Creature Walks
The creature moves right each frame. Variables in RAM track position. The main loop updates sprite control words per frame.
The creature appeared in Unit 4 but stayed frozen in place. Now it walks. Every frame, the x position increases by one pixel. The sprite slides across the terrain from left to right.
This is the first time the main loop does real work. Until now, the loop just waited for vertical blank and checked the mouse button. Now it also updates the creature’s position and writes new values into the sprite control words.
The creature walks right across the landscape. It doesn’t know about the terrain yet — it passes over cliffs and gaps as if they weren’t there. Collision comes later.
Variables in RAM
The creature’s position needs to persist between frames. Constants won’t work — the position changes every frame. Instead, two words in memory hold the current x and y:
;══════════════════════════════════════════════════════════════
; VARIABLES
;══════════════════════════════════════════════════════════════
even
creature_x: dc.w 0
creature_y: dc.w 0
dc.w 0 defines a word (16-bit) in memory, initialised to zero. The labels creature_x and creature_y give these memory locations names. The even directive ensures word alignment — the 68000 requires word and longword accesses to be on even addresses.
At startup, the program writes the starting position into these variables:
move.w #CREATURE_START_X,creature_x
move.w #CREATURE_START_Y,creature_yThe Main Loop
; === Main Loop ===
mainloop:
; Wait for vertical blank
move.l #$1ff00,d1
.vbwait:
move.l VPOSR(a5),d0
and.l d1,d0
bne.s .vbwait
; --- Update creature position ---
move.w creature_x,d0
add.w #CREATURE_SPEED,d0
move.w d0,creature_x
; --- Update sprite control words ---
bsr update_sprite
; --- Check exit ---
btst #6,$bfe001
bne.s mainloop
Three things happen every frame:
-
Wait for vertical blank — the same
VPOSRpolling loop as before. This ensures we update once per frame, not as fast as the CPU can run. -
Update position — read
creature_xfrom memory into D0, add the speed, write it back.ADD.W #CREATURE_SPEED,D0adds one pixel per frame. At 50 frames per second (PAL), the creature crosses the 320-pixel screen in about 6 seconds. -
Update sprite —
BSR update_spriterewrites the sprite control words using the new position. The DMA controller reads these words next frame and draws the sprite at the updated position.
The update_sprite subroutine is almost identical to set_sprite_pos from Unit 4, but now it reads from the variables instead of using constants.
Why Variables, Not Registers?
The 68000 has eight data registers (D0-D7) — wouldn’t it be faster to keep the position in a register? It would, but registers get destroyed by subroutine calls. draw_rect destroys D0-D5, and update_sprite uses several registers too. Storing the position in memory means it survives any subroutine call.
Later, when there are multiple creatures, each creature’s position will be a pair of words in a data table. RAM variables are the natural pattern.
Experiment: Change the Speed
CREATURE_SPEED equ 1 ; Try 1, 2, 41— slow walk (6 seconds to cross the screen)2— brisk walk (3 seconds)4— running (1.5 seconds)
The creature still walks off the right edge of the screen. It doesn’t stop or turn around — there’s no boundary check yet. That’s Unit 6.
The Complete Code
;──────────────────────────────────────────────────────────────
; EXODUS - A terrain puzzle for the Commodore Amiga
; Unit 5: Creature Walks
;
; The creature moves right each frame.
; Variables in RAM track position.
; The main loop updates sprite control words per frame.
;──────────────────────────────────────────────────────────────
;══════════════════════════════════════════════════════════════
; TWEAKABLE VALUES
;══════════════════════════════════════════════════════════════
COLOUR_SKY_DEEP equ $0016
COLOUR_SKY_UPPER equ $0038
COLOUR_SKY_MID equ $005B
COLOUR_SKY_LOWER equ $007D
COLOUR_SKY_HORIZON equ $009E
COLOUR_TERRAIN equ $0741
COLOUR_SPR0_1 equ $0FFF
COLOUR_SPR0_2 equ $0F80
COLOUR_SPR0_3 equ $0000
; Creature starting position (bitplane coordinates)
CREATURE_START_X equ 20
CREATURE_START_Y equ 110
CREATURE_SPEED equ 1 ; Pixels per frame
; Terrain
GROUND_L_X equ 0
GROUND_L_Y equ 152
GROUND_L_W equ 128
GROUND_L_H equ 104
GROUND_R_X equ 128
GROUND_R_Y equ 120
GROUND_R_W equ 192
GROUND_R_H equ 136
PLATFORM_X equ 24
PLATFORM_Y equ 104
PLATFORM_W equ 72
PLATFORM_H equ 8
;══════════════════════════════════════════════════════════════
; DISPLAY CONSTANTS
;══════════════════════════════════════════════════════════════
SCREEN_WIDTH equ 320
SCREEN_HEIGHT equ 256
BYTES_PER_ROW equ SCREEN_WIDTH/8
BITPLANE_SIZE equ BYTES_PER_ROW*SCREEN_HEIGHT
SPRITE_HEIGHT equ 12
;══════════════════════════════════════════════════════════════
; HARDWARE REGISTERS
;══════════════════════════════════════════════════════════════
CUSTOM equ $dff000
DMACON equ $096
INTENA equ $09a
INTREQ equ $09c
COP1LC equ $080
COPJMP1 equ $088
BPLCON0 equ $100
BPLCON1 equ $102
BPLCON2 equ $104
BPL1MOD equ $108
DIWSTRT equ $08e
DIWSTOP equ $090
DDFSTRT equ $092
DDFSTOP equ $094
BPL1PTH equ $0e0
BPL1PTL equ $0e2
SPR0PTH equ $120
SPR0PTL equ $122
COLOR00 equ $180
COLOR01 equ $182
COLOR17 equ $1a2
COLOR18 equ $1a4
COLOR19 equ $1a6
VPOSR equ $004
;══════════════════════════════════════════════════════════════
; CODE (Chip RAM)
;══════════════════════════════════════════════════════════════
section code,code_c
start:
lea CUSTOM,a5
; --- Take over the machine ---
move.w #$7fff,INTENA(a5)
move.w #$7fff,INTREQ(a5)
move.w #$7fff,DMACON(a5)
; --- Initialise creature position ---
move.w #CREATURE_START_X,creature_x
move.w #CREATURE_START_Y,creature_y
; --- Draw terrain into bitplane ---
move.w #GROUND_L_X,d0
move.w #GROUND_L_Y,d1
move.w #GROUND_L_W,d2
move.w #GROUND_L_H,d3
bsr draw_rect
move.w #GROUND_R_X,d0
move.w #GROUND_R_Y,d1
move.w #GROUND_R_W,d2
move.w #GROUND_R_H,d3
bsr draw_rect
move.w #PLATFORM_X,d0
move.w #PLATFORM_Y,d1
move.w #PLATFORM_W,d2
move.w #PLATFORM_H,d3
bsr draw_rect
; --- Patch bitplane pointer ---
lea bitplane,a0
move.l a0,d0
swap d0
lea bpl1pth_val,a1
move.w d0,(a1)
swap d0
lea bpl1ptl_val,a1
move.w d0,(a1)
; --- Patch sprite pointer ---
lea sprite_data,a0
move.l a0,d0
swap d0
lea spr0pth_val,a1
move.w d0,(a1)
swap d0
lea spr0ptl_val,a1
move.w d0,(a1)
; --- Install Copper list ---
lea copperlist,a0
move.l a0,COP1LC(a5)
move.w d0,COPJMP1(a5)
; --- Enable DMA ---
move.w #$83a0,DMACON(a5)
; === Main Loop ===
mainloop:
; Wait for vertical blank
move.l #$1ff00,d1
.vbwait:
move.l VPOSR(a5),d0
and.l d1,d0
bne.s .vbwait
; --- Update creature position ---
move.w creature_x,d0
add.w #CREATURE_SPEED,d0
move.w d0,creature_x
; --- Update sprite control words ---
bsr update_sprite
; --- Check exit ---
btst #6,$bfe001
bne.s mainloop
.halt:
bra.s .halt
;──────────────────────────────────────────────────────────────
; update_sprite — Write current position into sprite data
;
; Reads creature_x and creature_y variables.
;──────────────────────────────────────────────────────────────
update_sprite:
lea sprite_data,a0
move.w creature_y,d0
add.w #$2c,d0 ; VSTART (display offset)
move.w d0,d1
add.w #SPRITE_HEIGHT,d1 ; VSTOP
move.w creature_x,d2
add.w #$80,d2 ; HSTART (display offset)
; Word 0: VSTART[7:0] | HSTART[8:1]
move.b d0,d3
lsl.w #8,d3
move.w d2,d4
lsr.w #1,d4
or.b d4,d3
move.w d3,(a0)+
; Word 1: VSTOP[7:0] | control bits
move.b d1,d3
lsl.w #8,d3
moveq #0,d4
btst #8,d0
beq.s .no_vs8
bset #2,d4
.no_vs8:
btst #8,d1
beq.s .no_ve8
bset #1,d4
.no_ve8:
btst #0,d2
beq.s .no_h0
bset #0,d4
.no_h0:
or.b d4,d3
move.w d3,(a0)
rts
;──────────────────────────────────────────────────────────────
; draw_rect — Fill a byte-aligned rectangle in the bitplane
;──────────────────────────────────────────────────────────────
draw_rect:
lea bitplane,a0
mulu #BYTES_PER_ROW,d1
add.l d1,a0
lsr.w #3,d0
add.w d0,a0
lsr.w #3,d2
subq.w #1,d3
.row:
move.w d2,d5
subq.w #1,d5
move.l a0,a1
.col:
move.b #$ff,(a1)+
dbra d5,.col
add.w #BYTES_PER_ROW,a0
dbra d3,.row
rts
;══════════════════════════════════════════════════════════════
; COPPER LIST
;══════════════════════════════════════════════════════════════
copperlist:
dc.w DIWSTRT,$2c81
dc.w DIWSTOP,$2cc1
dc.w DDFSTRT,$0038
dc.w DDFSTOP,$00d0
dc.w BPLCON0,$1200
dc.w BPLCON1,$0000
dc.w BPLCON2,$0000
dc.w BPL1MOD,$0000
dc.w BPL1PTH
bpl1pth_val:
dc.w $0000
dc.w BPL1PTL
bpl1ptl_val:
dc.w $0000
dc.w SPR0PTH
spr0pth_val:
dc.w $0000
dc.w SPR0PTL
spr0ptl_val:
dc.w $0000
dc.w COLOR00,COLOUR_SKY_DEEP
dc.w COLOR01,COLOUR_TERRAIN
dc.w COLOR17,COLOUR_SPR0_1
dc.w COLOR18,COLOUR_SPR0_2
dc.w COLOR19,COLOUR_SPR0_3
dc.w $3401,$fffe
dc.w COLOR00,COLOUR_SKY_UPPER
dc.w $4401,$fffe
dc.w COLOR00,COLOUR_SKY_MID
dc.w $5401,$fffe
dc.w COLOR00,COLOUR_SKY_LOWER
dc.w $6001,$fffe
dc.w COLOR00,COLOUR_SKY_HORIZON
dc.w $6801,$fffe
dc.w COLOR00,$0000
dc.w $ffff,$fffe
;══════════════════════════════════════════════════════════════
; SPRITE DATA (Chip RAM)
;══════════════════════════════════════════════════════════════
even
sprite_data:
dc.w $0000,$0000 ; Control words (updated per frame)
dc.w %0000011111100000,%0000000000000000
dc.w %0000111111110000,%0000011111100000
dc.w %0001111111111000,%0000111111110000
dc.w %0001101110111000,%0001111111111000
dc.w %0001111111111000,%0000111111110000
dc.w %0000111001110000,%0000011111100000
dc.w %0000011111100000,%0000000000000000
dc.w %0000111111110000,%0000011111100000
dc.w %0001111111111000,%0000111111110000
dc.w %0001111111111000,%0000111111110000
dc.w %0000110000110000,%0000000000000000
dc.w %0000110000110000,%0000000000000000
dc.w $0000,$0000 ; End marker
;══════════════════════════════════════════════════════════════
; VARIABLES
;══════════════════════════════════════════════════════════════
even
creature_x: dc.w 0
creature_y: dc.w 0
;══════════════════════════════════════════════════════════════
; BITPLANE DATA
;══════════════════════════════════════════════════════════════
even
bitplane:
dcb.b BITPLANE_SIZE,0
If It Doesn’t Work
- Creature doesn’t move? Check that the main loop calls
update_spriteafter modifyingcreature_x. If the sprite control words aren’t rewritten, the DMA reads the old position. - Creature moves too fast? The vertical blank wait might not be working. Without it, the loop runs thousands of times per frame, moving the creature off-screen instantly.
- Creature jumps or flickers? Make sure
update_spriteruns after the vertical blank wait, not during active display. Writing sprite data while Denise is reading it causes visual glitches. - Position resets to zero? Check that
creature_xandcreature_yare defined withdc.w, notequ. Constants (equ) are assembler-time values;dc.wallocates actual RAM.
What You’ve Learnt
- RAM variables —
dc.wdefines a word in memory. The label provides a name for reading and writing the value at runtime. - Frame-based movement — adding a fixed amount to the position each frame creates smooth motion. The vertical blank wait ensures exactly one update per frame.
- Main loop structure — wait for blank, update state, update display. This pattern drives every real-time game.
- Register vs memory — registers are fast but temporary. Memory variables persist across subroutine calls and frames.
What’s Next
The creature walks off the right edge and never comes back. In Unit 6, a boundary check detects the screen edge and reverses the direction. The creature bounces back and forth across the terrain.