Routines
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6910: Move Down One Row
Used by the routines at AttributesPlayArea and DrawHUDPanel.
Adds 20 to an address, which moves it down by one character row in the attribute buffer or the screen buffer (for as long as it stays within the same third of the screen).
Input
HL Current address
Output
HL Address one row down
MoveDownOneRow 6910 LD A,L L=L + 20 (one row).
6911 ADD A,$20
6913 LD L,A
6914 LD A,H H=H + 00 (but include the carry from above).
6915 ADC A,$00
6917 LD H,A
6918 RET Return.
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