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This page has been machine-translated from the original page.

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I’m reading xv6 OS inspired by Your First OS Code Reading: Learning Kernel Internals with UNIX V6.

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Because UNIX V6 itself does not run on x86 CPUs, I decided to read the source code of kash1064/xv6-public: xv6 OS, a fork of the xv6 OS repository that adapts UNIXv6 to run on the x86 architecture.

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Last time, we looked at the behavior of the consoleinit function executed from main.

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This time, I’ll follow the behavior of the uartinit function.

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Table of Contents

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The uartinit function

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The uartinit function is defined in uart.c, and it performs initialization related to the serial port.

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#define COM1    0x3f8\nstatic int uart;    // is there a uart?\n\nvoid uartinit(void)\n{\n  char *p;\n\n  // Turn off the FIFO\n  outb(COM1+2, 0);\n\n  // 9600 baud, 8 data bits, 1 stop bit, parity off.\n  outb(COM1+3, 0x80);    // Unlock divisor\n  outb(COM1+0, 115200/9600);\n  outb(COM1+1, 0);\n  outb(COM1+3, 0x03);    // Lock divisor, 8 data bits.\n  outb(COM1+4, 0);\n  outb(COM1+1, 0x01);    // Enable receive interrupts.\n\n  // If status is 0xFF, no serial port.\n  if(inb(COM1+5) == 0xFF) return;\n  uart = 1;\n\n  // Acknowledge pre-existing interrupt conditions;\n  // enable interrupts.\n  inb(COM1+2);\n  inb(COM1+0);\n  ioapicenable(IRQ_COM1, 0);\n\n  // Announce that we're here.\n  for(p=\"xv6...\\n\"; *p; p++) uartputc(*p);\n}
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What is UART

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To begin with, UART stands for **Universal Asynchronous Receiver/Transmitter**.

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UART is a protocol for exchanging serial data between two devices.

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Reference: Overview of UART | Rohde & Schwarz

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Reference: Universal asynchronous receiver-transmitter - Wikipedia

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It does not seem to be used very much these days, but UART appears to have been the protocol used for communication over serial ports.

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The uartinit function seems to be setting up what is usually called a COM port.

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Using the serial port for communication

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If an OS uses a serial port for communication, it first needs to initialize the serial port.

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Reference: Serial Ports - OSDev Wiki

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Let’s look through the source code in order.

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#define COM1    0x3f8\n\n// Turn off the FIFO\noutb(COM1+2, 0);
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COM1 is defined as 0x3f8.

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This is the fixed I/O port address of the COM1 port.

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Each COM port register can be accessed as an offset from the I/O port address.

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COM1+2 points to the Interrupt Identification and FIFO control registers.

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Here, setting 0 disables FIFO operation inside the UART.

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When FIFO is disabled, received data is passed to the Receiver buffer register.

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Next, let’s look at the following code.

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// 9600 baud, 8 data bits, 1 stop bit, parity off.\noutb(COM1+3, 0x80);    // Unlock divisor\noutb(COM1+0, 115200/9600);\noutb(COM1+1, 0);\noutb(COM1+3, 0x03);    // Lock divisor, 8 data bits.\noutb(COM1+4, 0);\noutb(COM1+1, 0x01);    // Enable receive interrupts.
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COM1+3 corresponds to the Line control register.

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This register changes the communication parameters according to the bits that are set.

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\n \n \n \n \n \n \n \n \n

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Reference image: Serial UART, an in depth tutorial - Lammert Bies

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outb(COM1+3, 0x80); sets the eighth bit and enables DLAB.

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As a result, the following writes to COM1+0 and COM1+1 are redirected to the Divisor latch registers (R/W).

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Writing 115200/9600 and 0 there sets the UART speed to 9,600 bps.

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The following outb(COM1+3, 0x03); clears DLAB in the LCR while also setting 8 data bits.

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After that, it writes 0 to the Modem control register and enables interrupts.

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After going through the full initialization process, it checks the Line status register to confirm that the serial port is available.

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// If status is 0xFF, no serial port.\nif(inb(COM1+5) == 0xFF) return;\nuart = 1;
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Finally, it checks the current interrupt state by reading the RBR and IIR information, enables interrupts, and finishes.

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// Acknowledge pre-existing interrupt conditions;\n// enable interrupts.\ninb(COM1+2);\ninb(COM1+0);\nioapicenable(IRQ_COM1, 0);
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Summary

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This time, I traced through the serial-port initialization process.

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Next time, I’ll look at the pinit function.

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Reference books

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