System Support

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 4-10 lists the page register port addresses.

 

 

 

Table 4-10.

DMA Page Register Addresses

 

 

DMA Channel

Page Register I/O Port

Controller 1 (byte transfers)

 

Ch 0

087h

Ch 1

083h

Ch 2

081h

Ch 3

082h

 

 

Controller 2 (word transfers)

 

Ch 4

n/a

Ch 5

08Bh

Ch 6

089h

Ch 7

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.

DMA controller 1 can move up to 64 Kbytes of data per DMA transfer. DMA controller 2 can move up to 64 Kwords (128 Kbytes) of data per DMA transfer. Word DMA operations are only possible between 16-bit memory and 16-bit peripherals.

The RAM refresh is designed to perform a memory read cycle on each of the 512 row addresses in the DRAM memory space. Refresh operations are used to refresh memory on the 32-bit memory bus and the ISA bus. The refresh address is provided on lines SA00 through SA08. Address lines LA23..17, SA18,19 are driven low.

 

Technical Reference Guide

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