L64105 LSI Logic Corporation, L64105 Datasheet - Page 272

• Manufacturer Part Number: L64105
• Description: Mpeg-2 Audio/video Decoder
• Manufacturer: LSI Logic Corporation

8.4 Frame Store Modes
8.4.1 Normal (3-Frame Store) Mode
8-30
This section describes how frame stores are organized in the available
modes. Frame stores are maintained in the external SDRAM. The Video
Decoder decodes macroblocks from the Video ES Channel Buffer and
writes them to the frame stores as reconstructed pictures. Depending on
the bitstream, there are three store modes:
In the usual case, there are two frame stores used for decoding I
(intracoded) and P (forward predictive coded) pictures. These are
referred to as “anchor frame stores” in the description that follows. In
addition, there is a third frame store used for decoding B frames. The B
frames are decoded by performing motion compensation using the two
anchor frames as references (previous picture and future picture). The
sizes and locations of all three frame stores in the external SDRAM are
programmable and are set by the host through registers. The sizes of the
anchor frame stores are sufficient to hold the entire I or P frame. The
size of the third frame store, which is used for decoding B pictures, varies
depending on the decoding mode (i.e., normal mode or PAL Reduced
Memory Mode). You should understand the various restrictions on the
use of these modes. These restrictions are described in the following
sections. To aid the host in implementing system control functions, status
bits are provided that indicate which frame store is currently being used
for picture reconstruction and which frame store is currently being used
for display.
The on-chip Display Controller interfaces to the external SDRAM and
displays the pictures that the Video Decoder reconstructs.
Figure 8.4
indicates the timing relationship between the reconstruction of frames
from the incoming MPEG-1/MPEG-2 bitstream and the display of those
frames by the Display Controller.
Video Decoder Module
Normal or 3-Frame Store Mode for most MPEG streams
Reduced Memory Mode (RMM) for high-resolution pictures like PAL
(720 x 576)
2-Frame Store Mode for bitstreams without B pictures
depicts the organization of the three frame stores. It also