text mode cursor doesn't appear in qemu vga emulator

Issue

I have a problem with the function that updates cursor position in text mode the function definition and declaration are

#include <sys/io.h>
signed int VGAx = 0,VGAy=0;
void setcursor()
{
        uint16_t position = VGAx+VGAy*COLS;
        outb(0x0f, 0x03d4);
        outb((position<<8)>>8,0x03d5);
        outb(0x0e,0x03d4);
        outb(position>>8,0x03d5);
}

and the file sys/io.h

static inline unsigned char inb (unsigned short int port)
{
        unsigned char value;
        asm ("inb %0, %%al":"=rm"(value):"a"(port));
        return value;
}
static inline void outb(unsigned char value, unsigned short int port)
{
        asm volatile ("outb %%al, $0"::"rm"(value), "a"(port));
}

before using the function the cursor sometimes was blinking underscore and sometimes didn't appear while after using the function no cursor appeared

here is the main function that runs

#include <vga/vga.h>
int kmain(){
        setcursor()
        setbgcolor(BLACK);
        clc();
        setforecolor(BLUE);
        terminal_write('h');
        setcursor();
        return 0;
}

I tried using this function

void enable_cursor() {
    outb(0x3D4, 0x0A);
    char curstart = inb(0x3D5) & 0x1F; // get cursor scanline start

    outb(0x3D4, 0x0A);
    outb(0x3D5, curstart | 0x20); // set enable bit
}

which is provided here but I got this error inline asm: operand type mismatch for 'in' any help is appreciated

EDIT I tried to fix the wrong inb and outb:

static inline unsigned char inb (unsigned short int port)
{
        unsigned char value;
        asm volatile("inb %1, %0" : "=a"(value) : "Nd"(port));
        return value;
}
static inline void outb(unsigned char value, unsigned short int port)
{
        asm volatile ("outb %%al, $0"::"Nd"(value), "a"(port));
}

I guess this is the right definition but still no cursor appeard

EDIT 2 I followed the given answer and defined the io.h file as the following

static inline unsigned char inb (unsigned short int port)
{
        unsigned char value;
        asm volatile ("inb %1, %0" : "=a"(value) : "Nd"(port));
        return value;
}
static inline void outb(unsigned char value, unsigned short int port)
{
        asm volatile ("outb %0, %1"::"a"(value), "Nd"(port));
}

I would like to mention that I also addedenable_cursor(); to the beginning of kmain now the compile time error is fixed but no cursor appeared (which is the main problem)

EDIT 3 I would like to point out that a version of the whole code is availabe on gihub if any one want access to pieces of code that are no available in the question

Solution

inb and outb Function Bugs

This code for inb is incorrect:

static inline unsigned char inb (unsigned short int port)
{
        unsigned char value;
        asm ("inb %0, %%al":"=rm"(value):"a"(port));
        return value;
}

A few problems with it:

• It seems you have the parameters to inb reversed. See the instruction set reference for inb. Remember that in AT&T syntax (that you are using in your GNU Assembler code) the operands are reversed. The instruction set reference shows them in Intel format.
• The port number is either specified as an immediate 8 bit value or passed in the DX register. The proper constraint for specifying the DX register or an immediate 8 bit value for inb/outb is Nd. See my Stackoverflow answer here for an explanation of the constraint Nd.
• The destination that the value read is returned in is either AL/AX/EAX so a constraint =rm on the output that says an available register or memory address is incorrect. It should be =a in your case.

Your code should be something like:

static inline unsigned char inb (unsigned short int port)
{
        unsigned char value;
        asm volatile ("inb %1, %0" : "=a"(value) : "Nd"(port));
        return value;
}

Your assembler template for outb is incorrect:

static inline void outb(unsigned char value, unsigned short int port)
{
        asm volatile ("outb %%al, $0"::"rm"(value), "a"(port));
}

A couple problems with it:

• The port number is either specified as an immediate 8 bit value or passed in the DX register. The proper constraint for specifying the DX register or an immediate 8 bit value for inb/outb is Nd. See my Stackoverflow answer here for an explanation of the constraint Nd.
• The value to output on the port has to be specified in AL/AX/EAX so a constraint rm on the value that says an available register or memory address is incorrect. It should be a in your case. See the instruction set reference for outb

The code should probably look something like:

static inline void outb(unsigned char value, unsigned short int port)
{
        asm volatile ("outb %0, %1"::"a"(value), "Nd"(port));
}

Enabling and Disabling the Cursor

I had to look up the VGA registers about the cursor and found this document on the cursor start register which says:

   Cursor Start Register (Index 0Ah)

-------------------------------------------------
|  7  |  6  |  5  |  4  |  3  |  2  |  1  |  0  |
-------------------------------------------------
|     |     | CD  |     Cursor Scan Line Start  |
-------------------------------------------------

CD -- Cursor Disable

This field controls whether or not the text-mode cursor is displayed. Values are:

0 -- Cursor Enabled

1 -- Cursor Disabled

Cursor Scan Line Start

An important thing is that the cursor is disabled when the bit 5 is set. In your github setcursor function you do this:

outb(curstart | 0x20, 0x3D5);

curstart | 0x20 sets bit 5 (0x20 = 0b00100000). If you want to clear bit 5 and enable the cursor, then you bitwise NEGATE(~) the bitmask and bitwise AND (&) that with curstart. It should look like this:

outb(curstart & ~0x20, 0x3D5);

VGA Function Bugs

Once you have the cursor properly enabled it will render the cursor in the foreground color (attribute) for the particular video location it is currently over. One thing I noticed is that your clc routine does this:

vga_deref_80x24(VGAx,VGAy) = \
        vga_encode_80x24(' ',BgColor,BgColor);

The thing to observe is that you set the attribute for the foreground and background colors to BgColor . If you set the bgcolor to black before calling clc it will flash a black underline cursor on a black background rendering it invisible on any screen location. For the cursor to be visible it must be on a screen location where the foreground and background are different colors. One way to see if this works is to change the code to:

vga_deref_80x24(VGAx,VGAy) = \
        vga_encode_80x24(' ',BgColor,ForeColor);

I think it is a bug that you are clearing it with encoding vga_encode_80x24(' ',BgColor,BgColor); I think you mean to use vga_encode_80x24(' ',BgColor,ForeColor);

Now in your kmain function you need to set a ForeColor and BgColor before calling clc and they both must be different color to make the cursor visible. You have this code:

setbgcolor(BLACK);
clc();
setforecolor(BLUE);

It should now be:

setbgcolor(BLACK);
setforecolor(BLUE);
clc();

Now if the cursor is rendered anywhere on an unwritten location on the screen it will flash BLUE underline on BLACK background.

This should solve your cursor problem. However, I noticed that you also use encode vga_encode_80x24(' ',BgColor,BgColor); in your VGA scrolldown and terminal_control functions. I think this is a bug as well, and I think you should use encode vga_encode_80x24(' ',BgColor,ForeColor); instead. You do seem to set it properly in terminal_write.

If you want to change the color of the cursor at any location you could write a function that changes the foreground attribute under the cursor location without changing the background color. Make sure the two attributes (Foreground and background color) are different for the cursor to be visible. If you wish to hide the cursor you can set foreground and background color the same color for the screen location the cursor is currently at.

Answered By - Michael Petch