mirror of
https://github.com/sehugg/8bitworkshop.git
synced 2024-11-24 12:31:25 +00:00
142 lines
2.7 KiB
Plaintext
142 lines
2.7 KiB
Plaintext
|
|
processor 6502
|
|
include "vcs.h"
|
|
include "macro.h"
|
|
include "xmacro.h"
|
|
|
|
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
|
;
|
|
; This example draws a moving line using 16-bit fixed-point
|
|
; math and a missile object.
|
|
;
|
|
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
|
|
|
seg.u Variables
|
|
org $80
|
|
|
|
Temp .byte
|
|
X1 .byte ; start X coordinate of line
|
|
Y1 .byte ; start Y coordinate of line
|
|
Y2 .byte ; end Y coordinate of line
|
|
Slope .word ; 16-bit slope
|
|
XFrac .byte ; X fractional
|
|
|
|
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
|
|
|
seg Code
|
|
org $f000
|
|
|
|
Start
|
|
CLEAN_START
|
|
|
|
lda #80
|
|
sta X1
|
|
lda #80
|
|
sta Y1
|
|
lda #120
|
|
sta Y2
|
|
lda #0
|
|
sta Slope
|
|
lda #0
|
|
sta Slope+1
|
|
|
|
NextFrame
|
|
VERTICAL_SYNC
|
|
|
|
TIMER_SETUP 37
|
|
; Set missile 0 X start position
|
|
lda X1 ; starting X
|
|
ldx #2 ; missile 0
|
|
jsr SetHorizPos
|
|
sta WSYNC
|
|
sta HMOVE
|
|
TIMER_WAIT
|
|
|
|
lda #$54
|
|
sta COLUP0
|
|
ldy #0
|
|
sty XFrac
|
|
ScanLoop
|
|
cpy Y1
|
|
bcc NoLine ; out of bounds (< Y1)?
|
|
cpy Y2
|
|
bcs NoLine ; out of bounds (> Y2)?
|
|
; Add slope to X fractional error
|
|
lda XFrac
|
|
clc
|
|
adc Slope ; this sets carry flag
|
|
sta XFrac
|
|
lda Slope+1
|
|
adc #7 ; 7 + carry flag
|
|
tax ; -> X
|
|
lda DotWidths,x ; lookup register for missile width
|
|
sta Temp ; -> Temp
|
|
lda HMoveTable,x ; lookup HMOVE register for X offset
|
|
sta WSYNC
|
|
sta HMOVE ; apply moves on previous scanline
|
|
ldx #2
|
|
stx ENAM0 ; enable missile
|
|
ldx Temp
|
|
stx NUSIZ0 ; set missile width
|
|
sta HMM0 ; set HMM0 for next scanline
|
|
NextScan
|
|
iny
|
|
cpy #192
|
|
bcc ScanLoop ; any more scanlines?
|
|
beq DoneLine ; branch always taken
|
|
NoLine
|
|
sta WSYNC
|
|
lda #0
|
|
sta ENAM0 ; hide missile
|
|
jmp NextScan
|
|
DoneLine
|
|
|
|
TIMER_SETUP 29
|
|
|
|
; Change slope of line
|
|
lda Slope
|
|
clc
|
|
adc #3
|
|
sta Slope
|
|
lda Slope+1
|
|
adc #0
|
|
cmp #2
|
|
bmi NoLineReset
|
|
lda #$fe
|
|
NoLineReset
|
|
sta Slope+1
|
|
|
|
TIMER_WAIT
|
|
|
|
jmp NextFrame
|
|
|
|
SetHorizPos subroutine
|
|
sta WSYNC ; start a new line
|
|
bit 0 ; waste 3 cycles
|
|
sec ; set carry flag
|
|
DivideLoop
|
|
sbc #15 ; subtract 15
|
|
bcs DivideLoop ; branch until negative
|
|
eor #7 ; calculate fine offset
|
|
asl
|
|
asl
|
|
asl
|
|
asl
|
|
sta RESP0,x ; fix coarse position
|
|
sta HMP0,x ; set fine offset
|
|
rts ; return to caller
|
|
|
|
; HMOVE table from -7 to +8
|
|
HMoveTable
|
|
hex 7060504030201000f0e0d0c0b0a09080
|
|
|
|
; Table for NUSIZ registers
|
|
DotWidths
|
|
hex 40403030201000000010203030404040
|
|
|
|
; Epilogue
|
|
org $fffc
|
|
.word Start
|
|
.word Start
|
|
|