82801BA Datasheet, PDF (90/498 Pages) Intel Corporation – Intel 82801BA I/O Controller Hub 2 (ICH2) and Intel 82801BAM I/O Controller Hub 2 Mobile

Functional Description
Address Shifting When Programmed for 16-Bit I/O Count by Words
Table 5-9. 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 ICH2 maintains compatibility with the implementation of the DMA in the PC-AT that 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 5-10.
Table 5-10. 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: 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 processor 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.
5-28
82801BA ICH2 and 82801BAM ICH2-M Datasheet