Intel PCI User Manual
Software Developer’s Manual
303
Register Descriptions
13.4.27
Receive Descriptor Length
RDLEN (02808h; R/W)
This register determines the number of bytes allocated to the circular receive descriptor buffer. This
value must be 128-byte aligned (the maximum cache line size). Since each descriptor is 16 bytes in
value must be 128-byte aligned (the maximum cache line size). Since each descriptor is 16 bytes in
length, the total number of receive descriptors is always a multiple of eight.
Table 13-72. RDLEN Register Bit Description
13.4.28
Receive Descriptor Head
RDH (02810h; R/W)
This register contains the head pointer for the receive descriptor buffer. The register points to a 16-
byte datum. Hardware controls the pointer. The only time that software should write to this register
byte datum. Hardware controls the pointer. The only time that software should write to this register
is after a reset (RCTL.RST or CTRL.RST) and before enabling the receiver function (RCTL.EN).
If software were to write to this register while the receive function was enabled, the on-chip
If software were to write to this register while the receive function was enabled, the on-chip
descriptor buffers can be invalidated and other indeterminate operations might result. Reading the
descriptor head to determine which buffers are finished is not reliable.
Table 13-73. RDH Register Bit Description
31
20 19
7 6
0
Reserved
LEN
0
Field
Bit(s)
Initial
Value
Value
Description
Zero
6:0
0b
Zero value
Ignore on write. Reads back as 0b.
Ignore on write. Reads back as 0b.
LEN
19:7
0b
Receive Descriptor length
Provides the number of receive descriptors (in a multiple of eight).
Provides the number of receive descriptors (in a multiple of eight).
Reserved
31:20
0b
Reserved
Should be written with 0b to ensure future compatibility.
Reads as 0b.
Should be written with 0b to ensure future compatibility.
Reads as 0b.
31
16 15
0
Reserved
RDH
Field
Bit(s)
Initial
Value
Value
Description
RDH
15:0
0b
Receive Descriptor Head.
Reserved
31:16
0b
Reserved. Should be written with 0b for future compatibility.
Reads as 0b.
Reads as 0b.