# Flappy ASM [Courses](https://asm-editor.specy.app/learn/courses.md) › [M68K assembly](https://asm-editor.specy.app/learn/courses/m68k.md) › Examples Web page: https://asm-editor.specy.app/learn/courses/m68k/examples/flappy-asm > A game with a state machine in it: three states, gravity counted in sixteenths of a pixel, a press that only counts on the frame it began, and three pipes recycled into an endless course. The same program in: [MIPS](https://asm-editor.specy.app/learn/courses/mips/examples/flappy-asm.md), [RISC-V](https://asm-editor.specy.app/learn/courses/risc-v/examples/flappy-asm.md), [Z80](https://asm-editor.specy.app/learn/courses/z80/examples/flappy-asm.md) Build a small game in which a tap lifts the bird and gravity pulls it back down. Pipes scroll in from the right. Passing one earns a point; touching a pipe or the ground ends the round. Choose **Build**, then **Run**. Click inside the **Screen** so it has keyboard focus. Tap Space, W, Enter or the up arrow to start and flap; clicking the Screen also flaps. Release between taps: holding a key or mouse button gives only one flap. After a collision, tap again to restart. Choose **Stop** when you are done. The listing is long because it draws the scene as well as running the game. Use this map before reading it: | Follow this part | What it does | | -------------------------- | ------------------------------------------------------------------ | | `frame` → `readflap` | Find out whether a new tap began. | | `READY`, `playing`, `dead` | Start a round, advance it, or let the bird land. | | `movepipes` → `hittest` | Move the obstacles, award points and check collisions during play. | | `draw` | Draw the scene, show it with task 94, then pause with task 23. | Start with `frame`, `readflap`, `movepipes` and `hittest`. You can skim the shape and colour helpers under `Drawing` on a first pass. ```m68k ORG $1000 start: move.b #92,d0 move.b #17,d1 trap #15 ; mode 17: draw off screen until task 94 move.b #93,d0 move.b #1,d1 trap #15 ; a one pixel pen for every outline newgame: bsr reset * --- one frame --------------------------------------------------------------- * d7 carries "a flap began this frame" from the input read to the state machine, * so nothing called from here is allowed to touch it. frame: bsr readflap move.w d0,d7 move.w state,d0 cmp.w #PLAYING,d0 beq playing cmp.w #DEAD,d0 beq dead * --- waiting to start: the bird hangs still until the first flap -------------- tst.w d7 beq draw move.b #8,d0 trap #15 ; hundredths of a second since the run started move.w d1,seed ; so the course depends on when the player began bsr newpipes move.w #PLAYING,state move.w #FLAPV,birdv bra draw * --- playing ----------------------------------------------------------------- playing: tst.w d7 beq fall move.w #FLAPV,birdv ; one tap is one flap, however long the key is held clr.w wing ; and the wing beat starts again with it fall: move.w birdv,d0 add.w #GRAV,d0 cmp.w #MAXFALL,d0 ble capped move.w #MAXFALL,d0 capped: move.w d0,birdv ext.l d0 add.l d0,birdy ; birdy counts sixteenths of a pixel tst.l birdy bge insky clr.l birdy ; the top of the screen is a bump, not a death clr.w birdv insky: bsr movepipes bsr hittest tst.w d0 beq draw move.w #DEAD,state move.w points,d0 cmp.w record,d0 ble draw move.w d0,record bra draw * --- dead: the bird drops to the ground, then waits for a flap ---------------- dead: move.w birdv,d0 add.w #GRAV,d0 cmp.w #MAXFALL,d0 ble dcapped move.w #MAXFALL,d0 dcapped: move.w d0,birdv ext.l d0 add.l d0,birdy move.l birdy,d0 asr.l #4,d0 cmp.w #RESTY,d0 blt dfalling move.l #RESTY16,birdy clr.w birdv dfalling: tst.w d7 beq draw bra newgame * --- the frame is painted back to front, then shown all at once --------------- draw: bsr drawsky bsr drawpipes bsr drawground bsr drawbird bsr drawhud move.b #94,d0 trap #15 ; the finished frame appears here, all at once move.b #23,d0 move.l #DELAY,d1 trap #15 ; wait at least DELAY hundredths before the next frame bra frame *----------------------------------------------------------------------------- * Input *----------------------------------------------------------------------------- * Answers d0.w = 1 on the frame a flap begins. Both tasks report what is held * right now, so the "began" part is this program's: a press only counts when * the frame before it had nothing pressed. readflap: move.b #19,d0 move.l #KEYLIST,d1 trap #15 ; one $FF/$00 byte per key, in the same order move.l d1,d2 btst #24,d2 ; space bne pressed btst #16,d2 ; the up arrow bne pressed btst #8,d2 ; w bne pressed btst #0,d2 ; enter bne pressed move.b #61,d0 move.b #0,d1 trap #15 ; the mouse as it is right now btst #0,d0 ; bit 0 of the flags byte is the left button bne pressed clr.w held moveq #0,d0 rts pressed: tst.w held bne stillheld move.w #1,held moveq #1,d0 ; this frame is where the press began rts stillheld: moveq #0,d0 rts *----------------------------------------------------------------------------- * The world *----------------------------------------------------------------------------- * A new game, keeping the best score of the session. reset: move.w #READY,state move.l #STARTY16,birdy clr.w birdv clr.w points clr.w wing clr.w scroll bsr newpipes rts * Three pipes, evenly spaced off to the right, each with a gap of its own. newpipes: lea pipes,a0 move.w #FIRSTX,d2 moveq #2,d3 npnext: move.w d2,(a0)+ ; where its left edge is bsr randgap move.w d0,(a0)+ ; the middle of its gap clr.w (a0)+ ; and it has not been counted yet add.w #SPACING,d2 dbra d3,npnext rts * The middle of the next gap, from a sixteen bit congruential generator: one * seed always lays out the same course, and the seed comes from the clock. * Answers in d0.w and leaves every other register alone. randgap: moveq #0,d0 move.w seed,d0 mulu #25173,d0 add.l #13849,d0 move.w d0,seed ; the low word of the product is the next seed moveq #0,d0 move.w seed,d0 lsr.l #8,d0 ; its high byte is the half worth using mulu #GAPSPAN,d0 lsr.l #8,d0 ; 0 to GAPSPAN-1, without dividing add.w #GAPMIN,d0 rts * Everything scrolls left. A pipe that has left the screen jumps a whole cycle * to the right with a new gap, which is why three pipes are an endless course. movepipes: move.w scroll,d0 sub.w #SPEED,d0 bge scrolled add.w #TUFTGAP,d0 ; the tufts repeat, so this wrap is invisible scrolled: move.w d0,scroll lea pipes,a0 moveq #2,d3 mpnext: move.w (a0),d1 sub.w #SPEED,d1 cmp.w #NEGPIPE,d1 bgt mpmoved add.w #CYCLE,d1 bsr randgap move.w d0,2(a0) clr.w 4(a0) mpmoved: move.w d1,(a0) tst.w 4(a0) ; already counted? bne mpdone move.w d1,d2 add.w #PIPEW,d2 cmp.w #BIRDX,d2 bge mpdone move.w #1,4(a0) addq.w #1,points ; its right edge has passed the bird mpdone: lea 6(a0),a0 dbra d3,mpnext rts * Answers d0.w = 1 when the bird touches a pipe or the ground. Only the pipe * test uses an inset box; the ground checks the bird's full height. hittest: move.l birdy,d1 asr.l #4,d1 move.w d1,d2 add.w #BIRDH,d2 cmp.w #GROUNDY,d2 bge hit ; the ground is met by the whole bird add.w #HITIN,d1 ; the top of the box the pipes are tested with sub.w #HITIN,d2 ; and its bottom lea pipes,a0 moveq #2,d3 htnext: move.w (a0),d4 move.w d4,d5 add.w #PIPEW,d5 cmp.w #HITR,d4 bge htskip ; this pipe is still to the right of the bird cmp.w #HITL,d5 ble htskip ; and this one is already behind it move.w 2(a0),d6 sub.w #HALFGAP,d6 cmp.w d6,d1 blt hit ; above the gap add.w #GAPH,d6 cmp.w d6,d2 bgt hit ; below it htskip: lea 6(a0),a0 dbra d3,htnext moveq #0,d0 rts hit: moveq #1,d0 rts *----------------------------------------------------------------------------- * Drawing *----------------------------------------------------------------------------- * The sky, and two clouds drifting across it. drawsky: move.l #SKY,d1 move.b #80,d0 trap #15 move.l #SKY,d1 move.b #81,d0 trap #15 moveq #0,d1 moveq #0,d2 move.w #WIDTH,d3 move.w #HEIGHT,d4 move.b #87,d0 trap #15 move.l #CLOUDC,d1 move.b #80,d0 trap #15 move.l #CLOUDC,d1 move.b #81,d0 trap #15 ; pen and fill alike, so the lobes have no seams move.w cloud1x,d5 subq.w #1,d5 cmp.w #NEGCLOUD,d5 bgt cl1ok move.w #WIDTH,d5 ; gone on the left, back in on the right cl1ok: move.w d5,cloud1x move.w #56,d6 bsr cloud move.w cloud2x,d5 subq.w #1,d5 cmp.w #NEGCLOUD,d5 bgt cl2ok move.w #WIDTH,d5 cl2ok: move.w d5,cloud2x move.w #120,d6 bsr cloud rts * One cloud: three overlapping discs, at d5.w across and d6.w down. cloud: move.w d5,d1 move.w d6,d2 move.w d1,d3 add.w #70,d3 move.w d2,d4 add.w #34,d4 move.b #88,d0 trap #15 move.w d5,d1 add.w #40,d1 move.w d6,d2 sub.w #16,d2 move.w d1,d3 add.w #60,d3 move.w d2,d4 add.w #50,d4 move.b #88,d0 trap #15 move.w d5,d1 add.w #86,d1 move.w d6,d2 add.w #6,d2 move.w d1,d3 add.w #54,d3 move.w d2,d4 add.w #28,d4 move.b #88,d0 trap #15 rts * The three pipes. a0 walks the table, one pipe per call. drawpipes: move.l #PIPEDK,d1 move.b #80,d0 trap #15 move.l #PIPEC,d1 move.b #81,d0 trap #15 lea pipes,a0 bsr onepipe bsr onepipe bsr onepipe rts * A column down from the top and one up from the ground, each capped with a * wider lip, then a0 moves on to the pipe after it. onepipe: move.w (a0),d5 ; its left edge move.w 2(a0),d6 ; and the middle of its gap move.w d5,d1 moveq #0,d2 move.w d5,d3 add.w #PIPEW,d3 move.w d6,d4 sub.w #HALFGAP,d4 ; down to the top of the gap move.b #87,d0 trap #15 move.w d5,d1 sub.w #LIPOUT,d1 move.w d4,d2 sub.w #LIPH,d2 move.w d5,d3 add.w #PIPER,d3 move.b #87,d0 trap #15 ; the lip, still ending at the gap move.w d5,d1 move.w d6,d2 add.w #HALFGAP,d2 ; from the bottom of the gap move.w d5,d3 add.w #PIPEW,d3 move.w #GROUNDY,d4 move.b #87,d0 trap #15 move.w d5,d1 sub.w #LIPOUT,d1 move.w d6,d2 add.w #HALFGAP,d2 move.w d5,d3 add.w #PIPER,d3 move.w d2,d4 add.w #LIPH,d4 move.b #87,d0 trap #15 lea 6(a0),a0 rts * Sand, a band of grass on top of it, and tufts that scroll with the pipes. drawground: move.l #SAND,d1 move.b #80,d0 trap #15 move.l #SAND,d1 move.b #81,d0 trap #15 moveq #0,d1 move.w #GROUNDY,d2 move.w #WIDTH,d3 move.w #HEIGHT,d4 move.b #87,d0 trap #15 move.l #GRASS,d1 move.b #80,d0 trap #15 move.l #GRASS,d1 move.b #81,d0 trap #15 moveq #0,d1 move.w #GROUNDY,d2 move.w #WIDTH,d3 move.w #GRASSB,d4 move.b #87,d0 trap #15 move.l #GRASSDK,d1 move.b #80,d0 trap #15 move.l #GRASSDK,d1 move.b #81,d0 trap #15 * one tuft further left than the screen, so the row never runs out on that side move.w scroll,d5 sub.w #TUFTGAP,d5 moveq #11,d6 gtnext: move.w d5,d1 move.w #TUFTY,d2 move.w d5,d3 add.w #TUFTW,d3 move.w #TUFTB,d4 move.b #87,d0 trap #15 add.w #TUFTGAP,d5 dbra d6,gtnext rts * The bird: a body, a wing that beats through three positions, an eye and a * beak. Only the body's box is what `hittest` measures. drawbird: move.l birdy,d5 asr.l #4,d5 ; sixteenths of a pixel back down to pixels move.l #INK,d1 move.b #80,d0 trap #15 ; one dark outline for all of it move.l #BODY,d1 move.b #81,d0 trap #15 move.w #BIRDX,d1 move.w d5,d2 move.w #BIRDR,d3 move.w d5,d4 add.w #BIRDH,d4 move.b #88,d0 trap #15 moveq #0,d6 move.w wing,d6 move.w state,d0 cmp.w #DEAD,d0 beq wingset ; a dead bird stops beating addq.w #1,d6 cmp.w #12,d6 blt wingheld moveq #0,d6 wingheld: move.w d6,wing wingset: lsr.w #2,d6 ; twelve frames, three positions mulu #7,d6 add.w #WINGTOP,d6 move.l #WINGC,d1 move.b #81,d0 trap #15 move.w #WINGL,d1 move.w d5,d2 add.w d6,d2 move.w #WINGR,d3 move.w d2,d4 add.w #13,d4 move.b #88,d0 trap #15 move.l #PAPER,d1 move.b #81,d0 trap #15 move.w #EYEL,d1 move.w d5,d2 add.w #7,d2 move.w #EYER,d3 move.w d2,d4 add.w #14,d4 move.b #88,d0 trap #15 move.l #INK,d1 move.b #81,d0 trap #15 move.w #PUPL,d1 move.w d5,d2 add.w #11,d2 move.w #PUPR,d3 move.w d2,d4 add.w #7,d4 move.b #88,d0 trap #15 move.l #BEAKC,d1 move.b #81,d0 trap #15 move.w #BEAKL,d1 move.w d5,d2 add.w #19,d2 move.w #BEAKR,d3 move.w d2,d4 add.w #9,d4 move.b #87,d0 trap #15 rts * The score, and whatever state the game is in has to say. drawhud: move.w state,d0 cmp.w #READY,d0 beq hudready move.l #INK,d1 move.b #81,d0 trap #15 move.l #PAPER,d1 move.b #80,d0 trap #15 move.w #PLATEL,d1 move.w #14,d2 move.w #PLATER,d3 move.w #46,d4 move.b #87,d0 trap #15 lea line,a1 moveq #0,d1 move.w points,d1 bsr appnum lea line,a1 move.w #22,d6 bsr ctext move.w state,d0 cmp.w #DEAD,d0 beq huddead rts hudready: move.w #250,d1 move.w #392,d2 bsr plate lea title,a1 move.w #266,d6 bsr ctext lea hint1,a1 move.w #302,d6 bsr ctext lea hint2,a1 move.w #326,d6 bsr ctext lea hint3,a1 move.w #358,d6 bsr ctext rts huddead: move.w #150,d1 move.w #330,d2 bsr plate lea overmsg,a1 move.w #172,d6 bsr ctext lea line,a1 lea pointsmsg,a0 bsr append moveq #0,d1 move.w points,d1 bsr appnum lea line,a1 move.w #210,d6 bsr ctext lea line,a1 lea recordmsg,a0 bsr append moveq #0,d1 move.w record,d1 bsr appnum lea line,a1 move.w #238,d6 bsr ctext lea againmsg,a1 move.w #286,d6 bsr ctext rts * A plate across the middle of the screen, from d1.w down to d2.w. plate: move.w d1,d5 move.w d2,d6 move.l #INK,d1 move.b #81,d0 trap #15 move.l #PAPER,d1 move.b #80,d0 trap #15 move.w #PANELL,d1 move.w d5,d2 move.w #PANELR,d3 move.w d6,d4 move.b #87,d0 trap #15 rts *----------------------------------------------------------------------------- * Text *----------------------------------------------------------------------------- * Draws the NULL terminated string at (a1) centred across the screen with its * top at d6.w. The screen's font is an 8 by 16 cell, so half a character is the * four pixels each one takes off the centre. ctext: movea.l a1,a0 moveq #0,d5 ctlen: move.b (a0)+,d0 beq ctdraw addq.w #1,d5 bra ctlen ctdraw: asl.w #2,d5 move.w #CENTREX,d1 sub.w d5,d1 move.w d6,d2 move.b #95,d0 trap #15 rts * Appends the NULL terminated string at (a0) to the one at (a1), leaving a1 on * the new terminator so the next call carries straight on from there. append: move.b (a0)+,d0 beq appdone move.b d0,(a1)+ bra append appdone: clr.b (a1) rts * Appends d1.l, a number up to 65535, as decimal at (a1). Digits come out least * significant first, so they are written backwards into a scratch buffer and * the whole thing is appended at the end. appnum: lea numend,a0 clr.b -(a0) annext: divu #10,d1 move.l d1,d2 swap d2 ; the remainder is this digit add.w #$30,d2 move.b d2,-(a0) andi.l #$FFFF,d1 ; and the quotient is what is left to do bne annext bra append *----------------------------------------------------------------------------- * State *----------------------------------------------------------------------------- state: dc.w 0 birdy: dc.l STARTY16 ; sixteenths of a pixel, so gravity is smooth birdv: dc.w 0 points: dc.w 0 record: dc.w 0 wing: dc.w 0 held: dc.w 0 ; was a flap key or button down last frame? scroll: dc.w 0 cloud1x: dc.w 70 cloud2x: dc.w 390 seed: dc.w $1D4B * one pipe is its left edge, the middle of its gap, and whether it is counted pipes: dc.w 0,0,0 dc.w 0,0,0 dc.w 0,0,0 title: dc.b 'FLAPPY ASM',0 hint1: dc.b 'TAP SPACE, W, ENTER OR THE UP ARROW',0 hint2: dc.b 'OR CLICK ON THE SCREEN ITSELF',0 hint3: dc.b 'CLICK THE SCREEN PANEL SO IT TAKES THE KEYS',0 overmsg: dc.b 'GAME OVER',0 pointsmsg: dc.b 'SCORE ',0 recordmsg: dc.b 'BEST ',0 againmsg: dc.b 'FLAP TO PLAY AGAIN',0 line: ds.b 48 numbuf: ds.b 8 numend: ds.b 1 *----------------------------------------------------------------------------- * Tunables *----------------------------------------------------------------------------- WIDTH equ 640 HEIGHT equ 480 CENTREX equ 320 GROUNDY equ 416 ; the top of the ground GRASSH equ 10 GRASSB equ 426 ; GROUNDY+GRASSH TUFTY equ 410 TUFTB equ 418 TUFTW equ 20 TUFTGAP equ 56 CLOUDW equ 140 NEGCLOUD equ -140 ; -CLOUDW, where a cloud comes back on the right READY equ 0 PLAYING equ 1 DEAD equ 2 BIRDX equ 150 ; the bird only ever moves up and down BIRDW equ 46 BIRDH equ 34 BIRDR equ 196 ; BIRDX+BIRDW HITIN equ 6 ; the pipe hit box is smaller by this much on each side HITL equ 156 ; BIRDX+HITIN HITR equ 190 ; BIRDX+BIRDW-HITIN STARTY equ 196 STARTY16 equ 3136 ; STARTY*16 RESTY equ 382 ; GROUNDY-BIRDH, where a dead bird lands RESTY16 equ 6112 ; RESTY*16 GRAV equ 5 ; sixteenths of a pixel, per frame, per frame FLAPV equ -80 ; the speed one flap gives, upwards MAXFALL equ 150 WINGTOP equ 5 WINGL equ 159 ; BIRDX+9 WINGR equ 183 ; BIRDX+33 EYEL equ 174 ; BIRDX+24 EYER equ 188 ; BIRDX+38 PUPL equ 180 ; BIRDX+30 PUPR equ 187 ; BIRDX+37 BEAKL equ 190 ; BIRDX+40 BEAKR equ 208 ; BIRDX+58 PIPEW equ 70 PIPER equ 76 ; PIPEW+LIPOUT NEGPIPE equ -70 ; -PIPEW, where a pipe is recycled LIPH equ 18 LIPOUT equ 6 GAPH equ 140 HALFGAP equ 70 SPEED equ 4 ; pixels a frame, for everything that scrolls SPACING equ 240 ; between one pipe and the next CYCLE equ 720 ; SPACING*3, what a recycled pipe jumps by FIRSTX equ 620 GAPMIN equ 120 ; the highest the middle of a gap goes GAPSPAN equ 180 ; the offset ranges from 0 through 179 DELAY equ 3 ; hundredths of a second a frame KEYLIST equ $2026570D ; space, up arrow, w, enter PLATEL equ 284 PLATER equ 356 PANELL equ 104 PANELR equ 536 * colors are $00BBGGRR longs: blue high, then green, then red SKY equ $00E8C04E CLOUDC equ $00F8FBFF PIPEC equ $0039BB58 PIPEDK equ $00347A29 SAND equ $0095D8DE GRASS equ $0058C474 GRASSDK equ $0040A04C BODY equ $003ECEFA WINGC equ $00D2F5FF BEAKC equ $002882F0 INK equ $00101820 PAPER equ $00FFFFFF ``` *Testcase for the program above (JSON):* ```json { "runFor": 20000 } ``` ## Follow one frame `state` tells `frame` which part of the game to run. In `READY`, the bird waits for the first flap. That flap seeds a new set of pipes, sets the upward speed and changes `state` to `PLAYING`. On later frames, `playing` applies gravity, moves the pipes and checks for a hit. A hit changes `state` to `DEAD`; the bird then falls to the ground and waits for another flap. Every path reaches `draw`, which shows the completed picture and loops back to `frame`. ```m68k frame: bsr readflap move.w d0,d7 move.w state,d0 cmp.w #PLAYING,d0 beq playing cmp.w #DEAD,d0 beq dead ``` `readflap` returns 1 in `d0` when a new flap begins. The loop copies that answer to `d7` before using `d0` to inspect `state`. The routines called later leave `d7` alone, so the state code can still test the flap after those calls. ## Turn a held input into one flap Task 19 reports which keys are down, and task 61 reports whether the mouse button is down. Both describe the current state. If a key stays down for several frames, `readflap` must return 1 only on the first of them. The word `held` remembers whether any flap input was down on the previous read: ```m68k clr.w held moveq #0,d0 rts pressed: tst.w held bne stillheld move.w #1,held moveq #1,d0 rts ``` When all inputs are released, `held` becomes 0. On the next pressed read, the code sets `held` to 1 and returns a flap. Further reads while it stays pressed return 0. This is why you release and tap again for each lift. ## Move in sixteenths of a pixel `birdy` stores the vertical position in sixteenths of a pixel, and `birdv` stores the amount added to it each frame. Smaller screen Y values are higher up. A flap sets `birdv` to `FLAPV = -80`, an upward speed of 5 pixels per frame. During each `PLAYING` frame, gravity adds `GRAV = 5` to that speed _before_ the speed is added to `birdy`. The starting position is `STARTY16 = 196 × 16 = 3136`. The first flap in `READY` sets the speed but leaves the position at 3136. If no further flap occurs, the next three frames are: | `PLAYING` frame | `birdv` after gravity | `birdy` after movement | Drawn Y (`birdy >> 4`) | | --------------- | --------------------: | ---------------------: | ---------------------: | | 1 | -75 | 3061 | 191 | | 2 | -70 | 2991 | 186 | | 3 | -65 | 2926 | 182 | The fraction stays in `birdy` even though `drawbird` uses `asr.l #4` to get a whole pixel. That lets the speed change by 5 sixteenths each frame. Once gravity makes `birdv` positive, the bird descends; `MAXFALL` limits that downward speed. ## Reuse three pipe records `pipes` contains three records, each six bytes long. `a0` points to one record at a time: | Offset from `a0` | Word | Meaning | | ---------------- | ---------- | ---------------------------------------------------------- | | `0` | X | Left edge of the pipe. | | `2` | Gap centre | Vertical centre of its opening. | | `4` | Counted | 0 until its right edge passes the bird and scores a point. | `newpipes` starts their X positions at 620, 860 and 1100: `SPACING = 240` pixels apart. Each `PLAYING` frame, `movepipes` subtracts `SPEED = 4` from every X. Once a pipe has gone one width past the left edge, it jumps right by `CYCLE = 720`, which is three spacings. Its gap is chosen again and its counted word is cleared. It therefore joins the end of the same three-pipe course. `randgap` changes a word called `seed` with multiplication and addition, then uses its high byte to choose the next gap centre. The offset is 0 through 179, so `GAPMIN = 120` puts the centre at **120 through 299**. The first flap gets a seed from task 8's elapsed time, making the course depend on when you start. `GAPH = 140` makes each opening 140 pixels high, with `HALFGAP = 70` above and below its centre. If you change the opening height, change its half-height too. ## Check hits and show the result `hittest` checks the ground first. It uses the bird's full height: when `birdy / 16 + BIRDH` reaches `GROUNDY`, the bird hits the ground. For a pipe, it uses a smaller rectangle. `HITL` and `HITR` inset the bird's horizontal span by `HITIN = 6` pixels, and the tested top and bottom are also inset by 6. A pipe can cause a hit only while its X span overlaps this rectangle and the rectangle extends above or below the gap. Task 92 mode 17 sets up a hidden drawing image. `draw` paints sky, pipes, ground, bird and text in that order, then task 94 shows the completed image. The drawing helpers supply the clouds, grass, wing and score; their details can wait until you want to change the artwork. Task 23 then waits at least `DELAY = 3` hundredths of a second before the next frame. Drawing and instructions take time too, so this does not set an exact frame rate on every machine. Try one small change: set `DELAY` to 6. Before running it, predict how the game will feel. Will the bird and pipes move a different number of pixels _per frame_?
Show what to expect The minimum pause doubles, so the game will usually look slower in real time. `GRAV`, `birdv` and `SPEED` have not changed, so their movement per frame stays the same. The exact speed change in real time depends on the drawing and instruction time as well as the pause.
## Instructions in this Lecture - M68K: [`move`](https://asm-editor.specy.app/documentation/m68k/instruction/move.md), [`trap`](https://asm-editor.specy.app/documentation/m68k/instruction/trap.md), [`bsr`](https://asm-editor.specy.app/documentation/m68k/instruction/bsr.md), [`cmp`](https://asm-editor.specy.app/documentation/m68k/instruction/cmp.md), [`beq`](https://asm-editor.specy.app/documentation/m68k/instruction/beq.md), [`tst`](https://asm-editor.specy.app/documentation/m68k/instruction/tst.md), [`bra`](https://asm-editor.specy.app/documentation/m68k/instruction/bra.md), [`clr`](https://asm-editor.specy.app/documentation/m68k/instruction/clr.md), [`add`](https://asm-editor.specy.app/documentation/m68k/instruction/add.md), [`ble`](https://asm-editor.specy.app/documentation/m68k/instruction/ble.md), [`ext`](https://asm-editor.specy.app/documentation/m68k/instruction/ext.md), [`bge`](https://asm-editor.specy.app/documentation/m68k/instruction/bge.md), [`asr`](https://asm-editor.specy.app/documentation/m68k/instruction/asr.md), [`blt`](https://asm-editor.specy.app/documentation/m68k/instruction/blt.md), [`btst`](https://asm-editor.specy.app/documentation/m68k/instruction/btst.md), [`bne`](https://asm-editor.specy.app/documentation/m68k/instruction/bne.md), [`moveq`](https://asm-editor.specy.app/documentation/m68k/instruction/moveq.md), [`rts`](https://asm-editor.specy.app/documentation/m68k/instruction/rts.md), [`lea`](https://asm-editor.specy.app/documentation/m68k/instruction/lea.md), [`dbra`](https://asm-editor.specy.app/documentation/m68k/instruction/dbra.md), [`mulu`](https://asm-editor.specy.app/documentation/m68k/instruction/mulu.md), [`lsr`](https://asm-editor.specy.app/documentation/m68k/instruction/lsr.md), [`sub`](https://asm-editor.specy.app/documentation/m68k/instruction/sub.md), [`bgt`](https://asm-editor.specy.app/documentation/m68k/instruction/bgt.md), [`addq`](https://asm-editor.specy.app/documentation/m68k/instruction/addq.md), [`subq`](https://asm-editor.specy.app/documentation/m68k/instruction/subq.md), [`movea`](https://asm-editor.specy.app/documentation/m68k/instruction/movea.md), [`asl`](https://asm-editor.specy.app/documentation/m68k/instruction/asl.md), [`divu`](https://asm-editor.specy.app/documentation/m68k/instruction/divu.md), [`swap`](https://asm-editor.specy.app/documentation/m68k/instruction/swap.md), [`andi`](https://asm-editor.specy.app/documentation/m68k/instruction/andi.md) Previous: [The snake game](https://asm-editor.specy.app/learn/courses/m68k/examples/snake.md) Next: [Play a WAV and a MIDI file](https://asm-editor.specy.app/learn/courses/m68k/examples/play-sound-files.md)