TOS Emulator for QDOS

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A short note on the TOS emulator for QDOS. This product which was mentioned in the history of PDQ C. For those who are not aware of it, TOS is the native operating system of the Atari ST.

It seems ironic that there should be a product that allows QDOS to emulate the Atari ST operating system (at least in part). The QL emulator for the Atari ST is one of the ways forward for those who want to keep running QDOS, but want more powerful hardware. In fact the QDOS emulator on the Atari ST is quite happy to run this TOS emulator (if you can see what I mean)!

The aim of the TOS emulator is to allow TOS binary programs to be run without alteration under the QDOS operating system. This makes a number of programs developed for TOS available in the QDOS environment. The TOS emulator restricts itself to trying to run those programs that are "well-behaved", and do not try to use the graphic capabilities specific to the Atari ST (so unfortunately it cannot run ST games).

The TOS emulator works by intercepting system calls that a TOS program would make on the TOS operating system. As on the QL, TOS programs access operating system facilities by using the TRAP instruction. Under TOS, the traps used are:

        #1   GEMDOS calls
        #13  BIOS calls
        #14  XBIOS calls

QDOS uses the TRAP values of #0 to #4. This means that the only place there is a conflict is on TRAP #1. To get around this, when it first loads a TOS program, the TOS emulator patches all TRAP #1 instructions to be TRAP #5 instructions instead. It then invokes the QDOS facility that allows TRAP calls from #5 upwards to be re-directed to user supplied routines. The effect of this is that all TOS operating calls made by the TOS program are re-directed to the TOS emulator with virtually zero run-time overhead.

The TOS emulator program then tries to map these TOS system calls onto the appropriate QDOS system calls. This approach does has some limitations however:

The most obvious one is that if the subject program ever tries to by-pass TOS then it will fail to work as it is not really running under the operating system that it thinks it is.
A linked problem will be if it tries to access TOS system variables as they will not be there.
A problem occurs if the TOS program tries to make a TOS system call that the TOS emulator does not support. The Atari ST has facilities that cannot be emulated under a QDOS environment (in particular the GEM graphics interface).
The ST has the eqivalent to the QL vector calls, and these routines (the LINE A and LINE F routines) are not supported.

However, despite these limitations the TOS emulator does succeed in running a significant proportion of Atari ST programs that do not attempt to use GEM. The most significant of these is the Atari ST version of LATTICE C (version 3.04) which was going to form the basis of PDQ C.

Jeremy Allison (who wrote the TOS emulator) has agreed to put the code for this TOS emulator into the public domain. If anyone is interested in playing with it they can download the TOS Emulator for QL source You will have to be competent in assembler programming to be able to make use of this TOS emulator, so it is not for the faint-hearted. The source is in a format suitable for input to the GST Macro Assembler.

The current version was specifically aimed at supporting those system calls used by the Atari ST Lattice C compiler v3.04, and succeeds in this admirably. More work may well be required to get other programs to work reliably. For instance it does not (yet) successfully run the newer version of Lattice C (version 5) that has been released for the Atari ST. If you do make any improvements, then please pass them back to Dave Walker

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