INT 1A - Intel PCI BIOS v2.0c - WRITE CONFIGURATION BYTE
        AX = B10Bh
        BL = bus number
        BH = device/function number (bits 7-3 device, bits 2-0 function)
        DI = register number (0000h-00FFh)
        CL = byte to write
Return: CF clear if successful
        CF set on error
        AH = status
            00h successful
            87h bad register number
        EAX, EBX, ECX, and EDX may be modified
        all other flags (except IF) may be modified
Notes:  this function may require up to 1024 byte of stack; it will not enable
          interrupts if they were disabled before making the call
        the meanings of BL and BH on entry may be exchanged in future
          implementations
SeeAlso: AX=B18Bh

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