Serial Ports: Difference between revisions
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! IO Port Offset
! Setting of DLAB
! I/O Access
! Register mapped to this port
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| +0 || 0 ||
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| +1 || 0 || Read/Write || Interrupt Enable Register.
| +0 || 1 || With DLAB set to 1, this is the least significant byte of the divisor value for setting the baud rate.▼
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| +2 || - || Interrupt Identification and FIFO control registers▼
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| +6 || - || Read || Modem Status Register.
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| +7 || - || Read/Write || Scratch Register.
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===Terminals===
Once you can send and receive bytes with confidence, you probably want to connect the serial port to a terminal (or more likely a terminal emulator these days). Those send specific byte sequences when a key is pressed, and can interpret codes to move the cursor on the screen and change color for example.
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==Example Code==
===Initialization===
<
#define PORT 0x3f8 // COM1
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return 0;
}
</syntaxhighlight>
Notice that the initialization code above writes to [PORT + 1] twice with different values. This is once to write to the Divisor register along with [PORT + 0] and once to write to the Interrupt register as detailed in the previous section. The second write to the Line Control register [PORT + 3] clears the DLAB again as well as setting various other bits.
===Receiving data===
<
int serial_received() {
return inb(PORT + 5) & 1;
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return inb(PORT);
}
</syntaxhighlight>
===Sending data===
<
int is_transmit_empty() {
return inb(PORT + 5) & 0x20;
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outb(PORT,a);
}
</syntaxhighlight>
==Glossary==
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