Specifications

DMA Controller (V40)
DMA Base Address/Current Address (DBA/DCA)
Three DBA/DCA registers specify the 20-bit address. The format of
these registers is shown in Figure 9-5. The lower 16 bits of the
address can be accessed with byte or word instructions. The upper
four bits must be accessed with byte instructions. As is the case with
the DBC/DCC registers, the BASE bit of the DCH register defines the
operation of the DBA/DCA registers. A logical 0 in the BASE bit
specifies that values written to the Address registers are programmed
to both base and current values and data read will be current values. If
the BASE bit is a logical 1, the values written to the Address registers
are programmed into the base value only. The base value is read from
the Count registers if the BASE bit is set to a logical 1.
Register:DBA/DCA - Low
Address:Base + 4
Access:Read or Write
76543210
A0A1A2A3A4A5A6A7
Register:DBA/DCA - Middle
Address:Base + 5
Access:Read or Write
76543210
A8A9A10A11A12A13A14A15
Register:DBA/DCA - High
Address:Base + 6
Access:Read or Write
76543210
A16A17A18A19————
Figure 9–5. DMA Base and Current Address Registers.
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