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6980: Update the SRB for one column of a sprite
Used by the routine at 61B0. Sets the bits in the screen refresh buffer (SRB) that correspond to the four tiles in the left, middle or right column of a sprite.
DE Base address of the sprite UDG references
HL SRB address corresponding to the top row of the sprite
6980 LD B,$04 There are 4 rows (and 3 columns) in a sprite
6982 LD A,(DE) Pick up a sprite UDG reference
6983 INC D Next row of the sprite
6984 AND A Is this the blank UDG?
6985 JR Z,$6989 Jump if so (no need to update this SRB byte)
6987 SET 0,(HL) Set the appropriate bit in the SRB byte; this instruction is modified by the routine at 61B0 before this routine is called
6989 LD A,$04 Point HL at the byte of the SRB corresponding to the next row of the screen
698B ADD A,L
698C LD L,A
698D DJNZ $6982 Jump back until all 4 squares in the column are done
698F BIT 3,D Return with the zero flag reset if we have just updated the SRB for the rightmost column of the sprite
6991 RET
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