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void Enable_A20()
{   
  ERR:       
	char port_60_status = 0x00;
	
	__asm__ __volatile__ ("cli"); // Disable Interrupts
	
	
    // Wait for the controller to be ready for a command
    while(((inb(0x64) >> 0x01) & 0x01)== 0x01){} // Test bit 1 of 0x64 
    outb(0xD0, 0x64); // send 0xD0 to port 0x64

	// Wait for the controller to be ready with a byte of data
    while((inb(0x64) & 0x01) == 0x00){}
    port_60_status = inb(0x60); // Read & Save the current port status from port 60h

	// Wait for the controller to be ready for a command
    while(((inb(0x64) >> 0x01) & 0x01)== 0x01){} // Test bit 1 of 0x64 
    outb(0xD1, 0x64); // send 0xD1 to port 0x64

	// Wait for the controller to be ready for the data
    while(((inb(0x64) >> 0x01) & 0x01)== 0x01){} // Test bit 1 of 0x64 
    port_60_status &= 0x02; 
    outb(port_60_status, 0x60); // Turn on the A20 enable bit

	// Finally, we will attempt to read back the A20 status
	// to ensure it was enabled.

	// Wait for the controller to be ready for a command

    while(((inb(0x64) >> 0x01) & 0x01)== 0x01){} // Test bit 1 of 0x64 
    outb(0xD0, 0x64); // send 0xD1 to port 0x64

	// Wait for the controller to be ready with a byte of data
    while((inb(0x64) & 0x01) == 0x00){}
 
    if(((inb(0x60) >> 0x01) & 0x01) == 0x01) // if true, A20 is ENABLED!
    {
        __asm__ __volatile__ ("sti"); // Enable interrupts
    }
    else{goto ERR;} // A20 is NOT enabled, restart the process

}

Code: Select all

label: <do stuff>
       test <condition>
       jcc label
       <continue>

Code: Select all

while(<condition> == false) <do stuff>
<continue>

Code: Select all

struct RealMode_Regs {
    int myEAX;
    int myEBX;
    int myECX;
    int myEDX;
    int myEBP;
    int myDS;
    int myES;
    int myFlags;
}


void main(void) {

    /* Set 80*25 video mode and clear the screen using the BIOS */

    RealMode_Regs.eax = 0x0003;
    DO_BIOS_INTERRUPT(0x10);
    if( (RealMode_Regs.flags &= FLG_CARRY) != 0) {
        /* Failed to change video mode - assume headless?? */
       for(;;) { }
    }

    /* Lock up (need to implement more code here instead) */

    for(;;) { }
}


int readSector(void *buffer, int device, int head, int cylinder, int sector, int sectorCount) {
    int diskAttempts = 0;

    for(;;) {
        diskAttempts++;
        RealMode_Regs.eax = (0x02 << 8) | sectorCount;
        RealMode_Regs.ecx = (cylinder << 8) | sector;
        RealMode_Regs.edx = (head << 8) | device;
        RealMode_Regs.ebx = (unsigned int) (buffer & 0x000F);
        RealMode_Regs.es = (unsigned int) (buffer >> 4);
        DO_BIOS_INTERRUPT(0x13);                                               // Load sector
        if( (RealMode_Regs.flags &= FLG_CARRY) == 0) return;

        if( diskAttempts >= 4 ) return (RealMode_Regs.eax >> 8) & 0xFF;

        RealMode_Regs.eax = 0x0000;
        RealMode_Regs.edx = 0x0000;
        DO_BIOS_INTERRUPT(0x13);                                              // Reset disk system
    }
}