BBC Microcomputer Service Manual Oct 1985 Section 1 BBC Micro ...

The speed of printing on the screen is much increased by the use of
hardware scrolling. There is a register in the CRTC which is used to
define the start of screen address in the screen memory. Thus in order
to scroll the screen, it is only necessary to increment this register
by the number of characters per line and then write to the memory
address where the last screen data was. The number of address lines
from the CRTC, used to address the screen memory, has to be sufficient
to cater for the biggest screen (20 Kbytes). Thus 14 address lines have
to be used which means that when using the hardware scrolling
technique, the picture scrolls around in 32 Kbytes. Consider a scroll
of 8 Kbytes in a 20 Kbyte screen. The original start of screen for the
20 Kbyte mode was &3000. After an 8 Kbyte scroll, the current start of
screen address is &5000 with the end of the screen as seen by the CRTC
at &5000 plus 20 Kbytes, which comes to &9FFF, as illustated below.
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