2.0 Functional Description
Fusion 878A
2.12 DMA Controller
PCI Video Decoder
2-36
Conexant
100600B
2.12 DMA Controller
The Fusion 878A incorporates a unique DMA controller architecture that gives
the capture system great flexibility in its ability to deliver data to memory. It is
designed as a small RISC engine that runs on a set of instructions generated and
maintained in host system memory by the Fusion 878A device driver software.
The video and audio DMA controllers are identical except that the audio DMA
controller does not support planar mode instructions.
In this architecture, the DMA can dynamically change target memory address
from one video line to the next. This enables multiple memory targets to be
established for various components of each video frame. For example, an NTSC
video frame contains four discrete components which require separate target
memory locations:
•
Even-field video image data
•
Odd-field video image data
•
Line 21 closed captioning data
•
Line 15 teletext data
The Fusion 878A DMA can concurrently support a display memory target for
the even-field image and three separate system memory targets for the odd-field
image, line 21 data, and line 15 data images respectively.
The Fusion 878A device driver software creates a RISC program which runs
the DMA controller. The RISC program resides in host system memory. Through
the use of the PCI target, the RISC program puts its own starting address in a
Fusion 878A register and makes it available to the DMA controller. The DMA
controller then requests the PCI initiator to fetch an instruction. The RISC
instructions available are WRITE, SKIP, SYNC, and JUMP.
Decoded composite video data is stored in the Fusion 878A FIFO. The DMA
controller then presents the data to the PCI initiator and requests that the data be
output to the target memory. The PCI initiator outputs the pixel data on the PCI
bus after gaining access to the PCI bus. It is the responsibility of the DMA
controller to prevent and manage the overflow of the Fusion 878A FIFOs. This
process is illustrated in
.