DOS-Forum

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Full operating mode
Full mode supports up to 5 IDE controllers (10 drives). Full mode reserves a bit of RAM from top of base memory. This makes possible to use ROM Basic and software that requires top of interrupt vectors where XTIDE Universal BIOS parameters would be stored in lite mode. Lite mode supports only one IDE controller (2 drives) and stores parameters to top of interrupt vectors (30:0h) so no base RAM needs to be reserved. Lite mode cannot be used if some software requires top of interrupt vectors. Usually this is not a problem since only IBM ROM Basic uses them. Tandy 1000 models with 640 kiB or less memory need to use lite mode since top of base RAM gets dynamically reserved by video hardware. This happens only with Tandy integrated video controller, not with expansion graphics cards. It is possible to use full mode if reserving RAM for video memory + what is required for XTIDE Universal BIOS. This would mean 129 kiB but most software should work with 65 kiB reserved.

kiB to steal from RAM
Parameters for detected hard disks must be stored somewhere. In full mode they are stored to top of base RAM. At the moment 1 kiB is always enough but you might want to steal more if you want to use full mode with Tandy 1000. This menu item appears only when full operating mode is enabled.