PIIX datasheet(66/122 Pages) INTEL

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82371FB (PIIX) AND 82371SB (PIIX3)

2.5.1.10.

DMA Clear Byte Pointer Register

I/O Address:

Channels 0-3—00Ch; Channels 4-7—0D8h

Default Value:

All bits undefined

Attribute:

Write Only

Writing to this register executes the Clear Byte Pointer Command. This command is executed prior to

reading/writing a new address or word count to the DMA. The command initializes the byte pointer flip-

flop to a known state so that subsequent accesses to register contents address upper and lower bytes

in the correct sequence. The Clear Byte Pointer Command (or CPURST or the Master Clear

Command) clears the internal latch used to address the upper or lower byte of the 16-bit Address and

Word Count Registers.

Bit

Description

7:0

Clear Byte Pointer. No specific pattern. Command enabled with a write to the I/O port

address.

2.5.1.11.

DMC—DMA Master Clear Register

I/O Address:

Channel 0-3—00Dh; Channel 4-7—0DAh

Default Value:

All bits undefined

Attribute:

Write Only

This software instruction has the same effect as the hardware Reset.

Bit

Description

7:0

Master Clear. No specific pattern. Command enabled with a write to the I/O port address

2.5.1.12.

DCLM—DMA Clear Mask Register

I/O Address:

Channel 0-3—00Eh; Channel 4-7—0DCh

Default Value:

All bits undefined

Attribute:

Write Only

This command clears the mask bits of all four channels, enabling them to accept DMA requests.

Bit

Description

7:0

Clear Mask Register. No specific pattern. Command enabled with a write to the I/O port

address.

2.5.2.

TIMER/COUNTER REGISTER DESCRIPTION

2.5.2.1.

TCW—Timer Control Word Register

I/O Address:

043h

Default Value:

All bits undefined

Attribute:

Write Only

The Timer Control Word Register specifies the counter selection, the operating mode, the counter byte

programming order and size of the count value, and whether the counter counts down in a 16-bit or binary-

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