Spectrum ROM | Routines |
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Used by the routine at CIRCLE.
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 | 08924 | CALL STK_TO_BC | y-coordinate to B, x to C. | ||||
08927 | CALL PLOT_SUB | The subroutine is called. | |||||
08930 | JP TEMPS | Exit, setting temporary colours. | |||||
This entry point is used by the routine at DRAW_LINE.
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PLOT_SUB | 08933 | LD (23677),BC | The system variable COORDS is set. | ||||
08937 | CALL PIXEL_ADD | Pixel address to HL. | |||||
08940 | LD B,A | B will count A+1 loops to get a zero to the correct place in A. | |||||
08941 | INC B | ||||||
08942 | LD A,254 | The zero is entered. | |||||
PLOT_LOOP | 08944 | RRCA | Then lined up with the pixel bit position in the byte. | ||||
08945 | DJNZ PLOT_LOOP | ||||||
08947 | LD B,A | Then copied to B. | |||||
08948 | LD A,(HL) | The pixel-byte is obtained in A. | |||||
08949 | LD C,(IY+87) | P-FLAG is obtained and first tested for OVER. | |||||
08952 | BIT 0,C | ||||||
08954 | JR NZ,PL_TST_IN | Jump if OVER 1. | |||||
08956 | AND B | OVER 0 first makes the pixel zero. | |||||
PL_TST_IN | 08957 | BIT 2,C | Test for INVERSE. | ||||
08959 | JR NZ,PLOT_END | INVERSE 1 just leaves the pixel as it was (OVER 1) or zero (OVER 0). | |||||
08961 | XOR B | INVERSE 0 leaves the pixel complemented (OVER 1) or 1 (OVER 0). | |||||
08962 | CPL | ||||||
PLOT_END | 08963 | LD (HL),A | The byte is entered. Its other bits are unchanged in every case. | ||||
08964 | JP PO_ATTR | Exit, setting attribute byte. |
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