INT 15 - EISA SYSTEM ROM - READ PHYSICAL SLOT
        AX = D804h
        CL = slot number (including embedded and virtual)
Return: CF clear if successful
            AH = 00h
        CF set on error
            AH = error code
                80h invalid slot number
                83h empty slot
                86h invalid BIOS-FW function call
        SI:DI = 4-byte compressed ID (DI = bytes 0&1, SI = bytes 2&3)
Note:   call with AL=84h if using 32-bit CS addressing mode instead of 16-bit