Free Frame Descriptor And Buffer Descriptor Pools - Motorola MC68824 User Manual

II
Lastly, the TBC will set the TXDF bit in interrupt status word 0 (transmitted data frame), and
generate an interrupt if enabled.
Once the host has checked that the frame has been processed (the CFD bit is set), it clears the
confirmation indication word and either links this FD to the end of the free frame descriptors pool,
uses it in the next transmit frame, or frees up memory space.
4.3 RECEPTION OF FRAMES
Every frame detected on the token bus medium has its destination address field checked by each
station to see if that station should receive it. The following paragraphs describe the setting up
which is required by the host to allow the TBC to store a frame, as well as the mechanism used
by the TBC to do so.
4.3.1 Initialization Performed by Host
In order to allow the TBC to store the received frames, the host must set up free frame descriptor
and buffer descriptor pools as shown in the example of Figure 4-6.
To enable the TBC to store incoming frames, the host must prepare the following data structures:
1. The free frame descriptor pool pointer located in the private area must be updated at ini-
tialization time or through the SET TWO WORDS command. This pointer must point to a
valid free FD. Refilling of the FD pool is accomplished by writing the pointer to a new FD in
the last FD.
PRIVATE AREA
FREE FRAME DESCRIPTOR
POOL POINTER
FREE BUFFER DESCRIPTOR
POOL POINTER
FREE BUFFER
DESCRIPTOR POOL
FREE FRAME
DESCRIPTOR
POOL
Figure 4-6. Free Frame Descriptor and Buffer Descriptor Pools
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MC68824 USER'S MANUAL