Implementation of an Operating System:-Week 3 (Play with Outputs)

As the 3rd week of blog series of implementing an operating system, I will explain how to display text on the console as well as writing data to the serial port.

Furthermore, we will create our first driver, that is, code that acts as a layer between the kernel and the hardware, providing a higher abstraction than communicating directly with the hardware.

3.1) Interacting with the Hardware:

If the hardware uses memory-mapped I/O then you can write to a specific memory address and the hardware will be updated with the new data. One example of this is the framebuffer. For example, if you write the value 0x410F to address 0x000B8000, you will see the letter A in white color on a black background.

If the hardware uses I/O ports then the assembly code instructions out and in must be used to communicate with the hardware. The instruction out takes two parameters: the address of the I/O port and the data to send.

The instruction in takes a single parameter, the address of the I/O port, and returns data from the hardware. One can think of I/O ports as communicating with hardware the same way as you communicate with a server using sockets. The cursor (the blinking rectangle) of the framebuffer is one example of hardware controlled via I/O ports on a PC.

3.2) The Framebuffer:

The framebuffer is a hardware device that is capable of displaying a buffer of memory on the screen . The framebuffer has 80 columns and 25 rows, and the row and column indices start at 0 (so rows are labelled 0–24).

Writing Text

Bit:     | 15 14 13 12 11 10 9 8 | 7 6 5 4 | 3 2 1 0 |
Content: | ASCII | FG | BG |

The first cell corresponds to row zero, column zero on the console. Using an ASCII table, one can see that A corresponds to 65 or 0x41. Therefore, to write the character A with a green foreground (2) and dark grey background (8) at place (0,0), the following assembly code instruction is used:

mov [0x000B8000], 0x4128

The second cell then corresponds to row zero, column one and its address is therefore:

0x000B8000 + 16 = 0x000B8010

Writing to the framebuffer can also be done in C by treating the address 0x000B8000 as a char pointer, char *fb = (char *) 0x000B8000. Then, writing A at place (0,0) with green foreground and dark grey background becomes:

fb[0] = 'A';
fb[1] = 0x28;

The following code shows how this can be wrapped into a function:

/** fb_write_cell:
* Writes a character with the given foreground and background to position i
* in the framebuffer.
* @param i The location in the framebuffer
* @param c The character
* @param fg The foreground color
* @param bg The background color
void fb_write_cell(unsigned int i, char c, unsigned char fg, unsigned char bg)
fb[i] = c;
fb[i + 1] = ((fg & 0x0F) << 4) | (bg & 0x0F)

The function can then be used as follows:

#define FB_GREEN     2
#define FB_DARK_GREY 8
fb_write_cell(0, 'A', FB_GREEN, FB_DARK_GREY);

Moving the Cursor

Since the position is 16 bits large, and the out assembly code instruction argument is 8 bits, the position must be sent in two turns, first 8 bits then the next 8 bits. The framebuffer has two I/O ports, one for accepting the data, and one for describing the data being received. Port 0x3D4is the port that describes the data and port 0x3D5is for the data itself.

To set the cursor at row one, column zero (position 80 = 0x0050), one would use the following assembly code instructions:

out 0x3D4, 14      ; 14 tells the framebuffer to expect the highest 8 bits of the position
out 0x3D5, 0x00 ; sending the highest 8 bits of 0x0050
out 0x3D4, 15 ; 15 tells the framebuffer to expect the lowest 8 bits of the position
out 0x3D5, 0x50 ; sending the lowest 8 bits of 0x0050

The out assembly code instruction can’t be executed directly in C. Therefore it is a good idea to wrap out in a function in assembly code which can be accessed from C via the cdecl calling standard :

global outb             ; make the label outb visible outside this file

; outb - send a byte to an I/O port
; stack: [esp + 8] the data byte
; [esp + 4] the I/O port
; [esp ] return address
mov al, [esp + 8] ; move the data to be sent into the al register
mov dx, [esp + 4] ; move the address of the I/O port into the dx register
out dx, al ; send the data to the I/O port
ret ; return to the calling function

By storing this function in a file called io.s and also creating a header io.h, the out assembly code instruction can be conveniently accessed from C:

#ifndef INCLUDE_IO_H
#define INCLUDE_IO_H

/** outb:
* Sends the given data to the given I/O port. Defined in io.s
* @param port The I/O port to send the data to
* @param data The data to send to the I/O port
void outb(unsigned short port, unsigned char data);

#endif /* INCLUDE_IO_H */

Moving the cursor can now be wrapped in a C function:

#include "io.h"

/* The I/O ports */
#define FB_COMMAND_PORT 0x3D4
#define FB_DATA_PORT 0x3D5

/* The I/O port commands */

/** fb_move_cursor:
* Moves the cursor of the framebuffer to the given position
* @param pos The new position of the cursor
void fb_move_cursor(unsigned short pos)
outb(FB_DATA_PORT, ((pos >> 8) & 0x00FF));
outb(FB_DATA_PORT, pos & 0x00FF);

The Driver

int write(char *buf, unsigned int len);

The write function writes the contents of the buffer buf of length len to the screen. The write function should automatically advance the cursor after a character has been written and scroll the screen if necessary.

3.3) The Serial Ports

The serial port is easy to use, and, more importantly, it can be used as a logging utility in Bochs. If a computer has support for a serial port, then it usually has support for multiple serial ports, but we will only make use of one of the ports. This is because we will only use the serial ports for logging. Furthermore, we will only use the serial ports for output, not input. The serial ports are completely controlled via I/O ports.

Configuring the Serial Port

  • The speed used for sending data (bit or baud rate)
  • If any error checking should be used for the data (parity bit, stop bits)
  • The number of bits that represent a unit of data (data bits)

Configuring the Line

First the speed for sending data will be set. The serial port has an internal clock that runs at 115200 Hz. Setting the speed means sending a divisor to the serial port, for example sending 2 results in a speed of 115200 / 2 = 57600 Hz.

The divisor is a 16 bit number but we can only send 8 bits at a time. We must therefore send an instruction telling the serial port to first expect the highest 8 bits, then the lowest 8 bits. This is done by sending 0x80 to the line command port. An example is shown below:

#include "io.h" /* io.h is implement in the section "Moving the cursor" */

/* The I/O ports */

/* All the I/O ports are calculated relative to the data port. This is because
* all serial ports (COM1, COM2, COM3, COM4) have their ports in the same
* order, but they start at different values.

#define SERIAL_COM1_BASE 0x3F8 /* COM1 base port */

#define SERIAL_DATA_PORT(base) (base)
#define SERIAL_FIFO_COMMAND_PORT(base) (base + 2)
#define SERIAL_LINE_COMMAND_PORT(base) (base + 3)
#define SERIAL_MODEM_COMMAND_PORT(base) (base + 4)
#define SERIAL_LINE_STATUS_PORT(base) (base + 5)

/* The I/O port commands */

* Tells the serial port to expect first the highest 8 bits on the data port,
* then the lowest 8 bits will follow

/** serial_configure_baud_rate:
* Sets the speed of the data being sent. The default speed of a serial
* port is 115200 bits/s. The argument is a divisor of that number, hence
* the resulting speed becomes (115200 / divisor) bits/s.
* @param com The COM port to configure
* @param divisor The divisor
void serial_configure_baud_rate(unsigned short com, unsigned short divisor)
(divisor >> 8) & 0x00FF);
divisor & 0x00FF);

The way that data should be sent must be configured. This is also done via the line command port by sending a byte. The layout of the 8 bits looks like the following:

Bit:     | 7 | 6 | 5 4 3 | 2 | 1 0 |
Content: | d | b | prty | s | dl |

We will use the mostly standard value 0x03, meaning a length of 8 bits, no parity bit, one stop bit and break control disabled. This is sent to the line command port, as seen in the following example:

/** serial_configure_line:
* Configures the line of the given serial port. The port is set to have a
* data length of 8 bits, no parity bits, one stop bit and break control
* disabled.
* @param com The serial port to configure
void serial_configure_line(unsigned short com)
/* Bit: | 7 | 6 | 5 4 3 | 2 | 1 0 |
* Content: | d | b | prty | s | dl |
* Value: | 0 | 0 | 0 0 0 | 0 | 1 1 | = 0x03
outb(SERIAL_LINE_COMMAND_PORT(com), 0x03);

Configuring the Buffers

Bit:     | 7 6 | 5  | 4 | 3   | 2   | 1   | 0 |
Content: | lvl | bs | r | dma | clt | clr | e |

We use the value 0xC7 = 11000111 that:

  • Enables FIFO
  • Clear both receiver and transmission FIFO queues
  • Use 14 bytes as size of queue

Configuring the Modem

The modem configuration byte is shown in the following figure:

Bit:     | 7 | 6 | 5  | 4  | 3   | 2   | 1   | 0   |
Content: | r | r | af | lb | ao2 | ao1 | rts | dtr |

We don’t need to enable interrupts, because we won’t handle any received data. Therefore we use the configuration value 0x03 = 00000011 (RTS = 1 and DTS = 1).

Writing Data to the Serial Port

Reading the contents of an I/O port is done via the in assembly code instruction. There is no way to use the in assembly code instruction from C, therefore it has to be wrapped (the same way as the out assembly code instruction):

global inb

; inb - returns a byte from the given I/O port
; stack: [esp + 4] The address of the I/O port
; [esp ] The return address
mov dx, [esp + 4] ; move the address of the I/O port to the dx register
in al, dx ; read a byte from the I/O port and store it in the al register
ret ; return the read byte
/* in file io.h */

/** inb:
* Read a byte from an I/O port.
* @param port The address of the I/O port
* @return The read byte
unsigned char inb(unsigned short port);

Checking if the transmit FIFO is empty can then be done from C:

#include "io.h"

/** serial_is_transmit_fifo_empty:
* Checks whether the transmit FIFO queue is empty or not for the given COM
* port.
* @param com The COM port
* @return 0 if the transmit FIFO queue is not empty
* 1 if the transmit FIFO queue is empty
int serial_is_transmit_fifo_empty(unsigned int com)
/* 0x20 = 0010 0000 */
return inb(SERIAL_LINE_STATUS_PORT(com)) & 0x20;

Writing to a serial port means spinning as long as the transmit FIFO queue isn’t empty, and then writing the data to the data I/O port.

Configuring Bochs

To save the output from the first serial serial port the Bochs configuration file bochsrc.txt must be updated. The com1 configuration instructs Bochs how to handle first serial port:

com1: enabled=1, mode=file, dev=com1.out

The output from serial port one will now be stored in the file com1.out.

The Driver

A final recommendation is that you create some way of distinguishing the severeness of the log messages, for example by prepending the messages with DEBUG, INFO or ERROR.

Undergraduate at University of Kelaniya Software Engineering