Chapter 4 System Support
Compaq Deskpro Personal Computers
Third Edition – April 2001
4-20
The DMA logic is accessed through two types of I/O mapped registers; page registers and
controller registers.
4.4.2.1 DMA Page Registers
The DMA page register contains the eight most significant bits of the 24-bit address and works in
conjunction with the DMA controllers to define the complete (24-bit)address for the DMA
channels. Table 4-11 lists the page register port addresses.
Table 4-11. DMA Page Register Addresses
Table 4-11.
DMA Page Register Addresses
DMA Channel Page Register I/O Port
Controller 1 (byte transfers)
Ch 0
Ch 1
Ch 2
Ch 3
087h
083h
081h
082h
Controller 2 (word transfers)
Ch 4
Ch 5
Ch 6
Ch 7
n/a
08Bh
089h
08Ah
Refresh 08Fh [see note]
NOTE:
The DMA memory page register for the refresh channel must be
programmed with 00h for proper operation.
The memory address is derived as follows:
24-Bit Address - Controller 1 (Byte Transfers)
8-Bit Page Register 8-Bit DMA Controller
A23..A16 A15..A00
24-Bit Address - Controller 2 (Word Transfers)
8-Bit Page Register 16-Bit DMA Controller
A23..A17 A16..A01, (A00 = 0)
Note that address line A16 from the DMA memory page register is disabled when DMA controller
2 is selected. Address line A00 is not connected to DMA controller 2 and is always 0 when word-
length transfers are selected.
By not connecting A00, the following applies:
♦ The size of the the block of data that can be moved or addressed is measured in 16-bits
(words) rather than 8-bits (bytes).
♦ The words must always be addressed on an even boundary.
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