Routines
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B18C: Step an object through four frames
Used by the routines at REMOTE_ROBOT, SOCKET_SPARKLE and SPARK_CHASER.
Adds one to the low two bits of the graphic and leaves the rest alone, so 76 goes to 77, 78, 79 and back to 76. Knight Lore's next_graphic_no_mod_4.
Input
IX the object
NEXT_FRAME_MOD4 B18C LD A,(IX+$00) the graphic with its low two bits plus 1, modulo 4
B18F LD C,A
B190 AND $FC
B192 LD B,A
B193 LD A,C
B194 INC A
B195 AND $03
B197 OR B
This entry point is used by the routine at TOGGLE_FRAME.
NEXT_FRAME_MOD4_0 B198 LD (IX+$00),A store it (TOGGLE_FRAME comes here too)
B19B RET
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