**************************************** * * Bouncing Balls in Hires * 6502 Assembly for the Apple II * * Stephen A. Edwards * **************************************** org $8000 ; Numeric constants NBALLS equ 30 ; Number of balls to bounce COLUMNS equ 40 ; Number of columns/bytes per row WHITE equ $7F ; White hires byte RKEY equ $D2 ; Code for when "R" is pressed RWALL equ $f700 LWALL equ $0400 BOTTOM equ $B700 BOTTOM2 equ $6E00 ; Bottom * 2, for bouncing ; Addresses BALL equ $6 ; Current ball number GBASL equ $26 ; Graphics base address GBASH equ $27 HCOLOR1 equ $1c ; Color value HGRX equ $e0 ; two-byte value HGRY equ $e2 HGR1SCRN equ $2000 ; Start of hires page 1 KBD equ $c000 ; key code when MSB set KBDSTRB equ $c010 ; clear keyboard buffer TXTCLR equ $c050 ; graphics mode TXTSET equ $c051 ; text mode LOWSCR equ $c054 ; page 1 HIRES equ $c057 ; hires mode balls jsr hclear ; Draw the boundary lda #WHITE ldy #0 jsr hline ; White line on the top row ldy #191 jsr hline ; White line on the bottom row lda #%00000011 ldy #0 jsr vline ; White line on the left column lda #%01100000 ldy #COLUMNS-1 jsr vline ; White line on the right column ; Initialize the ball positions and speeds ; Zero the first four tables ldx #NBALLS * 4 - 1 lda #0 init0 sta BALLXL,x dex bpl init0 ; Initialize non-zero entries ldx #NBALLS-1 ldy #1 initl tya ; Linear feedback shift register lsr bcc skip eor #$c0 skip tay and #$1F adc #1 sta BALLXH,x sta BALLDX,x tya lsr adc #10 sta BALLYH,x dex bpl initl ; Draw each ball drawall lda #NBALLS-1 sta BALL da1 jsr xorball dec BALL bpl da1 update lda #NBALLS-1 sta BALL uploop jsr xorball ; erase the ball in its current position ldx BALL ; Update horizontal position lda BALLXL,x clc adc BALLDX,x bpl noxunderflow adc #56 ; Correct for underflow; carry is clear sta BALLXL,x dec BALLXH,x bne xdone ; Hit the left wall? beq bouncex ; Yes: bounce noxunderflow sta BALLXL,x sec sbc #56 bcc xdone ; No overflow? sta BALLXL,x inc BALLXH,x ldy BALLXH,x cpy #COLUMNS-2 ; Hit the right wall? bne xdone bouncex sec lda #0 sbc BALLDX,x sta BALLDX,x xdone ; Update the ball's vertical position ; Apply gravity inc BALLDYH,x ; Update position clc lda BALLYL,x adc BALLDYL,x sta BALLYL,x lda BALLYH,x adc BALLDYH,x sta BALLYH,x cmp #>BOTTOM bcc nobounce ; BALLY = 2 * bottom - BALLY sec lda #BOTTOM2 sbc BALLYH,x sta BALLYH,x sec lda #0 sbc BALLDYL,x sta BALLDYL,x lda #0 sbc BALLDYH,x sta BALLDYH,x nobounce jsr xorball ; draw the ball in its new position dec BALL bpl uploop bit KBD ; Key pressed? bmi pressed jmp update ; No, update everything pressed lda KBD cmp #RKEY ; Was R pressed? bne quit bit KBDSTRB ; Clear the key jmp balls ; Restart quit bit KBDSTRB ; Clear the key bit TXTSET ; Switch to text mode rts ; Draw a horizontal line ; A = color byte to repeat, e.g., $7F ; Y = row (0-191) ($FF on exit) ; ; Uses GBASL, GBASH hline pha lda LKLO,y sta GBASL lda LKHI,y sta GBASH ldy #COLUMNS-1 ; Width of screen in bytes pla hl1 sta (GBASL),y dey bpl hl1 rts ; Draw a vertical line on all but the topmost and bottommost rows ; A = byte to write in each position ; Y = column ; ; Uses GBASL, GBASH, HCOLOR1 vline sta HCOLOR1 ldx #190 ; Start at second-to-last row vl1 lda LKLO,x ; Get the row address sta GBASL lda LKHI,x sta GBASH lda HCOLOR1 sta (GBASL),y ; Write the color byte dex ; Previous row bne vl1 rts ; Clear and switch to hires page 1 ; Uses GBASL and GBASH hclear ldx #>HGR1SCRN ; $20, also the number of pages to clear stx GBASH lda #0 ; Clear to black sta GBASL tay hclr1 sta (GBASL),y iny bne hclr1 ; Done with the page? inc GBASH dex bne hclr1 ; Done with all pages? bit HIRES ; Switch to hires mode bit TXTCLR rts ; Draw or erase a ball ; BALL = ball number ; BALLYH+BALL = top row ; BALLXH+BALL = left column (byte offset) ; BALLXL+BALL = bits 4-6 select one of the 7 shifted versions ; ; Uses GBASL, GBASH, HGRY, HGRX xorball ldy BALL lda BALLYH,y ; Get row sta HGRY lda BALLXH,y ; Get column sta HGRX lda BALLXL,y ; Get Shift (0,8,...,48) and #$38 tax ; Offset into sprite table (pixel * 8) xsplot ldy HGRY ; Get the row address lda LKLO,y sta GBASL lda LKHI,y sta GBASH iny sty HGRY ldy HGRX ; XOR the two bytes onto the screen ; (GBASL, GBASH) = row address ; Y = byte offset into the row ; X = index into sprite tables lda (GBASL),y eor BALL0,x sta (GBASL),y iny lda (GBASL),y eor BALL1,x sta (GBASL),y inx txa and #7 bne xsplot ; Stop at a multiple of 8 bytes rts ; Hires row address lookup tables, after Pelczarski, Graphically Speaking LKHI hex 2024282c3034383c2024282c3034383c hex 2125292d3135393d2125292d3135393d hex 22262a2e32363a3e22262a2e32363a3e hex 23272b2f33373b3f23272b2f33373b3f hex 2024282c3034383c2024282c3034383c hex 2125292d3135393d2125292d3135393d hex 22262a2e32363a3e22262a2e32363a3e hex 23272b2f33373b3f23272b2f33373b3f hex 2024282c3034383c2024282c3034383c hex 2125292d3135393d2125292d3135393d hex 22262a2e32363a3e22262a2e32363a3e hex 23272b2f33373b3f23272b2f33373b3f LKLO hex 00000000000000008080808080808080 hex 00000000000000008080808080808080 hex 00000000000000008080808080808080 hex 00000000000000008080808080808080 hex 2828282828282828a8a8a8a8a8a8a8a8 hex 2828282828282828a8a8a8a8a8a8a8a8 hex 2828282828282828a8a8a8a8a8a8a8a8 hex 2828282828282828a8a8a8a8a8a8a8a8 hex 5050505050505050d0d0d0d0d0d0d0d0 hex 5050505050505050d0d0d0d0d0d0d0d0 hex 5050505050505050d0d0d0d0d0d0d0d0 hex 5050505050505050d0d0d0d0d0d0d0d0 ; Eight bytes per sprite, first bytes BALL0 db %00111100 db %01111111 db %01111111 db %01111111 db %01111111 db %01111111 db %01111111 db %00111100 db %01111000 db %01111110 db %01111110 db %01111110 db %01111110 db %01111110 db %01111110 db %01111000 db %01110000 db %01111100 db %01111100 db %01111100 db %01111100 db %01111100 db %01111100 db %01110000 db %01100000 db %01111000 db %01111000 db %01111000 db %01111000 db %01111000 db %01111000 db %01100000 db %01000000 db %01110000 db %01110000 db %01110000 db %01110000 db %01110000 db %01110000 db %01000000 db %00000000 db %01100000 db %01100000 db %01100000 db %01100000 db %01100000 db %01100000 db %00000000 db %00000000 db %01000000 db %01000000 db %01000000 db %01000000 db %01000000 db %01000000 db %00000000 ; Second byte BALL1 db %00000000 db %00000001 db %00000001 db %00000001 db %00000001 db %00000001 db %00000001 db %00000000 db %00000000 db %00000011 db %00000011 db %00000011 db %00000011 db %00000011 db %00000011 db %00000000 db %00000001 db %00000111 db %00000111 db %00000111 db %00000111 db %00000111 db %00000111 db %00000001 db %00000011 db %00001111 db %00001111 db %00001111 db %00001111 db %00001111 db %00001111 db %00000011 db %00000111 db %00011111 db %00011111 db %00011111 db %00011111 db %00011111 db %00011111 db %00000111 db %00001111 db %00111111 db %00111111 db %00111111 db %00111111 db %00111111 db %00111111 db %00001111 db %00011110 db %01111111 db %01111111 db %01111111 db %01111111 db %01111111 db %01111111 db %00011110 ; Position and velocity information for each ball BALLXL ds NBALLS ; 0,8,...,48 = horizontal shift BALLYL ds NBALLS BALLDYL ds NBALLS BALLDYH ds NBALLS BALLDX ds NBALLS ; 8 = one pixel horizontal BALLXH ds NBALLS ; column/byte BALLYH ds NBALLS ; row