EP80579 Datasheet, PDF (1376/1916 Pages) Intel Corporation – Intel® EP80579 Integrated Processor Product Line

Intel^® EP80579 Integrated Processor
Figure 37-29.Transmit Descriptor Ring Structure
Circular Buffer
Base
Head
Transmit
Queue
Tail
Base + Size
The transmit descriptor ring is described by the following registers:
The Transmit Descriptor Base Address High Register and the Transmit
Descriptor Base Address Low Register (TDBAH and TDBAL) - These registers
indicate the start of the descriptor ring buffer in the host memory; this 64-bit
address is aligned on a 16B boundary. Hardware ignores the lower 4 bits.
Transmit Descriptor Length Register (TDLEN) - This register determines the
number of bytes allocated to the circular buffer. This value must be 0 modulo 128.
Transmit Descriptor Head Register (TDH) - This register holds a value which is
an offset from the base, and indicates the in-progress descriptor. There can be up
to 64K descriptors in the circular buffer. Reading this register returns the value of
“head” corresponding to descriptors already loaded in the output FIFO.
Transmit Descriptor Tail Register (TDT) - This register holds a value which is
an offset from the base, and indicates the location beyond the last descriptor
hardware can process. This is the location where software writes the first new
descriptor.
The base register indicates the start of the circular descriptor queue and the length
register indicates the maximum size of the descriptor ring. The lower seven bits of
length are hard-wired to 0. Byte addresses within the descriptor buffer are computed
as follows: address = base + (pointer * 16), where pointer is the value in the hardware
head or tail register.
The size chosen for the head and tail registers permit a maximum of 64K descriptors,
or approximately 16K packets for the transmit queue given an average of four
descriptors per packet.
Once activated, hardware fetches the descriptor indicated by the hardware head
register. The hardware tail register points one beyond the last valid descriptor. Software
reads the head register to determine which packets (those logically before the head)
have been transmitted.
Intel^® EP80579 Integrated Processor Product Line Datasheet
1376
August 2009
Order Number: 320066-003US