82596DX Datasheet, PDF (30/77 Pages) Intel Corporation – HIGH-PERFORMANCE 32-BIT LOCAL AREA NETWORK COPROCESSOR

82596DX SX
RECEIVE UNIT (RU)
The Receive Unit is the logical unit that receives frames and stores them in memory The RU is modeled as a
logical machine that takes at any given time one of the following states
 Idle The RU has no memory resources and is discarding incoming frames This is the initial state
 No Resources The RU has no memory resources and is discarding incoming frames This state differs
from Idle in that the RU accumulates statistics on the number of discarded frames
 Suspended The RU has memory available for storing frames but is discarding them The suspend state
can only be reached if the CPU forces it through the SCB or sets the suspend bit in the RFD
 Ready The RU has memory available and is storing incoming frames
The CPU can affect RU operation in three ways by issuing a RU Control Command by setting bits in the
Frame Descriptor Command word of the frame being received or by setting the EL bit of the current buffer’s
Buffer Descriptor
When programming the 82596 RU it is important to consider the asynchronous way the 82596 processes
receive frames If an RU Start is issued to the 82596 RU it may be busy processing other incoming packets In
order to avoid asynchronous race conditions the following guidelines are recommended to the 82596 pro-
grammer
 If the RU is already in the Ready state and a new RFA is required to be started it is unwise to immediately
issue another RU Start command If the new RFA needs to be started first issue an RU Suspend com-
mand wait for the RU to become Suspended then issue the new RU Start This will insure that all incoming
frames are received correctly
 In general it is a good idea to make sure any RU command has been accepted and executed before
issuing a new control command to the RU
SYSTEM CONTROL BLOCK (SCB)
The SCB is a memory block that plays a major role in communications between the CPU and the 82596 Such
communications include the following
 Commands issued by the CPU
 Status reported by the 82596
Control commands are sent to the 82596 by writing them into the SCB and then asserting CA The 82596
examines the command performs the required action and then clears the SCB command word Control
commands perform the following types of tasks
 Operation of the Command Unit (CU) The SCB controls the CU by specifying the address of the Command
Block List (CBL) and by starting suspending resuming or aborting execution of CBL commands
30