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Use BIOS INT 10h function 13h with correct far pointers, character counts, attributes, page selection, cursor flags, and adapter compatibility.
Understand the AT BIOS block-move descriptor table, 24-bit addresses, transfer limits, status flags, and why XMS or a DPMI host usually owns memory.
Use the BIOS wait service with checked status, realistic timing precision, bounded fallbacks, and clear separation from application deadlines.
Use BIOS INT 16h blocking and polling calls without dropping enhanced keys, confusing scan codes, or stealing keyboard state from DOS and resident software.
Use INT 10h video services while preserving mode, active-page, cursor, and adapter state instead of assuming every PC BIOS behaves like VGA.
Distinguish IBM-compatible warm reboot flags from INT 19h bootstrap calls, DOS cache state, cold reset behavior, and machine-specific firmware paths.
Build a robust DOS application around INT 33h: probe safely, interpret text and graphics coordinates, track button edges, and use callbacks carefully.
Distinguish processor exceptions from DOS interrupts, understand the original 8086 vector contract, and build handlers that preserve state and return safely.