RBIL61 - Flags
The use of -> instead of = signifies that the indicated register or register
pair contains a pointer to the specified item, rather than the item itself.
Register pairs (such as AX:BX) indicate that the item is split across the
registers, with the high-order half in the first register.

One or more letters may follow the interrupt number; they have the following
meanings:  U - undocumentedInformation about a product which is not publicly available from the manufacturer, and must be determined by reverse-engineering (disassembly, trial-and-error, etc.).	 Undocumented information tends to change -- often dramatically -- between successive revisions of a product, since the manufacturer has no obligation to maintain compatibility in behavior which is not explicitly stated. function, u - partially documented function,
P - available only in protected mode, R - available only in real or V86see Virtual-86 Mode mode,
C - callout or callback (usually hooked rather than called),
O - obsolete (no longer present in current versions)