ADVANCE INFORMATION
VCT 38xxA
The DISPLAY_PAGE_REQUEST command searches
and displays a page according to the requested dis-
play subcode. The search starts from page table and
continues through the subpage chain if there is any. A
rolling header will be displayed if the requested sub-
page cannot be found in memory.
3.6. WST Display Controller
The display controller reads data from a display page
buffer in the internal/external SRAM. The display page
buffer is organized in rows which are separated into
level 1 data such as character codes and spacing
attributes and into level 2 data, such as character set
extension and non-spacing attributes. To limit the
memory amount for level 2 data, a slightly modified
stack model is used, in which one pointer bit for every
character location indicates the presence of additional
parallel attributes. Fig. 3–9 shows the organization of
the stack row buffer. In this stack model the number of
non-spacing attributes per row is limited to 40, which
agrees with the WST and CEPT specification.
A requested display subcode FFFF (don’t care sub-
code) will only search and display the first subpage in
chain, thus there is no rolling subpage anymore. A
DISPLAY_PAGE_REQUEST command with subcode
F0xx (follow subcode) will search and display the last
received subpage in chain, thus it is possible to
request all subpages in background while still showing
rolling subpages in display.
subpage
pointer
Page Table
Level 2 Buffer
Char 3 Attr.
Level 1 Buffer
Pointer
page 100
subcode 0003
page pointer
0
0
1
0
0
1
0
0
0
1
1
Char 1
Char 2
Char 3
Char 4
Char 5
Char 6
1 Char 3 Attr.
Char 3 Attr.
1
0
1
1
0
1
Char 3 Attr.
Char 6Attr.
Char 6 Attr.
Char 6 Attr.
Char 10 Attr.
subpage
pointer
page 100
subcode 0001
Char 7
Char 8
Char 9
Char 10
1 Char 10 Attr.
Char 10 Attr.
Char 36 Attr.
1
0000
Char 36
Char 37
Char 38
Char 39
Char 40
0
0
0
0
0
1
0
0
0
0
page 100
subcode 0002
Fig. 3–8: Subpage organization
Fig. 3–9: Stack Row Buffer
The display controller includes two row buffers. The
first row buffer holds a copy of a teletext row from the
display page buffer. This decreases the data rate
through the SRAM interface by a factor of 10 or 8,
because new teletext row data is needed only after
10 lines in PAL or 8 lines in NTSC mode. The second
row buffer stores all display attributes in parallel, to
allow level 2 display without additional decoding.
Micronas
53
MICRONAS [ MICRONAS ]
