dos33fsprogs/demos/seasons/autumn.s
Vince Weaver b1238af49d re-arranged the entire directory structure
this will probably upset people
2021-01-05 15:29:31 -05:00

158 lines
3.0 KiB
ArmAsm

; Autumn, based on the code in Hellmood's Autumn
; by deater (Vince Weaver) <vince@deater.net>
; Zero Page Addresses
XCOORDL = $F6
XCOORDH = $F7
YCOORDL = $F8
YCOORDH = $F9 ; shapetable ROT
EBP1 = $FA
EBP2 = $FB
EBP3 = $FC
EBP4 = $FD
COLORL = $FE
COLORH = $FF
; Soft Switches
KEYPRESS= $C000
KEYRESET= $C010
; ROM routines
; Some of these are in the Applesoft ROMs so need at least an Apple II+
; or later to run
TEXT = $FB36 ; Set text mode
HGR2 = $F3D8 ; Set full-screen hi-res mode using page 2 ($4000)
; 280x192 6-colors
HPLOT0 = $F457 ; Plot point, (Y,X) = Horizontal, (A=Vertical)
HCOLOR = $F6EC ; Set color in X, must be 0..7
autumn:
;===================
; init screen ; Instruction Length
jsr HGR2 ; 3
autumn_forever:
; save old Xcoord value to X/Y for later
; push/pop is 1 byte each but have to get
; value into accumulator first
ldx XCOORDL ; 2
ldy XCOORDH ; 2
; 16-bit subtraction x=x-y
; need to set carry before subtraction on 6502
txa ; 1
sec ; 1
sbc YCOORDL ; 2
sta XCOORDL ; 2
tya ; 1
sbc YCOORDH ; 2
; 16-bit arithmatic shift right of X
; 6502 has no asr instruction
; cmp #$80 sets carry if high bit set
cmp #$80 ; 2 ; XCOORDH still in A from before
ror ; 1
sta XCOORDH ; 2
ror XCOORDL ; 2
; 16-bit add, ycoord=ycoord+oldx
clc ; 1
txa ; 1
adc YCOORDL ; 2
sta YCOORDL ; 2
tya ; 1
adc YCOORDH ; 2
; 16-bit arithmatic shift right of y-coord
cmp #$80 ; 2 ; YCOORDH still in A from before
ror ; 1
sta YCOORDH ; 2
ror YCOORDL ; 2
; 32 bit rotate of low bit shifted out of Y-coordinate
ror EBP1 ; 2
ror EBP2 ; 2
ror EBP3 ; 2
ror EBP4 ; 2
; branch if carry set
bcs label_11f ; 2
; 16-bit increment of color
inc COLORL ; 2
bne no_oflo ; 2
inc COLORH ; 2
no_oflo:
; 16-bit add of X-coord by 0x80
; this keeps the drawing roughly to the 280x192 screen
; carry should still be clear (inc doesn't set it)
lda XCOORDL ; 2
adc #$80 ; 2
sta XCOORDL ; 2
bcc no_oflo2 ; 2
inc XCOORDH ; 2
no_oflo2:
; 16-bit negate of Y-coord
sec ; 1
lda #0 ; 2
sbc YCOORDL ; 2
sta YCOORDL ; 2
lda #0 ; 2
sbc YCOORDH ; 2
sta YCOORDH ; 2
label_11f:
; skipping the color manipulation done here by original
; mix colors a bit?
; 16-bit shift of color
; 2nd is a rotate as asl doesn't shift in cary
lda COLORL ; 2
asl ; 1 ; shl %ax
rol COLORH ; 2
eor COLORH ; 2
sta COLORL ; 2
; get color mapping
; using a color lookup table looks best but too many bytes
; you can approximate something close to the lookup with
; two extra instructions
and #$7 ; 2
ora #$2 ; 2
tax ; 1
; if ycoord negative, loop
lda YCOORDH ; 2
bmi autumn_forever ; 2
; if top bits of xcoord, loop
lda XCOORDH ; 2
and #$f0 ; 2
bne autumn_forever ; 2
put_pixel:
; actually set the color
jsr HCOLOR ; 3
; set up paramaters for HPLOT call
ldx XCOORDL ; 2
ldy XCOORDH ; 2
lda YCOORDL ; 2
jsr HPLOT0 ; 3
jmp autumn_forever ; 3 ; if we get under 128 bytes