258 lines
2.8 KiB
ArmAsm
258 lines
2.8 KiB
ArmAsm
DRAW_PAGE = $E6
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ZL = $06
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ZH = $07
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SEEDLO = $4E
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SEEDHI = $4F
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NUM1 = $F0
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NUM1L = $F1
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NUM2 = $F2
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NUM2L = $F3
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RESULT = $F4
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RESULT2 = $F5
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RESULT3 = $F6
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RESULT4 = $F7
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XL = $F8
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XH = $F9
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YL = $FA
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YH = $FB
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MINUS = $FC
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Z = $FD
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HGR2 = $F3D8
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HPLOT0 = $F457 ; plot (Y,X), (A)
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HGLIN = $F53A ; plot to (A,X), (Y)
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HCLEAR0 = $F3F2 ; clear current hgr page to blackh
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HCOLOR = $F6F0 ; color in X (must be 0..7)
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lda #$ff
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sta XH
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ldx #0
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jmp check_bounds
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populate_loop:
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; Z = XXYY YYYY
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; C=Z*.125
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; A=A+(A-64)*C
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; B=B+(B-64)*C
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; X is in NUM1 for this
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lda #0
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sta MINUS
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sec
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lda XH
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sbc #64
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bpl no_minus
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set_minus:
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inc MINUS
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sec
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lda #0
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sbc XL
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sta NUM1
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lda #0
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sbc XH
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sta NUM1+1
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jmp do_mult
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no_minus:
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sta NUM1+1
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lda XL
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sta NUM1
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do_mult:
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lda #0
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sta NUM2+1
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lda Z
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sta NUM2
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txa
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pha
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jsr mult16
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pla
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tax
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; result has result but due to our fixed point
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; we want to shift the whole thing left by 2
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; then grab RESULT+2 as high and RESULT+1 as low
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rol RESULT+1
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rol RESULT+2
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ror RESULT+1
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lda RESULT+2
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lda RESULT+1 ; low
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sta NUM1
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lda RESULT+2 ; hight
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sta NUM1+1
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; add to X; subtract if minus
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lda MINUS
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beq not_minus
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sec
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lda #00
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sbc NUM1
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sta NUM1
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lda #00
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sbc NUM1+1
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sta NUM1+1
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not_minus:
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clc
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lda XL
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adc NUM1
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sta XL
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lda XH
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adc NUM1+1
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sta XH
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; Z=Z+.125
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clc
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lda Z
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adc #$8
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sta Z
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; check to see if out of bounds
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check_bounds:
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lda XH
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bmi redo_point
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lda YH
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bpl all_good
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redo_point:
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; store a break in the lines
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lda #0
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sta $1000,X
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sta $1100,X
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sta Z ; needed?
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inx
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jsr rand16
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sta XH
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jsr rand16
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sta YH
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all_good:
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; write out
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lda XH
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sta $1000,X
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lda YH
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sta $1100,X
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inx
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beq done_pop
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jmp populate_loop
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done_pop:
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;=========================
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;=========================
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;=========================
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jsr HGR2
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ldx #3
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jsr HCOLOR
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ldx #0
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star_loop:
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txa
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and #$1
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beq page2
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page1: ; draw page1, show page2
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bit $C055
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lda #$20
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bne adjust_page
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page2:
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bit $C054
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lda #$40
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adjust_page:
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sta DRAW_PAGE
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; clear background
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jsr HCLEAR0
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txa
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pha
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lda $1100,X ; ycoord
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beq skip_draw
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pha
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ldy $1000,X ; xcoord
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tya
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tax
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ldy #0
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pla
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jsr HPLOT0 ; plot (Y,X), (A)
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skip_draw:
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pla
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tax
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inx
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jmp star_loop
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; make sure seed isn't 0000
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rand16:
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; batari rand16
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lda SEEDHI
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lsr
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rol SEEDLO
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bcc noeor
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eor #$B4
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noeor:
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sta SEEDHI
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eor SEEDLO
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; we only want 127 bits
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and #$7f
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rts
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;=====================
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;=====================
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; 16x16 -> 32 multiply
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;=====================
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;=====================
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; destroys NUM2
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mult16:
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lda #0 ; Initialize RESULT to 0
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sta RESULT+2
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ldx #16 ; There are 16 bits in NUM2
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L1:
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lsr NUM2+1 ; top part of 16 bit shift right
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ror NUM2
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bcc L2 ; 0 or 1?
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tay ; If 1, add NUM1 (hi byte of RESULT is in A)
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clc
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lda NUM1
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adc RESULT+2
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sta RESULT+2
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tya
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adc NUM1+1
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L2:
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ror ; "Stairstep" shift
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ror RESULT+2
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ror RESULT+1
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ror RESULT
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dex
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bne L1
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sta RESULT+3
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rts
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