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INT 21 - Phar Lap 386/DOS-Extender - ISSUE REAL-MODE INTERRUPT
        AX = 2511h
        DS:EDX -> pointer to parameter block (see below)
Return: all segment registers unchanged
        EDX unchanged
        all other registers contain values set by the real-mode int handler
        the flags are set as they were left by the real-mode interrupt handler
        real-mode register values are returned in the parameter block
SeeAlso: AX=2503h,2505h

Format of parameter block:
Offset  Size    Description
 00h    WORD    interrupt number
 02h    WORD    real-mode DS value
 04h    WORD    real-mode ES value
 06h    WORD    real-mode FS value
 08h    WORD    real-mode GS value
 0Ah    DWORD   real-mode EAX value
 0Eh    DWORD   real-mode EDX value
Note: all other real-mode values set from protected-mode registers