A register pair is a pair of register bound together with a colon and used as a single value or parameter.

Usually to provide a 32-bit number or a memory pointer using two 16-bit registers. The first register in the pair is the high portion (most significant) of the pair. And the second register is the low portion (least significant).

Here are some examples:

    ES:DI   ; probably a pointer to memory segment ES at offset DI
    DX:AX   ; maybe a pointer or 32-bit number.
    CX:BX   ; don't think I use this much in the library, probable a 32-bit number.
    EDX:EAX ; why are you trying to use 32-bit 386 registers for 64-bit numbers?
    GS:EDI  ; other than extensions, the library really does not like 386 parameters :-(