Descriptor Overview; Descriptor Endianness; Descriptors - Altera Cyclone V Device Handbook

17-36

Descriptor Overview

Descriptor Overview
This section describes the HPS EMAC DMA descriptor.
The DMA in the Ethernet subsystem transfers data based on a single descriptor, as explained in
DMA Controller. The descriptor is created in the system memory.
The descriptor format can have 8 DWORDS (32 bytes). The descriptor contains two buffers, two byte-count
buffers, and two address pointers, which enable the adapter port to be compatible with various types of
memory management schemes.
Related Information
• DMA Controller
•

Descriptors

Detailed bit map of the descriptor structure

Descriptor Endianness

The descriptor addresses must be aligned to the used bus width (Word for 32-bit bus). The data bus is
configured as little-endian.
Related Information
Descriptors on page 17-36
Detailed bit map of the descriptor structure
Descriptors
The descriptor structure can have 8 DWORDS (32-bytes). The features of the descriptor structure are:
• The descriptor structure is implemented to support buffers of up to 8 KB (useful for Jumbo frames).
• There is a re-assignment of control and status bits in TDES0, TDES1, RDES0 (advanced timestamp or
IPC full offload configuration), and RDES1.
• The transmit descriptor stores the timestamp in TDES6 and TDES7 when you select the advanced
timestamp.
• This receive descriptor structure is also used for storing the extended status (RDES4) and timestamp
(RDES6 and RDES7) when advanced timestamp, IPC Full Checksum Offload Engine, or Layer 3 and
Layer 4 filter feature is selected.
• You can select one of the following options for descriptor structure:
• If timestamping is enabled in Register 448 (Timestamp Control Register) or Checksum Offload is
enabled in Register 0 (MAC Configuration Register), the software needs to allocate 32-bytes (8
DWORDS) of memory for every descriptor. For this, the software should set Bit 7 (Descriptor Size)
of Register 0 (Bus Mode Register).
• If timestamping or Checksum Offload is not enabled, the extended descriptors (DES4 to DES7) are
not required. Therefore, the software can use descriptors with the default size of 16 bytes (4 DWORDS).
For this, the software should reset Bit 7 (Descriptor Size) of Register 0 (Bus Mode Register) to 0.
• When descriptor is selected without Timestamp or Receive IPC Full Checksum Offload Engine (Type 2)
feature, the descriptor size is always 4 DWORDs (DES0-DES3). Therefore, the software can use descriptors
with the default size of 16 bytes.
Altera Corporation
on page 17-23
on page 17-36
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Ethernet Media Access Controller
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2013.12.30