Mysterious Orange Line — Before leaving the subject of HIRES, we must discuss the “Mysterious Orange (or Pink) Line.” This vertical line appears from time to ti

Mysterious Orange Line — Before leaving the subject of HIRES, we must discuss the “Mysterious Orange (or Pink) Line.” This vertical line appears from time to time along the left edge of the screen (Reference 8.2). The phenomenon is rooted in the hardware. Notice in Fig. 8-28* that when the video is shifted one clock cycle by A11-9, 70 nS of garbage is shifted such that it falls within the unblanked portion of the screen (point Z). This unwanted half-dot ends up in the video output at point AA (the left edge of the screen). Can we determine where this dot comes from and what its value is? It comes from bit 6 of the memory location that maps to the video address just to the left of the left-most byte displayed on the screen. If bit 6 of this critical location is set, and if bit 7 of the left-most byte is set, then there will be a half-dot at the left edge of the screen (see the last signal plotted in Fig. 8-29). The phase of this dot is such that it will be pinkish-orange in color. The critical addresses themselves can be determined from Fig. 5-8*. They are all within the 16K HIRES page. Of these locations, 128 are legitimate screen locations along the right edge of the screen. The remaining 64 are unused locations. The mapping is indicated in Fig. 8-30. You can see the Mysterious Line by running the following 20 second program. 10 REM MYSTERIOUS ORANGE LINE 20 POKE —16297,0 30 POKE —16304,0 40 POKE —16302,0 100 P = 8192 110 FOR I = P TO P + P 120 POKE 1,128 : NEXT I 130 FOR N = 0 TO 7168 STEP 1024 140 FOR I = 127 + P + N TO 1023 + P + N STEP 128 150 POKE 1,64 : NEXT I 160 FOR I = 39 + P + N TO 935 + P + N STEP 128 170 POKE 1,64 : NEXT I 180 FOR I = 79 + P + N TO 975 + P + N STEP 128 190 POKE 1,64 : NEXT I 200 NEXT N 999 END Fig. 8-30. Mysterious orange line. MYSTERIOUS ORANGE LINE UNUSED MEMORY LOCATIONS STORE THE TOP THIRD OF THE MYSTERIOUS LINE. LEGITIMATE SCREEN LOCATIONS ON THE RIGHT STORE THE BOTTOM TWO-THIRDS OF THE MYSTERIOUS LINE.