*Specific ROM switching
*8-bit printer port (with bit3 of CRTC register 12)
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== Soft Scroll ==
The 8-bit register SSCR (at address 6804h) controls soft scrolling by pixels rather than by characters. Setting this register to 0 (the default value at power-up) will disable the soft scroll feature. The soft scrolling mechanism affects the entire main screen, regardless of the split screen feature, but does not affect sprites.
* Bit3..0 of SSCR defines a horizontal delay between 0 and 15 high-resolution (mode 2) pixels, shifting the screen image to the right by the programmed value. This causes pixels to be lost behind the right border and random data to appear on the left. Also the programmer must ensure that the delay value is a multiple of the number of bits per pixel.
* Bit6..4 of SSCR is added to the least significant 3 bits of the scan line address, determining which of the eight 2k blocks contains the data for the first scan line on the screen. This shifts the display up by the programmed number of scan lines, causing the first lines to be lost and extra lines to appear at the bottom.
* Bit7 of SSCR, when set, extends the border to cover the first 2 bytes (16 high-resolution pixels) of each scan line, masking the garbage data caused by the horizontal soft scroll. Software using horizontal soft scroll should always set this bit to maintain a consistent screen width.
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