INT 7A - IBM 3270 Workstation Program API - KEYBOARD SERVICES
        AH = 09h
        BX = 8020h (synchronous request)
        CX = 0000h
        DX = ID of keyboard manager
        AL = service
            01h connect to keyboard
            02h disconnect from keyboard
            03h read from keyboard
            04h send keystroke to session
            05h disable input
            06h enable input
            07h update status code
        ES:DI -> control block
Return: ???

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