Most QL software is well written, and provided that you use standard SuperBASIC commands and make no assumptions such as about addresses in memory, or the size and location of the screen, it would appear to work happily on all different QL and QDOS compatible set-ups. Unfortunately, as always, there are some exceptions to this rule, and the areas which appear in the following sections would appear to cause the greatest problems:
As the QL was developed, the designers tried to leave everything open-ended so that nothing could be taken for granted. However, towards the end of 1984 (the first year of the QL’s long history) things appeared to have settled down with the JS ROM in the UK and USA, and MG ROM version elsewhere. Without any later ROMs in sight, software writers got very lazy and rather than write a few lines of machine code to check for the address for various things (such as the start of the screen): they assumed that it would always remain where it had been, and so started to use absolute addressing.
Likewise, programmers assumed that the QL’s operating system would always appear in ROM and they could therefore write routines which tried to overwrite parts of memory, regardless of whether they were pointing to ROM or RAM - after all, it could not harm the operating system as you cannot write to ROM, can you (!).
Another problem are programs which use RESPR to reserve memory for themselves at a particular place - the boot programs typically include lines such as:
10 Addr=273102 20 A=RESPR(0):A=RESPR(A-Addr) 30 LBYTES flp1_Program_cde,Addr:CALL Addr
Unfortunately, later developments in the QL world, namely emulators and the Gold Card (for example), have moved the QL’s operating system into RAM, meaning that it can be overwritten, thus crashing the whole machine. Parts of memory are moved around freely, making use of the calls incorporated into QDOS by the QL’s designers and even the speed of the QL has altered. Despite the best attempts of the manufacturers of the Gold Card and emulators, this has resulted in one or two incontestabilities with older software.
Both QDOS emulators and the Gold Card have greatly increased the speed at which the QL works, making some programs unusable. Luckily the effects of this are limited by various commands which slow the operating speed of emulators and the Gold Card down.
A6.3 The Operating System
Some software in the market was written with specific versions of the QL ROM in mind. For example, one program which was quite a useful SuperBASIC utility, would appear to only work on JS and MG ROMs. This software can only become redundant as more and more users upgrade their systems to take advantage of the latest developments in the QDOS operating system.
Some older software is not address independent, which means that it has to be loaded at a specific place in memory. This can prove impossible on machines with expanded memory, but luckily commands do exist to reduce the memory size. We have however come across one program, which although it is address independent, refuses to work correctly whenever the system on which it is running has anything more than the QL’s original 128K (even if only 128K is set aside for use by the program).
A6.5 The Stack Pointer
As any machine code programmer will be aware, the processor’s address register a7 is used by the operating system as the stack pointer. Some software attempts to set this to an absolute address when the program begins (even though there is no need for this). If such a program is not executed as a task, then it is likely to fall over on Minerva and SMSQ/E.
SuperBASIC compilers are an excellent means of getting the best of two worlds: the flexibility and clarity of a SuperBASIC program, but with the speed of machine code. Unfortunately, although the SuperBASIC compilers have mainly kept pace with the development of QDOS, some software originally compiled with earlier versions of compilers has not been upgraded, meaning that it may not be compatible with the latest ROM versions.
There are two types of SuperBASIC compiler which have been produced for the QL, namely true compilers (Turbo and Supercharge) which produce independent machine code, not relying in any way on SuperBASIC structures (and the code produced is therefore much more portable between different QDOS machines) and so-called pseudo compilers (Qliberator) which produce machine code which still uses SuperBASIC structures and calls.
Although the former produce much quicker code, they are not as versatile as the latter as they expect SuperBASIC commands to be used in a certain manner and cannot therefore recognise the enhancements introduced to SuperBASIC commands by Minerva SMSQ/E and emulators.
There is also a problem in that programs compiled with Turbo will not work on versions 2.25 - 2.31 of SMSQ. You will also find that some Turbo compiled programs will not work if started from a copy of BASIC other than Job 0 (on both SMS and Minerva) and also neither compiler can currently compile a program which from within a multiple copy of BASIC.
Patch programs have been released which enable Turbo compiled programs to be used on a system which has a large amount of memory (such as the Q40). Later versions of the Turbo Toolkit (v3d27 and later) are also required for better compatibility.
However, one of the remaining problems with SMSQ/E which is displayed by Turbo compiled programs is the failure of the in- built Integer to ASCII conversion routine to cope with negative integers. This means that Turbo compiled programs remain unreliable on current versions of SMSQ/E - refer to PRINT for an example.
A6.7 High Resolution Displays
More and more QL implementations are now able to use much higher screen resolutions than the original 512x256 pixels (or 256x256 pixels in MODE 8). Unfortunately many programs were written before this facility was available and therefore will not work correctly on higher resolution displays. Even if they do work, the program may be confined to a small section of the screen, normally the top left hand corner, and use fonts which are much too small to read.
The only answer to this (unless a new version of the program is released) is to use a lower resolution screen (see DISP_SIZE) or to put up with the slight inconvenience. Aurora has other problems too - see Appendix 5.3 for more details.
You should however be aware that on some monitors, a lower resolution screen may still not fill up the whole area shown by the monitor - this does not cause a problem in itself and there is nothing you can do about it - it is due to the difference in ratio between 512x256 pixel displays and standard PC (or ATARI / MAC / UNIX …) displays.
Appendix 16.4 is also of interest in connection with High Resolution Displays.
A6.8 String Lengths
The maximum length of strings varies on each QL implementation (even though you can use DIM to dimension a string up to 32767 characters, this does not mean that you will be able to use all of those characters!!). SMSQ/E allows a maximum string length of 32765 characters, whilst Minerva allows a maximum string length of 32764. QDOS ROMs allow a maximum of 32766 characters. On the other hand FILL$ is allowed to be used to create slightly different string lengths - on SMSQ/E this is 32764 characters, on Minerva 32767 (except on v1.98 where a limit of 32764 characters was implemented) and on QDOS ROMs, FILL$ can produce strings up to 32767 characters long.
The outcome of this is that the maximum length that should be used for a string should be 32764 characters which is the limit imposed by the Turbo compiler.
A6.9 Later Processors & Gold Cards
Various QL implementations use a chip as the main processor which is not a 68008 chip (the chip that the QL was originally designed to use). These later chips have various facilities, such as caches which can cause problems with some software (see CACHE_OFF). You may also note that some leisure software does not work on a GOLD CARD QL - this is normally where the command SCR2DIS has been issued - some leisure software insists that the second screen is enabled!!
Specific points to watch out for on the different QDOS implementations are covered in the appendices dealing with each one.
There are also various points explained in the main keywords section of this book.