Play with `Outputs`

Making a Small OS

10 min readAug 6, 2021

In this part we will going to display text on our Bochs emulator

Interacting with the Hardware

In most cases, there are two ways to interact with the hardware. They are memory-mapped I/O and I/O ports.

If the hardware uses memory-mapped I/O then we can write to a specific memory address and the hardware will be updated with the new data. One example of this is 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.

Writing Text

Now we are going to display one single letter in our consol. For this process we need to update our kmain.c file and call from loader.s file to kmain file.

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;
/** 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)
    }

kmain.c contain fb_write_cellfunction and we need to call that from main function

void main(){
#define FB_GREEN     2
#define FB_DARK_GREY 8

    fb_write_cell(0, 'A', FB_GREEN, FB_DARK_GREY);
}

Now me can run make runcommand in terminal and we can see following output

Moving the Cursor

Moving the cursor of the framebuffer is done via two different I/O ports. The cursor’s position is determined with a 16 bits integer: 0 means row zero, column zero; 1 means row zero, column one; 80 means row one, column zero and so on. 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 0x3D4 [29] is the port that describes the data and port 0x3D5 [29] is 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 [25]:

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
    outb:
        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:

io.h

io.s

The Driver

The driver should provide an interface that the rest of the code in the OS will use for interacting with the framebuffer. There is no right or wrong in what functionality the interface should provide, but a suggestion is to have a write function with the following declaration:

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.

The Serial Ports

The serial port [30] is an interface for communicating between hardware devices and although it is available on almost all motherboards, it is seldom exposed to the user in the form of a DE-9 connector nowadays. 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 first data that need to be sent to the serial port is configuration data. In order for two hardware devices to be able to talk to each other they must agree upon a couple of things. These things include:

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

Configuring the line means to configure how data is being sent over the line. The serial port has an I/O port, the line command port, that is used for configuration.

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 */    /* SERIAL_LINE_ENABLE_DLAB:
     * Tells the serial port to expect first the highest 8 bits on the data port,
     * then the lowest 8 bits will follow
     */
    #define SERIAL_LINE_ENABLE_DLAB         0x80    /** 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)
    {
        outb(SERIAL_LINE_COMMAND_PORT(com),
             SERIAL_LINE_ENABLE_DLAB);
        outb(SERIAL_DATA_PORT(com),
             (divisor >> 8) & 0x00FF);
        outb(SERIAL_DATA_PORT(com),
             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  |

A description for each name can be found in the table below (and in [31]):

NameDescriptiondEnables (d = 1) or disables (d = 0) DLABbIf break control is enabled (b = 1) or disabled (b = 0)prtyThe number of parity bits to usesThe number of stop bits to use (s = 0 equals 1, s = 1 equals 1.5 or 2)dlDescribes the length of the data

We will use the mostly standard value 0x03 [31], 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);
    }

The article on OSDev [31] has a more in-depth explanation of the values.

Configuring the Buffers

When data is transmitted via the serial port it is placed in buffers, both when receiving and sending data. This way, if you send data to the serial port faster than it can send it over the wire, it will be buffered. However, if you send too much data too fast the buffer will be full and data will be lost. In other words, the buffers are FIFO queues. The FIFO queue configuration byte looks like the following figure:

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

A description for each name can be found in the table below:

NameDescriptionlvlHow many bytes should be stored in the FIFO buffersbsIf the buffers should be 16 or 64 bytes largerReserved for future usedmaHow the serial port data should be accessedcltClear the transmission FIFO bufferclrClear the receiver FIFO buffereIf the FIFO buffer should be enabled or not

We use the value 0xC7 = 11000111 that:

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

The WikiBook on serial programming [32] explains the values in more depth.

Configuring the Modem

The modem control register is used for very simple hardware flow control via the Ready To Transmit (RTS) and Data Terminal Ready (DTR) pins. When configuring the serial port we want RTS and DTR to be 1, which means that we are ready to send data.

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 |

A description for each name can be found in the table below:

NameDescriptionrReservedafAutoflow control enabledlbLoopback mode (used for debugging serial ports)ao2Auxiliary output 2, used for receiving interruptsao1Auxiliary output 1rtsReady To TransmitdtrData Terminal Ready

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

Writing data to the serial port is done via the data I/O port. However, before writing, the transmit FIFO queue has to be empty (all previous writes must have finished). The transmit FIFO queue is empty if bit 5 of the line status I/O port is equal to one.

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
    inb:
        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

We recommend that you implement a write function for the serial port similar to the write function in the driver for the framebuffer. To avoid name clashes with the write function for the framebuffer it is a good idea to name the functions fb_write and serial_write to distinguish them.

We further recommend that you try to write a printf-like function, see section 7.3 in [8]. The printf function could take an additional argument to decide to which device to write the output

Finally kmain.c file as follow

The text we want to display is “Starting Vidu OS” and its character count is 16

Now open the terminal and run the “make run” command and then open bochs and continue in terminal.