Routines
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26001: Clear bottom character row
Used by the routines at move_alien_bullet and alien_hit.
clear_bottom_char_row 26001 LD DE,20704 Top scan of bottom row of the screen, where the ship is
Clear one character row Takes the address of the top scan byte of the leftmost char in a row of 32 chars and sets them to zero. So it clears 32 chars, 8 scans high, starting at DE.
Used by the routines at draw_lives_remaining, spaceship_handler, hit_spaceship, move_one_row_aliens_down and remove_barriers. I:DE Top scan of screen char to start clearing at
clear_char_row 26004 LD C,8 We're going to clear 8 scans at screen address in DE
26006 XOR A
clear_bottom_char_row_0 26007 LD B,32 Screen width is 32 bytes, write zero into each byte
26009 PUSH DE
clear_bottom_char_row_1 26010 LD (DE),A
26011 INC DE
26012 DJNZ clear_bottom_char_row_1
26014 POP DE
26015 INC D Down one scan (this won't work across boundaries)
26016 DEC C Back for next scan
26017 JR NZ,clear_bottom_char_row_0
26019 RET
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