Intel Intel Gigabit Ethernet Controllers, PCI-X manual Receive Descriptor Ring Structure

Receive and Transmit Description

The receive descriptor head and tail pointers reference 16-byte blocks of memory. Shaded boxes in the figure represent descriptors that have stored incoming packets but have not yet been recognized by software. Software can determine if a receive buffer is valid by reading descriptors in memory rather than by I/O reads. Any descriptor with a non-zero status byte has been processed by the hardware, and is ready to be handled by the software.

Circular Buffer Queues

Base

Head

Owned By Hardware

Receive Queue

Tail

Base + Size

Figure 3-2. Receive Descriptor Ring Structure

Note: The head pointer points to the next descriptor that is written back. At the completion of the descriptor write-back operation, this pointer is incremented by the number of descriptors written back. HARDWARE OWNS ALL DESCRIPTORS BETWEEN [HEAD AND TAIL]. Any descriptor not in this range is owned by software.

The receive descriptor ring is described by the following registers:

•Receive Descriptor Base Address registers (RDBAL and RDBAH)

These registers indicate the start of the descriptor ring buffer. This 64-bit address is aligned on a 16-byte boundary and is stored in two consecutive 32-bit registers. RDBAL contains the lower 32-bits; RDBAH contains the upper 32 bits. Hardware ignores the lower 4 bits in

RDBAL.

•Receive Descriptor Length register (RDLEN)

This register determines the number of bytes allocated to the circular buffer. This value must be a multiple of 128 (the maximum cache line size). Since each descriptor is 16 bytes in length, the total number of receive descriptors is always a multiple of 8.

•Receive Descriptor Head register (RDH)

This register holds a value that is an offset from the base, and indicates the in–progress descriptor. There can be up to 64K descriptors in the circular buffer. Hardware maintains a shadow copy that includes those descriptors completed but not yet stored in memory.

Software Developer’s Manual 27