ABZ Teletext Editor Technical Data
		    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Teletext Page Data Format
=========================
A teletext file is a multiple of &400 bytes long, with each chunk of &400
being a single teletext frame. Each frame can contain 23 lines, 24 lines or
25 lines of text in the following layout.

 Byte
          25 lines        24 lines        23 lines
 000-039  Text line 1     Text line 1     Text line 1
 040-079  Text line 2     Text line 2     Text line 2
  etc
 880-919  Text line 23    Text line 23    Text line 23
 920-959  Text line 24    Text line 24    Title
 960-999  Text line 25    Title           Page links 0-9

1000-1019             Page links 10-14
1020-1021             Carousel timer, centiseconds
1022                  b0=CLS,   b1=Clock,   b2=Timer
1023                  Frames counter

The number of lines of text is found by counting the number of lines of
printable characters, that is, bytes larger than 31. This can be found
with the FNttx_rows() function listed below.

The title in row 24 or row 25 is terminated by a control character (<32)
and is displayed on the top screen line. A frame with 25 full lines of
text is displayed filling the full screen with no clock or counter, etc.

Page links 0 to 9 are followed when the digits 0 to 9 are pressed in the
ViewData display program. Page link 10 is the 'up' link and is followed
when '^' is pressed. Page link 11 is the 'follow-on' link and is followed
when SPACE is pressed, or the carousel counter times out.

Links 10 to 14 are also known as the INDEX, RED, GREEN, YELLOW, BLUE links
followed by pressing f0-f4.

If a page link is 0 then it does not exist, if it is a negative number it
is page -1-(link), ie -1 links to 0, -2 links to 1, etc.

If byte 920 is <32, then it is a version number stored by early versions
of the ABZ teletext editor. It will normally be a displayable teletext
character and be >31.

Some network teletext servers serve teletext pages with the BCD page
subnumber in bytes 1022-1023.



            Teletext BASIC library - Teletext data manipulation
            ===================================================
                       File: TtxtLib - Update: 1.00
                  Author: J.G.Harston - Date: 18-Feb-2010

The Teletext library provides various functions for manipulating and
displaying teletext data.

DEFFNttx_rows(address%) - returns number of rows
-------------------------------------------------
DEFFNttx_rows(D%):A%=919:REPEAT A%=A%+1:UNTIL A%>999 OR D%?A%<32:=A%DIV40

FNttx_rows examines the teletext frame in memory at address% and returns the
number of displayable rows. This will normally be 23, 24 or 25.


DEFPROCttx_display(address%) - display a teletext frame
--------------------------------------------------------
DEFPROCttx_display(D%):LOCAL B%,R%
CLS:R%=FNttx_rows(D%):IF R%<25:PRINT FNttx_title(D%,R%)
FOR A%=1TO4:B%=D%?(230*A%):D%?(230*A%)=13:PRINT$(D%+230*(A%-1));
D%?(230*A%)=B%:NEXT:IF R%<24:ENDPROC
A%=D%+959-40*(R%=25):B%=?A%:?A%=13:PRINT$(D%+920);:?A%=B%:ENDPROC

PROCttx_display displays the teletext frame in memory at address%.


DEFFNttx_title(address%, rows%) - returns frame title
------------------------------------------------------
DEFFNttx_title(D%,R%):LOCAL B%:D%=D%+920-40*(R%=24)
A%=D%-1:REPEAT A%=A%+1:UNTIL?A%<32:B%=?A%:?A%=13:A$=$D%:?A%=B%:=A$

FNttx_title returns the title of the teletext frame in memory at address%.
You have to pass the number of rows in the frame, found with FNttx_rows.


Internal variables and other settings
=====================================
Editor flags:
  Z%=&ABCDEFGH
      \/\  /\/
      |  \/ &GH - Page buffer number
      |  |
      |  &CDEF00 - ST% - start of data buffer,
      |            offset from (PAGE AND &FF000000)
      |
     &AB - maxpg% - maximum no. of pages (0-127)

!Comdat Info:  (pointed to by B%)
    00-03 in D%
    00      Flags: b0 Check files
                   b1
    01      Flags: b0 Don't ask user for date
    02      (0)    b1 Check NetFS date first
    03      (0)    b2\Number of directory
                   b3/extension characters
                   b4 Ignore NetFS time
                   b5\
                   b6 Exit option
                   b7/
    04-xx   Pages Directory
    yy      (CR)
    2B-2E   Exit page number
    2F-38   Teletext User Name
    39      (CR)
    3A-     Teletext program directory

Exit options:
  b2 - *I AM BOOT
  b1 - CH."ViewData"
  b0 - CH."Hello"

  0 - END
  1 - CHAIN "Hello"
  2 - V%=100:W%=exitpage%:CHAIN TP$+"ViewData"
  3 - unused.
  4 - *I AM BOOT,  END
  5 - *I AM BOOT,  *EXEC,  CHAIN "Hello"
  6 - *I AM BOOT,  *EXEC,  CHAIN TP$+"ViewData"
  7 - *I AM BOOT,  *EXEC,  unused.