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=NoCls, b1=NoClock, b2=NoTimer
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.

If a page link is 0 then it does not exist, if it is -1 then it is a link
to page 0.

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.


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

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

  0 - END	
  1 - CHAIN "Hello"	
  2 - V%=100:W%=expg%: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.

Editor flags:
  Z%=&ABCDEFGH
      \/\  /\/
      |  \/ &GH - Page number
      |  |
      |  &CDEF00 - ST% - start of data buffer 
      |
     &AB - maxpg% - maximum no. of pages (0-127)