62 lines
1.4 KiB
ArmAsm
62 lines
1.4 KiB
ArmAsm
;; Clear screen and initialise line link table
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;; (Compute's Mapping the 64 p215-216)
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clear_screen:
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;; Clear line link table
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;; (Compute's Mapping the 64 p215)
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lda #$00
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ldy #24
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clearscreen_l1:
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sta screen_line_link_table,y
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dey
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bpl clearscreen_l1
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;; Y now = #$FF
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;; Clear screen RAM.
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;; We should do this at HIBASE, which annoyingly
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;; is no ZP, so we need to make a vector
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;; (Compute's Mapping the 64 p216)
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;; Get pointer to the screen into current_screen_line_ptr
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;; as it is the first appropriate place for it found when
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;; searching through the ZP allocations listed in
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;; Compute's Mapping the 64
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sta current_screen_line_ptr+0
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lda hibase
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sta current_screen_line_ptr+1
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ldx #$03 ; countdown for pages to update
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iny ; Y now = #$00
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lda #$20 ; space character
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clearscreen_l2:
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sta (current_screen_line_ptr),y
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iny
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bne clearscreen_l2
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;; To draw only 1000 bytes, add 250 to address each time
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lda current_screen_line_ptr
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clc
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adc #<250
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sta current_screen_line_ptr
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lda current_screen_line_ptr+1
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adc #>250
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sta current_screen_line_ptr+1
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lda #$20 ; get space character again
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dex
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bpl clearscreen_l2
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;; Clear colour RAM
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;; (Compute's Mapping the 64 p216)
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lda text_colour
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clearscreen_l3:
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sta $d800,y
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sta $d900,y
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sta $da00,y
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sta $db00-24,y ; so we only erase 1000 bytes
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iny
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bne clearscreen_l3
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;; (Compute's Mapping the 64 p216)
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jmp home_cursor
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