Flappy bird
The same program in MIPS, RISC-V, Z80.
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.
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 68K',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 $00FFFFFFFollow 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.
frame:
bsr readflap
move.w d0,d7
move.w state,d0
cmp.w #PLAYING,d0
beq playing
cmp.w #DEAD,d0
beq deadreadflap 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:
clr.w held
moveq #0,d0
rts
pressed:
tst.w held
bne stillheld
move.w #1,held
moveq #1,d0
rtsWhen 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.