
Frame Store Modes
8-33
The host enables RMM by setting the Reduced Memory Mode bit in
Register 248
(page 4-78). This register is read by the Video Decoder
only when it encounters an anchor frame (I or P picture). The host
determines the amount of memory allocated to the B frame store by
writing a value into the Number of Segments in RMM eld in
Register 289
(page 4-89). Each segment consists of a frame store for
eight lines of the frame. The minimum and maximum number of
segments can be calculated using the following formulas:
For a full-size PAL image, the minimum number of segments is 40. If
sufcient SDRAM memory is available, the recommended number of
segments for adequate performance is 44. The maximum number of
segments for a PAL image is 54.
Allocating more segments than the minimum will always boost decoder
performance by allowing “decode ahead” without waiting for the display
process to free up segments. Allocating extra segments is especially
recommended for applications, like letterbox display, which demand
higher decoder performance.
There are certain restrictions which must be followed when RMM is used.
SDRAM memory is dynamically allocated in this mode, and the SDRAM
memory used to reconstruct the lines of the rst eld are reallocated to
reconstruct other lines once the rst eld lines have been displayed.
3:2 pulldowns (Repeat_rst_eld parameter in picture coding extension)
will be processed while in RMM. However, it should be noted that the
actual display in this case will be; First Field, Last Field, Last Field. Thus,
the Repeat First Field is actually implemented as a Freeze Last Field
during RMM while displaying B-pictures.
Another restriction is the display modes that are allowed with RMM. Only
the following modes may be used in conjunction with RMM:
Display Mode 4: Interlaced Chroma Field Repeat and No Filter
Display Mode 5: Interlaced Chroma Field Repeat and Filter
Display Mode 6: Interlaced Chroma Line Repeat
Display Mode 7: Interlaced Chroma Line Repeat and Filter
Min NumSegments
Total Lines
8
-----------------------------
4
+
=
Total Lines
8
-----------------------------
1
–
Max Num Segments
54
≤≤