225 lines
8.6 KiB
ArmAsm
225 lines
8.6 KiB
ArmAsm
; Support rotating the BG1 graphics by leveraging the fact that a rotation function can be decomposed
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; into an addition of two function parametertized by the angle of rotation: pixel = *(f(x, a) + f(y, a))
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;
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; The pre-build a number of rotation tables and then populate the direct page values and Y-register values
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; for each line of the blitter, such that a single lda (00),y instruction fetches the appropriate data
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;
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; This is about as fast of a rotation as we can do.
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;
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; When possible, off-screen locations are calculate to produce an address of $FFFE, so that the last two bytes
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; of the BG1 data buffer provides the "fill value".
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ANGLEBNK EXT
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_ApplyBG1XPosAngle
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:ptr equ $FC
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:stbl equ $FA
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phd ; save the direct page because we are going to switch to the
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pei BlitterDP ; blitter direct page space and fill in the addresses
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lda BG1Scaling
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pld
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and #$000F
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asl
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tax
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lda ScalingTables,x
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sta :stbl
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lda #^ANGLEBNK
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sta :ptr+2
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sty :ptr ; Store in the new direct page
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ldx #162
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:loop
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txy
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lda (:stbl),y ; Map the through the scaling factor
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tay
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lda [:ptr],y ; Load the underlying value
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sta 00,x ; store the value
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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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_ApplyBG1YPosAngle
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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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:angle_tbl equ tmp4
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:scale_ptr equ tmp5
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sty :angle_tbl
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lda BG1Scaling ; Set the scaling table
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and #$0007
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asl
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tax
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lda ScalingTables,x
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sta :scale_ptr
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lda BG1StartYMod208
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sta :ytbl_idx ; Start copying from the first entry in the table
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lda StartYMod208 ; 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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; Copy out the y-values from the rotation table into a temporary buffer
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; Copy the rotation values into the code fields
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phb
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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 ; push the bank on the stack
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plb
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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 (duplicate every 4 lines)
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lsr
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lsr
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asl
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clc
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adc :angle_tbl
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sec
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sbc #ANGLEBNK
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jsr CopyAngleYTableToBG1Addr ; or CopyBG1YTableToBG1Addr2
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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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plb
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rts
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; Unrolled copy routine to move y_angle entries into BG1_ADDR position with an additional
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; shift. This has to be split into two
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;
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; A = index into the array (x2)
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; Y = starting line * $1000
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; X = number of lines (x2)
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CopyAngleYTableToBG1Addr
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:ptr equ $FC
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:stbl equ $FA
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; tax
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; ldal ANGLEBNK+XX,x
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; sta BG1_ADDR+$F000,y
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phy ; save y; used when writing
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phx
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; Scale the mapping
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tay
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jsr SaveBG1AngleValues
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plx ; x is used directly in this routine
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ply
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jmp ApplyBG1OffsetValues
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SaveBG1AngleValues
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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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:do16 tax
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ldal ANGLEBNK+06,x
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stal BG1YCache+30
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:x15 ldal ANGLEBNK+06,x
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stal BG1YCache+28
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:x14 ldal ANGLEBNK+06,x
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stal BG1YCache+26
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:x13 ldal ANGLEBNK+06,x
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stal BG1YCache+24
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:x12 ldal ANGLEBNK+04,x
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stal BG1YCache+22
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:x11 ldal ANGLEBNK+04,x
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stal BG1YCache+20
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:x10 ldal ANGLEBNK+04,x
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stal BG1YCache+18
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:x09 ldal ANGLEBNK+04,x
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stal BG1YCache+16
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:x08 ldal ANGLEBNK+02,x
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stal BG1YCache+14
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:x07 ldal ANGLEBNK+02,x
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stal BG1YCache+12
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:x06 ldal ANGLEBNK+02,x
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stal BG1YCache+10
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:x05 ldal ANGLEBNK+02,x
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stal BG1YCache+08
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:x04 ldal ANGLEBNK+00,x
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stal BG1YCache+06
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:x03 ldal ANGLEBNK+00,x
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stal BG1YCache+04
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:x02 ldal ANGLEBNK+00,x
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stal BG1YCache+02
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:x01 ldal ANGLEBNK+00,x
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stal BG1YCache+00
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:none rts
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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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