
84
Functional Operation
Chapter 3
AMD-8111 HyperTransport I/O Hub Data Sheet
24674
Rev. 3.00
April 2003
AMD Preliminary Information
FFFFh which corresponds to 0.671 seconds. A write to STVAL restarts the timer with the new
contents of STVAL.
3.10.4
Media Access Control
The Media Access Control (MAC) engine incorporates the essential protocol requirements for
operation of an Ethernet/IEEE 802.3-compliant node and provides the interface between the FIFO
subsystem and the MII.
This section describes operation of the MAC engine when operating in half-duplex mode. The
operation of the device in full-duplex mode is described in the section titled
Full-Duplex Operation
.
The MAC engine is compliant to Section 4 of IEEE standard 802.3, 1998 Edition.
The MAC engine provides programmable enhanced features designed to minimize host supervision,
bus utilization, and pre- or post-message processing. These features include the ability to disable
retries after a collision, dynamic FCS generation on a frame-by-frame basis, automatic pad field
insertion and deletion to enforce minimum frame size attributes, automatic retransmission without
reloading the FIFO, and automatic deletion of collision fragments. The MAC also provides a
mechanism for automatically inserting, deleting, and modifying IEEE 802.3ac VLAN tags.
The two primary attributes of the MAC engine are:
Transmit and receive message data encapsulation
–
Framing (frame boundary delimitation, frame synchronization)
–
Addressing (source and destination address handling)
–
Error detection (physical medium transmission errors)
Media access management
–
Medium allocation (collision avoidance, except in full-duplex operation)
–
Contention resolution (collision handling, except in full-duplex operation)
3.10.4.1
Transmit and Receive Message Data Encapsulation
The MAC engine provides minimum frame size enforcement for transmit and receive frames. When
APAD_XMT (CMD2, bit 6) is set to 1, transmit messages are padded with sufficient bytes
(containing 00h) to ensure that the receiving station will observe an information field (destination
address, source address, length/type, data, and FCS) of 64 bytes. When ASTRP_RCV (CMD2, bit 13)
is set to 1, the receiver automatically strips pad bytes from the received message by observing the
value in the length field and by stripping excess bytes if this value is below the minimum data size (46
bytes). Both features can be independently over-ridden to allow messages shorter than the minimum
frame size (less than 64 bytes of frame data) to be transmitted and/or received. The use of this feature
reduces bus utilization because the pad bytes are not transferred into or out of main memory.