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Routines |
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Used by the routines at FLOOD_FILL and DRAW_LINE.
y is 0-127, so bit 7 going high means it would leave the canvas: the move is undone and A returned with its low bit clear. Otherwise the low bit is set. The callers test that bit rather than the flags, which is why both exits go through the same tail.
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| INC_Y | 812B | INC E | Up one; did y pass 127? | |
| 812C | BIT 7,E | |||
| 812E | JR Z,Y_MOVED | |||
| 8130 | DEC E | Yes: undo it, and return A with bit 0 clear | ||
| 8131 | LD H,A | |||
| 8132 | XOR A | |||
| 8133 | LD A,H | |||
| 8134 | RET | |||
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This entry point is used by the routine at DEC_Y.
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| Y_MOVED | 8135 | LD H,A | No: return A with bit 0 set (DEC_Y shares this) | |
| 8136 | OR $01 | |||
| 8138 | LD A,H | |||
| 8139 | RET | |||
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