Advanced Teletext System User Guide

5 Teletext OSWORD Calls
5.1 Introduction
Teletext OSWORD calls allow you access to the facilities of Teletext from
your own assembly language programs, and to access some of the lower level
facilities available with the Teletext system.
The basis of the Teletext system is the 'page grabber'. This is a piece of
machine code which constantly looks for Teletext pages. It can look for up to
20 pages, automatically storing them in the designated area of memory when it
finds them. It also looks for, and stores, the TSDP. The Teletext OS commands
only use one of the page grabber's 20 records to grab pages requested with
*PAGE. From assembly language however it is possible to use all 20, as the
Terminal does.
All the facilities of the Teletext system are controlled by one OSWORD call
&7A. The first byte of the parameter block for an OSWORD &7A is a
command code. The commands are described in Section 5.2.
Teletext System Control
OSWORD with A = &7A
The call address is &FFF1
On entry, X (lo-byte) and Y (hi-byte) point to the parameter block:
Important: Before you use the Teletext OSWORD call, you must issue a
*TTXON command, or an OSWORD &7A with command = 13, to tell the
Teletext Adaptor to start accepting Teletext data. When you have finished with
Teletext, issue a *TTXOFF command, or OSWORD &7A with command = 12,
and command = 14, to switch off the Adaptor's Teletext reception.
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