Chapter 12: 10/100 IEEE802.3 Media Access Controller
TX7901 User’s Manual (Rev. 6.30T – Nov, 2001)
12-40
12.4.3 Transmit
Descriptors
The format of the Transmit Descriptors is shown below. It is made of two doublewords, and
the fields are described in more detail in Table 12-35.
OWN
Byte-Count Buffer 1
Byte-Count Buffer 2
Control Bits
Status Bits
Buffer Address 1
Buffer Address 2
Figure 12-8 Transmit Descriptor Format
12.4.3.1 Transmit
Descriptor
The Transmit Descriptor contains the transmit frame status, the frame length, and the
Descriptor ownership information in the first word (Word 0). The 32 bits of two Transmit
Buffer Addresses is in the second word (Word 1). Bits [31:0] in Word 0 are read only. Bits
[30:0] are valid only when TxEOF is set and OWN is clear.
Table 12-35 Transmit Descriptor Field Descriptions
Word 0 Fields
Bit(s)
Field
Description
63
OWN
When set, indicates that the Descriptor is owned by the MAC. When reset, indicates that the
Descriptor is owned by the C790. MAC clears this bit either when it completes the frame
transmission or when the buffers that are allocated in the Descriptor are empty.
The Ownership bit of the first Descriptor of the frame should be set after all subsequent
Descriptors belonging to the same frame have been set. This avoids a possible race
condition between the MAC fetching a Descriptor and the driver setting an ownership bit.
62:59
-
Must be set to “0”.
58:48
Buffer1 Size (2K)
Indicates the size, in bytes, of the first data buffer. If this field is 0, the MAC ignores the
buffer and uses buffer2.
47:37
Buffer2 Size (2K)
Indicates the size, in bytes, of the second data buffer. If TxChain is set (Address Chained),
the MAC ignores this buffer and fetches the next Descriptor.
36
TxERing
Transmit End of Ring
When set, indicates that the Descriptor pointer has reached its final Descriptor. The MAC
returns to the base address of the list, creating a Descriptor ring.
35
TxChain
Second Address Chained
When set, indicates that the second address in the Descriptor is the next Descriptor
address, rather than the second buffer address. If TxERing is set, TxChain is ignored.
34
TxSOF
Start of Frame
When set, indicates that the buffer contains the first byte of a frame.
33
TxEOF
End of Frame
When set, indicates that the buffer contains the last byte of a frame.
32
TxEnCRCDes
CRC Enable
When TxDesSelEn is set, this bit overrides the TxEnCRC bit in the TFCReg.
31
–
Reserved
30:20
TxFrmLen[10:0]
Transmission Frame Length
19:15
–
Reserved
14:13
TxFrmType[1:0]
Transmit Frame Type
00 Ethernet
01 IEEE
10 VLAN I
11 VLAN II
12
TxExDefer
Excessive Deferral
When set, indicates that the transmission was aborted because of an excessive deferral as
defined by the defer bit in the transmit frame configuration register. This bit is not valid for
Summary of Contents for TMPR7901
Page 1: ...TX System RISC TX79 Family TMPR7901 Symmetric 2 way superscalar 64 bit CPU ...
Page 14: ...Handling Precautions ...
Page 15: ......
Page 17: ...1 Using Toshiba Semiconductors Safely 1 2 ...
Page 41: ...4 Precautions and Usage Considerations 4 2 ...
Page 42: ...TX7901 User s Manual Rev 6 30T November 2001 DOCUMENT NUMBER M 99 00004 07 ...
Page 43: ......
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