82801DB Datasheet, PDF (94/587 Pages) Intel Corporation – Intel 82801DB I/O Controller Hub 4 (ICH4)

Functional Description
5.4.3
5.4.3.1
Summary of DMA Transfer Sizes
Table 5-8 lists each of the DMA device transfer sizes. The column labeled “Current Byte/Word
Count Register” indicates that the register contents represents either the number of bytes to transfer
or the number of 16-bit words to transfer. The column labeled “Current Address Increment/
Decrement” indicates the number added to or taken from the Current Address register after each
DMA transfer cycle. The DMA Channel Mode Register determines if the Current Address Register
will be incremented or decremented.
Address Shifting When Programmed for 16-Bit I/O Count by Words
Table 5-8. DMA Transfer Size
DMA Device Date Size And Word Count
8-Bit I/O, Count By Bytes
16-Bit I/O, Count By Words (Address Shifted)
Current Byte/Word Count
Register
Bytes
Words
Current Address
Increment/Decrement
1
1
The ICH4 maintains compatibility with the implementation of the DMA in the PC AT which used
the 82C37. The DMA shifts the addresses for transfers to/from a 16-bit device count-by-words.
Note that the least significant bit of the Low Page Register is dropped in 16-bit shifted mode. When
programming the Current Address Register (when the DMA channel is in this mode), the Current
Address must be programmed to an even address with the address value shifted right by one bit.
The address shifting is shown in Table 5-9.
Table 5-9. Address Shifting in 16-bit I/O DMA Transfers
Output
Address
A0
A[16:1]
A[23:17]
8-Bit I/O Programmed Address
(Ch 0–3)
A0
A[16:1]
A[23:17]
16-Bit I/O Programmed Address
(Ch 5–7)
(Shifted)
0
A[15:0]
A[23:17]
NOTE: The least significant bit of the Page Register is dropped in 16-bit shifted mode.
5.4.4
Autoinitialize
By programming a bit in the DMA Channel Mode Register, a channel may be set up as an
autoinitialize channel. When a channel undergoes autoinitialization, the original values of the
Current Page, Current Address, and Current Byte/Word Count registers are automatically restored
from the Base Page, Address, and Byte/Word Count Registers of that channel following TC. The
Base registers are loaded simultaneously with the Current registers by the microprocessor when the
DMA channel is programmed and remain unchanged throughout the DMA service. The mask bit is
not set when the channel is in autoinitialize. Following autoinitialize, the channel is ready to
perform another DMA service, without processor intervention, as soon as a valid DREQ is
detected.
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Intel^® 82801DB ICH4 Datasheet