has a palette assigned to it ($00 by default). Thanks to this setup, a
+    ;; single byte can encode four palettes by addressing each square:
+    ;; %44332211. The tool also tells us in which of these smaller squares our
+    ;; element resides. With this info, and since we have two bits available for
+    ;; each of these smaller squares and, therefore, can encode up to 4
+    ;; palettes, it means that we just have to assign either on the range of
+    ;; 00-11 to the pair of bits representing the smaller square that NEXXT is
+    ;; telling us that our background element resides in. In this case it's the
+    ;; first square, so we just need to assign $01 to the bits reserved to this
+    ;; square if I want to change the palette to 1. This is what we pass as a
+    ;; value.
+    WRITE_PPU_DATA $23CE, %00000001
+
+    rts
+.endproc
+
+;; init_sprites loads all the sprites we want from our game.
+.proc init_sprites
+    ;; This is tied to the number of sprites stored in the `initial_sprite_data`
+    ;; section. Remember that this can be 64 *maximum*: 64 sprites * 4 bytes per
+    ;; sprite = 256 bytes; which is the reserved space in memory we have for
+    ;; sprites: $0200-$02ff.
+    NUM_SPRITES = 2
+
+    ;; The loading is quite straight-forward. We just store whatever is on
+    ;; initial_sprite_data + x into $0200 + x, where x is our index that gets
+    ;; increased on each iteration. We will stop the loop whenever x reaches
+    ;; NUM_SPRITES * 4 (each sprite takes 4 bytes, more below).
+    ldx #$00
+@load_sprites_loop:
+    lda initial_sprite_data, x
+    sta $0200, x
+    inx
+    cpx #(4 * NUM_SPRITES)
+    bne @load_sprites_loop
+    rts
+initial_sprite_data:
+    ;; Each sprite consists of 4 bytes:
+    ;;  1. The Y position.
+    ;;  2. The sprite position on the pattern table in hexadecimal. Make sure to
+    ;;     get this position right by using YY-CHR (or whatever program you are
+    ;;     using to manipulate the CHR file). You can get this wrong also
+    ;;     depending if you are using 8x8 or 8x16 tiles mode (check the bit set
+    ;;     on PPUCTRL on the `main` code).
+    ;;  3. The attributes for the sprite. The first two bits control which
+    ;;     palette is used to draw the sprite (in our case palette $00). Bits 2
+    ;;     to 4 are not used. Bit 5 sets the sprite behind the background if set
+    ;;     to 1; bit 6 flips the sprite horizontally; and bit 7 flips the sprite
+    ;;     vertically.
+    ;;  4. The X position.
+    ;;
+    ;; In our case, notice that we are using the same sprite (located at $00),
+    ;; but the second sprite is moved eight pixels right and flipped
+    ;; horizontally. This is because the sprite located at $00 is just half of
+    ;; the real "meta-sprite", and so we can build the entire "meta-sprite" by
+    ;; simply flipping the same part horizontally. This is one of the many
+    ;; tricks we can use to save space on CHR-ROM.
+    .byte $B0, $00, %00000000, $7A
+    .byte $B0, $00, %01000000, $82
+.endproc
+
+;;;
+;; Non-Maskable Interrupts handler. After being enabled by the initialization
+;; code, you can count on this code being called at the end of each PPU
+;; rendering frame during the Vertical Blanking Interval (VBLANK). This time
+;; span is pretty tight (around 2273 CPU cycles), so you better update
+;; everything to be rendered before that if you don't want graphical glitches.
+;;;
+nmi:
+    ;; As mentioned on the `main` subroutine, rendering will be skipped until
+    ;; the proper flag is set.
+    bit $20
+    bpl @next
+
+    ;; We are instructed that we can start rendering stuff. Transfer the sprites
+    ;; via OAM. This is the same we did when we resetted sprites in our `reset`
+    ;; code.
+    lda #$00
+    sta $2003                   ; OAMADDR
+    lda #$02
+    sta $4014                   ; OAMDMA
+
+    ;; And unset the render flag so the `main` code is unblocked.
+    lda #%01111111
+    and $20
+    sta $20
+@next:
+    rti
+
+;;;
+;; Interrupt Requests handler. This is triggered by the NES' sound processor
+;; (APU) or by some specific cartridge hardware (e.g. something specific to a
+;; mapper). In our case we don't have to do anything here, so we just return
+;; from the interrupt.
+;;;
+irq:
+    rti
+
+;;;
+;; Include into this all the data that needs to go into the CHR ROM. One typical
+;; implementation for this is by using the `.incbin` macro, which will blindly
+;; copy the bitmap that you have generated through a program such as YY-CHR into
+;; the CHR ROM. Note that not all games used the CHR-ROM for storing their
+;; assets and instead used the PRG-ROM for that as well (e.g. The Legend of
+;; Zelda). There are multiple technical reasons to do this, but this falls out
+;; of the scope of this file and my expertise, to be honest.
+;;;
+.segment "CHARS"
+    .incbin "../assets/basic.chr"