80 lines
1.5 KiB
ArmAsm
80 lines
1.5 KiB
ArmAsm
; Function defined on pp272-273 of C64 Programmers Reference Guide
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chrout:
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;; Crude implementation of character output
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;; Save X and Y values
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;; (Confirmed by writing a test program that X and Y
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;; don't get modified, in agreement with C64 PRG's
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;; description of CHROUT)
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sta last_printed_character_ascii
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txa
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pha
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tya
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pha
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lda last_printed_character_ascii
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;; Write character on the screen
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ldy current_screen_x
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;; Carriage return
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cmp #$0d
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beq screen_advance_to_next_line
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;; Clear screen
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cmp #$93
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bne not_clearscreen
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jsr clear_screen
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jmp chrout_done
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not_clearscreen:
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;; Convert PETSCII to screen code
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cmp #$40
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bcc chrout_l1
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cmp #$41+26
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bcs chrout_l1
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and #$1f
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chrout_l1:
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sta (current_screen_line_ptr),y
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;; Advance the column, and scroll screen down if we need
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;; to insert a 2nd line in this logical line.
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;; (eg Compute's Mapping the 64 p41)
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ldx current_screen_y
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iny
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sty current_screen_x
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cpy #40
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bne no_screen_advance_to_next_line
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jsr screen_grow_logical_line
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cpy #79
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bcc no_screen_advance_to_next_line
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jmp screen_advance_to_next_line
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no_screen_advance_to_next_line:
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chrout_done:
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;; Restore X and Y
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pla
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tay
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pla
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tax
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rts
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screen_grow_logical_line:
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;; XXX - Scroll screen down to make space
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rts
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screen_advance_to_next_line:
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;; Go to start of line
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lda #0
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sta current_screen_x
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;; Advance line number
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inc current_screen_y
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;; Work out if we have gone off the bottom of the screen?
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;; XXX -- implement this check and scrolling of the screen
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;; and updating the screen line links.
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jmp chrout_done
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