Acorn Computers Limited BBC A, Hardware Manual, Page: 25 / 47

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<Figure 3.2 to go here>
PALETTE
64 BJT RAM
(4x16)
RGB FROM SAA5050
MULTIPLEXER
CURSOR CONTROL
RGB out
FIGURE 3.2 RGB SYNTHESIS IN VIDEO ULA
A feature of the BBC Microcomputer is its ability to perform a "hardware scroll". The start-address-register
within the 6845 CRTC is used to refer to the start address of screen memory. By altering the contents of
this register it is possible to scroll the "active" screen display area. In the BBC Microcomputer, the
hardware scroll is implemented in a more sophisticated fashion.
Not only is this start address register used, but also an offset can be added to the high order refresh
address lines of the CRTC. This offset is specified by the system VIA (IC3), under the control of the
operating system, and a two-bit code produced via an addressable latch IC32 (74LS259). This two bit
code (C0 and C1) controls a combination of four NAND gates (parts of IC36, 40 and 27) and a full adder
IC39 (74LS283) such that the required offset is added to the refresh address lines of the CRTC. The
offset to be added is calculated such that by incrementing the CRTC refresh address lines by this
amount, the active display area is shifted by one line to reveal the next line where the new data is to be
written. Using this technique, the microcomputer can scroll the screen very rapidly.
In screen modes 0, 1 and 2, 20 kilobytes of RAM is required and since sufficient address lines are
provided to access 32