User Manual
BCM1250/BCM1125/BCM1125H
10/21/02
B r o a d c o m C o r p o r a t i o n
Document
1250_1125-UM100CB-R
Section 7: DMA Page
153
Figure 27: DMA Descriptor Chain
shows a descriptor chain. Each descriptor points to a single buffer, and contains a pointer to the next
descriptor. Ownership of descriptors is passed in the same way as ring mode, the count refers to the number
of valid descriptors. The last valid descriptor must contain a link pointer to the location where software will place
the next descriptor, when the count reaches zero the controller will remember the link value and fetch the new
descriptor from that address when the count becomes non-zero.
In both ring and chain modes reading the
dma_cur_dscr_addr
register gives address of the descriptor
currently being processed and the current count of descriptors owned by the controller. If the count field in this
register is non-zero then the address is of the descriptor that the controller is currently using. If the count is
zero then the address is where the controller will fetch the next descriptor from when the count becomes non-
zero.
Descriptor
Buffer
Size
Current
Cou
nt