Attribute Memory

Each bank has an attribute memory select bit associated with it which allows for the control of NREG. NREG is used to select between attribute memory and common memory on plug-in cards (NCS0 and NCS1). When NREG is high, common memory is accessed. When NREG is low, attribute memory is accessed. If the attribute memory select bit is set for a given memory bank and that bank is selected, attribute memory will be accessed. If the attribute memory select bit is reset for that bank, common memory will be ac cessed. Therefore the state of the NREG signal is just the inverse of the state of the attribute memory select bit. The attribute memory select bit for Bank C is located in C Reg1 at index -99h. The bit is C Reg1[4].

The attribute memory select bits for the four E banks and the four D banks are located in a separate register called the Attribute Memory Select register located at index -9E. The register is defined as follows:

Index   Name                        Bits    Comments
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-9Eh    Attribute Memory Select     [7]     Attribute memory select for Bank E3
                                    [6]     Attribute memory select for Bank E2
                                    [5]     Attribute memory select for Bank E1
                                    [4]     Attribute memory select for Bank E0
                                    [3]     Attribute memory select for Bank D3
                                    [2]     Attribute memory select for Bank D2
                                    [1]     Attribute memory select for Bank D1
                                    [0]     Attribute memory select for Bank D0
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