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Routines |
| Prev: 22CB | Up: Map | Next: 2307 |
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The address of this routine is found in the parameter table.
This routine consists of a main subroutine plus one line to call it and one line to exit from it. The main routine is used twice by CIRCLE and the subroutine is called by DRAW_LINE. The routine is entered with the coordinates of a pixel on the calculator stack. It finds the address of that pixel and plots it, taking account of the status of INVERSE and OVER held in the P-FLAG.
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| PLOT | 22DC | CALL STK_TO_BC | y-coordinate to B, x to C. | |
| 22DF | CALL PLOT_SUB | The subroutine is called. | ||
| 22E2 | JP TEMPS | Exit, setting temporary colours. | ||
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This entry point is used by the routine at DRAW_LINE.
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| PLOT_SUB | 22E5 | LD ($5C7D),BC | The system variable COORDS is set. | |
| 22E9 | CALL PIXEL_ADD | Pixel address to HL. | ||
| 22EC | LD B,A | B will count A+1 loops to get a zero to the correct place in A. | ||
| 22ED | INC B | |||
| 22EE | LD A,$FE | The zero is entered. | ||
| PLOT_LOOP | 22F0 | RRCA | Then lined up with the pixel bit position in the byte. | |
| 22F1 | DJNZ PLOT_LOOP | |||
| 22F3 | LD B,A | Then copied to B. | ||
| 22F4 | LD A,(HL) | The pixel-byte is obtained in A. | ||
| 22F5 | LD C,(IY+$57) | P-FLAG is obtained and first tested for OVER. | ||
| 22F8 | BIT 0,C | |||
| 22FA | JR NZ,PL_TST_IN | Jump if OVER 1. | ||
| 22FC | AND B | OVER 0 first makes the pixel zero. | ||
| PL_TST_IN | 22FD | BIT 2,C | Test for INVERSE. | |
| 22FF | JR NZ,PLOT_END | INVERSE 1 just leaves the pixel as it was (OVER 1) or zero (OVER 0). | ||
| 2301 | XOR B | INVERSE 0 leaves the pixel complemented (OVER 1) or 1 (OVER 0). | ||
| 2302 | CPL | |||
| PLOT_END | 2303 | LD (HL),A | The byte is entered. Its other bits are unchanged in every case. | |
| 2304 | JP PO_ATTR | Exit, setting attribute byte. | ||
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