mirror of
https://github.com/lscharen/iigs-game-engine.git
synced 2024-11-15 10:05:02 +00:00
205 lines
6.8 KiB
ArmAsm
205 lines
6.8 KiB
ArmAsm
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; Initialization and setup routines for the second background
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_InitBG1
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jsr _ApplyBG1XPos
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jsr _ApplyBG1YPos
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rts
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; Everytime either BG1 or BG0 X-position changes, we have to update the direct page values. We
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; *could* do this by adjusting the since address offset, but we have to change up to 200 values
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; when the vertical position changes, and only 41 when the horizontal value changes. Plus
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; these are all direct page values
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;
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; Note: This routine can be optimized as an unrolled loop of PEI instructions
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_ApplyBG1XPos
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lda #162
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sec
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sbc StartXMod164
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bpl *+6
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clc
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adc #164
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tay
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phd ; save the direct page because we are going to switch to the
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lda BlitterDP ; blitter direct page space and fill in the addresses
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tcd
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tya
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ldx #162
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:loop
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sta 00,x ; store the value
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dec
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dec
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bpl *+6
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clc
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adc #164
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dex
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dex
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bpl :loop
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pld
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rts
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; Everytime either BG1 or BG0 Y-position changes, we have to update the Y-register
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; value in all of the code fields (within the visible screen)
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_ApplyBG1YPos
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:virt_line equ tmp0
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:lines_left equ tmp1
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:draw_count equ tmp2
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:ytbl_idx equ tmp3
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stz :rtbl_idx ; Start copying from the first entry in the table
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lda StartY ; This is the base line of the virtual screen
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sta :virt_line ; Keep track of it
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lda ScreenHeight
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sta :lines_left
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:loop
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lda :virt_line
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asl
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tax
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ldal BTableLow,x ; Get the address of the first code field line
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tay
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sep #$20
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ldal BTableHigh,x
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pha
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plb ; This is the bank that will receive the updates
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rep #$20
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lda :virt_line
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and #$000F
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eor #$FFFF
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inc
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clc
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adc #16
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min :lines_left
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sta :draw_count ; Do this many lines
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asl
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tax
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lda :ytbl_idx ; Read from this location in the BG1YTable
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asl
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jsr CopyBG1YTableToBG1Addr
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lda :virt_line ; advance to the virtual line after the segment we just
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clc ; filled in
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adc :draw_count
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sta :virt_line
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lda :ytbl_idx ; advance the index into the YTable
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adc :draw_count
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sta :ytbl_idx
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lda :lines_left ; subtract the number of lines we just completed
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sec
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sbc :draw_count
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sta :lines_left
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jne :loop
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phk
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plb
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rts
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; Unrolled copy routine to move BG1YTable entries into BG1_ADDR position.
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;
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; A = intect into the BG1YTable array (x2)
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; Y = starting line * $1000
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; X = number of lines (x2)
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CopyBG1YTableToBG1Addr
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jmp (:tbl,x)
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:tbl da :none
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da :do01,:do02,:do03,:do04
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da :do05,:do06,:do07,:do08
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da :do09,:do10,:do11,:do12
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da :do13,:do14,:do15,:do16
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:do15 tax
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bra :x15
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:do14 tax
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bra :x14
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:do13 tax
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bra :x13
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:do12 tax
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bra :x12
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:do11 tax
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bra :x11
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:do10 tax
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bra :x10
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:do09 tax
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bra :x09
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:do08 tax
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bra :x08
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:do07 tax
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bra :x07
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:do06 tax
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bra :x06
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:do05 tax
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bra :x05
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:do04 tax
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bra :x04
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:do03 tax
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bra :x03
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:do02 tax
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bra :x02
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:do01 tax
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bra :x01
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:do16 tax
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ldal BG1YTable+30,x
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sta BG1_ADDR+$F000,y
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:x15 ldal BG1YTable+28,x
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sta BG1_ADDR+$E000,y
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:x14 ldal BG1YTable+26,x
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sta BG1_ADDR+$D000,y
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:x13 ldal BG1YTable+24,x
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sta BG1_ADDR+$C000,y
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:x12 ldal BG1YTable+22,x
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sta BG1_ADDR+$B000,y
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:x11 ldal BG1YTable+20,x
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sta BG1_ADDR+$A000,y
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:x10 ldal BG1YTable+18,x
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sta BG1_ADDR+$9000,y
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:x09 ldal BG1YTable+16,x
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sta BG1_ADDR+$8000,y
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:x08 ldal BG1YTable+14,x
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sta BG1_ADDR+$7000,y
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:x07 ldal BG1YTable+12,x
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sta BG1_ADDR+$6000,y
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:x06 ldal BG1YTable+10,x
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sta BG1_ADDR+$5000,y
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:x05 ldal BG1YTable+08,x
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sta: BG1_ADDR+$4000,y
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:x04 ldal BG1YTable+06,x
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sta BG1_ADDR+$3000,y
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:x03 ldal BG1YTable+04,x
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sta BG1_ADDR+$2000,y
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:x02 ldal BG1YTable+02,x
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sta BG1_ADDR+$1000,y
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:x01 ldal BG1YTable+00,x
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sta: BG1_ADDR+$0000,y
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:none rts
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