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8250 UART Programming
motherboard of your computer. It may take some time to hunt down these settings, and
it is important to know what these values are when you are trying to write your software.
Often these values can be found in the BIOS setup screens of your computer, or if you can
pause the messages when your computer turns on, they can be found as a part of the boot
process of your computer.
For a "typical" PC system, the following are the Port I/O addresses and IRQs for each serial
COM port:
Common UART IRQ and I/O Port Addresses
COM Port IRQ Base Port I/O address
COM1 IRQ4 $3F8
COM2 IRQ3 $2F8
COM3 IRQ4 $3E8
COM4 IRQ3 $2E8
If you notice something interesting here, you can see that COM3 and COM1 share the same
interrupt. This is not a mistake but something you need to keep in mind when you are
writing an interrupt service routine. The 15 interrupts that were made available through
the 8259 PIC chips still have not been enough to allow all of the devices that are found on a
modern computer to have their own separate hardware interrupt, so in this case you will
need to learn how to share the interrupt with other devices. I'll cover more of that later
when we get into the actual software to access the serial data ports, but for now remember
not to write your software strictly for one device.
The Base Port I/O address is important for the next topic we will cover, which is directly
accessing the UART registers.
3.6 UART Registers
The UART chip has a total of 12 different registers that are mapped into 8 different Port
I/O locations. Yes, you read that correct, 12 registers in 8 locations. Obviously that means
there is more than one register that uses the same Port I/O location, and affects how the
UART can be configured. In reality, two of the registers are really the same one but in a
different context, as the Port I/O address that you transmit the characters to be sent out of
the serial data port is the same address that you can read in the characters that are sent to
the computer. Another I/O port address has a different context when you write data to it
than when you read data from it... and the number will be different after writing the data
to it than when you read data from it. More on that in a little bit.
One of the issues that came up when this chip was originally being designed was that the
designer needed to be able to send information about the baud rate of the serial data with
16 bits. This actually takes up two different "registers" and is toggled by what is called the
"Divisor Latch Access Bit" or "DLAB". When the DLAB is set to "1", the baud rate registers
can be set and when it is "0" the registers have a different context.
Does all this sound confusing? It can be, but lets take it one simple little piece at a time.
The following is a table of each of the registers that can be found in a typical UART chip:
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