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Intel 8255x 10/100 Mbps Ethernet Controller Family Open Source Software Developer Manual 39
Host Software Interface
6.3.2.3 SCB General Pointer
The SCB General Pointer is a 32-bit entity, which points to various data structures depending on
the command in the CUC or RUC field. The two tables below indicate what the SCB pointer means
for the different commands.
Bits 18:16 RUC
This field contains the RU Command. Valid values are:
000 NOP. NOP is a no operation command and does not alter current state of
unit.
001 RU Start. RU Start enables the receive unit. The pointer to the RFA must be
placed in the SCB General Pointer before using this command. The device pre-
fetches the first RFD in preparation of receiving incoming frames that pass its
address filtering.
010 RU Resume. The RU Resume command resumes frame reception (only
when in suspended state).
011 Receive DMA Redirect. This command is only valid for the 82558 and later
devices. The buffers are indicated by an RBD chain, which is pointed to by an
offset stored in the general pointer register (in the RU base).
100 RU Abort. The RU Abort command immediately stops RU receive
operation.
101 Load Header Data Size (HDS). After a load HDS command is issued, the
device expects to only find header RFDs or to be used in Receive DMA mode
until it is reset. This value defines the size of the header portion of the RFDs or
receive buffers. The HDS value is defined by the lower 14 bits of the SCB
General Pointer; thus, bits 15 through 31 should always be set to zeros when
using this command. The value of HDS should be an even non-zero number.
110 Load RU Base. The internal RU Base Register is loaded with the value that
was placed in the SCB General Pointer just before this command was issued.
Table 14. SCB Command Word Bits Descriptions
Bit Symbol Description
Table 15. SCB General Pointer for the CU Command
RUC
Field
RU Command SCB General Pointer Added to
0 NOP Don’t care
1 CU Start
Pointer to first command block in the command
block list
CU Base
2 CU Resume Don’t care
3 CU HPQ Start
Pointer to first command block in the HPQ
command block list
CU Base
4
Load Dump
Counters Address
Absolute address written to by Dump Counters
and Dump & Reset Counters commands
5 Dump Counters Don’t care
6 Load CU Base 32-bit Base Register for CU data structures
7
Dump & Reset
Counters
Don’t care
10 CU Static Resume Don’t care
11 CU HPQ Resume Don’t care