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        Port 021  -  8259A Interrupt Mask Register

        0=interrupt enabled, 1=interrupt disabled.

        1xxx xxxx  IRQ 7  Parallel printer, on I/O channel
        x1xx xxxx  IRQ 6  Disk controller, on I/O channel
        xx1x xxxx  IRQ 5  Harddisk, on I/O channel
        xxx1 xxxx  IRQ 4  COM1, on I/O channel
        xxxx 1xxx  IRQ 3  COM2, on I/O channel
        xxxx x1xx  IRQ 2  I/O channel - availeble for use
        xxxx xx1x  IRQ 1  Keyboard
        xxxx xxx1  IRQ 0  System timer