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Function 4B03h          Load Overlay

    Loads a program as an overlay. DOS loads the overlay into memory
    already allocated by the program.

        Entry   AX    = 4B03h
                DS:DX = Pointer to ASCIIZ program name
                ES:BX = Pointer to a LoadOverlay structure

        Return  Expect all registers to be changed, except CS:IP
                AX = Error code, if CF is set
                     | 01h Invalid function number
                     | 02h File not found
                     | 03h Path not found
                     | 04h No handle available
                     | 05h Access denied
                     | 08h Insufficient memory
                     | 0Ah Invalid environment

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    Before function 4B03h is invoked, DS:DX must point to a zero-
    terminated string that specifies the program to load. The program
    name must be a valid DOS filename. ES:BX must point to a LoadOverlay
    structure.