CMOS RAM usage

Byte ???        bits 0..2  LCD screen brightness           } values toggle under action of
                bits 3..5  secondary LCD screen brightness } Bright/Dim key
                bits 6..7  Unused at present

Byte ???        bits 0..7  LCD screen contrast


New service call
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Issued when 'access lock' key is pressed. This should allow the implementation of a
third party security module.
