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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.
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.
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.
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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