Routines
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E86E: Find a pixel's screen byte and its mask
Used by the routines at BUFFER_PIXEL and PLOT_LINE_START.
Input
B Row, from the bottom
C Column
Output
HL The screen byte
A The pixel's mask (bit 7 the left-hand pixel); D is left 0
SCREEN_PIXEL E86E CALL PIXEL_ADDRESS the screen byte
E871 LD A,C $80, moved right by the column's low three bits
E872 AND $07
E874 LD D,A
E875 INC D
E876 LD A,$01
SCREEN_PIXEL_0 E878 RRCA
E879 DEC D
E87A JR NZ,SCREEN_PIXEL_0
E87C RET
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