When ResiDOS starts up, it displays the current mode on the title screen. If you have a 48K Spectrum, then 48K mode is the only possibility. For 128K Spectrums, ResiDOS will usually start up in 128K mode.
In ResiDOS, 128K mode behaves in the same way as what is often referred to as USR 0 mode; this mode is usually required by demos, and modern programs from Russia and other countries. (Normally, 128K Spectrums enter this mode by typing USR 0 in 128K BASIC).
This mode gives access to all the special hardware of the 128K machines (the sound and extra memory), but without access to the 128K BASIC editor and commands. Under ResiDOS, the 128K RAMdisk commands and SPECTRUM command are also available (in fact, they also work in 48K mode).
This mode is recommended for loading 128K programs. Some 128K programs will not load in this mode (since they check to see if 128K BASIC is active). In such cases, you must disable ResiDOS and restart into 128K BASIC with the %ZX command.
IMPORTANT NOTE: The current version of the Task Manager does not understand that some tasks might run in 128K mode and use the whole 128K of memory, and will not save 128K snapshots. It is possible to have a single task loaded which uses the extra memory of the 128K Spectrum, and switch between that and other tasks. However, if two 128K programs are loaded, there will be problems. This will be addressed in a future version.
Some games will not work correctly in 128K mode. For these you can use the SPECTRUM command to force the computer into 48K mode until the next reset.
If you always want to work in this mode, the command SPECTRUM+ will make 48K mode active every time the computer is switched on. To disable this mode, and re-enter 128K mode on the next reset, use the SPECTRUM- command.
It is possible to temporarily disable ResiDOS and then re-enable it, whilst leaving any program in memory unchanged. This can be useful if you have some piece of hardware which is incompatible with ResiDOS, such as Microdrives, or a Wafadrive etc. It is also useful for switching from the %ZX mode's 128K BASIC back into ResiDOS without having to switch off.