104 lines
3.7 KiB
ArmAsm
104 lines
3.7 KiB
ArmAsm
; This is the method that is most useful from the high-level code. We want the
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; freedom to blit a range of lines. This subroutine can assume that all of the
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; data in the code fields is set up properly.
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;
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; X = first line (inclusive), valid range of 0 to 199
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; Y = last line (exclusive), valid range >X up to 200
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;
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; The lines are based on the appearance of lines in the play field, so blitting lines 0 through
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; 19 will draw the first 20 lines on the play field, regardless of where the playfield is physically
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; on the SHR screen or the current value of StartY
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_BltRange
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:exit_ptr equ tmp0
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:jmp_low_save equ tmp2
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phb ; preserve the bank register
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clc`
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dey
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tya ; Get the address of the line that we want to return from
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adc StartYMod208 ; and create a pointer to it
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asl
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tay
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lda BTableLow,y
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sta :exit_ptr
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lda BTableHigh,y
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sta :exit_ptr+2
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txa ; get the first line (0 - 199)
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adc StartYMod208 ; add in the virtual offset (0, 207) -- max value of 406
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asl
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tax ; this is the offset into the blitter table
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sep #$20 ; 8-bit Acc
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lda BTableHigh,x ; patch in the bank
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stal blt_entry+3
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lda BTableLow+1,x ; patch in the page
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stal blt_entry+2
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; The way we patch the exit code is subtle, but very fast. The CODE_EXIT offset points to
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; an JMP/JML instruction that transitions to the next line after all of the code has been
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; executed.
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;
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; The trick we use is to patch the low byte to force the code to jump to a special return
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; function (jml blt_return) in the *next* code field line.
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ldy #CODE_EXIT+1 ; this is a JMP or JML instruction that points to the next line.
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lda [:exit_ptr],y
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sta :jmp_low_save
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lda #FULL_RETURN ; this is the offset of the return code
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sta [:exit_ptr],y ; patch out the low byte of the JMP/JML
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; Now we need to set up the Bank, Stack Pointer and Direct Page registers for calling into
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; the code field
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lda EngineMode
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bit #ENGINE_MODE_TWO_LAYER
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beq :skip_bank
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; TODO: Switch to loading the selected BG1 bank. No special "Alt" bank
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;
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; lda RenderFlags
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; bit #RENDER_ALT_BG1
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; beq :primary
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;
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; lda BG1AltBank
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; bra :alt
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;
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;:primary lda BG1DataBank
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;:alt
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lda BG1DataBank
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pha
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plb
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:skip_bank
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rep #$20
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phd ; Save the application direct page
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lda BlitterDP ; Set the direct page to the blitter data
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tcd
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sei ; disable interrupts
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_R0W1
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tsc ; save the stack pointer
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stal stk_save+1
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blt_entry jml $000000 ; Jump into the blitter code $XX/YY00
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blt_return _R0W0
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stk_save lda #0000 ; load the stack
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tcs
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cli ; re-enable interrupts
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pld ; restore the direct page
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sep #$20
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ldy #CODE_EXIT+1
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lda :jmp_low_save
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sta [:exit_ptr],y
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rep #$20
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plb ; restore the bank
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rts
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