Serial Ports: Difference between revisions
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(Added in where to find the COM port IO addresses) |
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You should be able to find the IO port addresses of the COM ports in the [[Memory Map (x86)#BIOS_Data_Area_.28BDA.29|BIOS Data Area]] |
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Once you have the base address of your COM port, you add an offset value to get to one of the data registers. One of the registers hold what is termed the DLAB or Divisor Latch Access Bit. When this bit is set, offsets 0 and 1 are mapped to the low and high bytes of the Divisor register for setting the baud rate of the port. When this bit is clear, offsets 0 and 1 are mapped to their normal registers. The DLAB bit only affects port offsets 0 and 1, the other offsets ignore this setting. |
Once you have the base address of your COM port, you add an offset value to get to one of the data registers. One of the registers hold what is termed the DLAB or Divisor Latch Access Bit. When this bit is set, offsets 0 and 1 are mapped to the low and high bytes of the Divisor register for setting the baud rate of the port. When this bit is clear, offsets 0 and 1 are mapped to their normal registers. The DLAB bit only affects port offsets 0 and 1, the other offsets ignore this setting. |
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