Intel Extensible Firmware Interface, Specification

Page: 435 / 494

32/64-bit UNDI Specification
Version 1.02 12/12/00 417
G.4.1 Command Linking & Queuing
When linking commands, the CDBs must be stored consecutively in system memory without any
gaps in between. Do not set the Link bit in the last CDB in the list. The Link bit must be set in all
other CDBs in the list.
Linked CDBs
CDB
Set Link bit.
CDB
Set Link bit.
CDB
Do not set
Link bit.
0x00
0x1F
0x40
0x5F
0x20
0x3F
Figure G-7. Linked CDBs
When the H/W UNDI is executing commands, the State bits in the Status field in the !PXE
structure will be set to Busy (3).
When H/W or S/W UNDI is executing commands and a new command is issued, a StatCode of
PXE_STATCODE_BUSY
and a StatFlag of
PXE_STATFLAG_COMMAND_FAILURE
is set in the
CDB. For linked commands, only the first CDB will be set to Busy, all other CDBs will be
unchanged. When a linked command fails, execution on the list stops. Commands after the failing
command will not be run.
When queuing commands, only the first CDB needs to have the Queue Control flag set. If queuing
is supported and the UNDI is busy and there is room in the command queue, the command (or list
of commands) will be queued.