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There is one more thing we need to do before we switch to long mode, which is set video mode. 

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Printing on the screen is done by the bios service in real mode. 

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Once we switch to long mode, we are unable to call bios function. So we set up a video mode 

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and print characters on that mode. 

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There are a lot of video mode we can use.  Our system choose text mode in which printing characters is really simple

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.

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All we need to do is specify the ascii code for the character we want to print

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and the attribute for that character such as foreground color and background color. computer.

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We write them on the specific address and the data will appear on the screen.

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OK, let's see how to set up text mode.

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To set up text mode, we use interrupt 10 

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and function code 0 in ah register which means we want to set video mode. 

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Then we need to choose video mode, text mode in this case, by copying 3 to al register.  

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So we copy

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3 to ax to set it up.

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Ok now we have set up text mode and we can print message on screen without calling BIOS service. 

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Let’s see how to print characters. 

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Remove the print function.

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The base address for text mode is b8000. The size of screen we can print on is 80*25

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.

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So we have 25 lines and we can print 80 characters each line.

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Every character takes up 2 bytes of space. 

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The first position on the screen corresponds to the two bytes at b8000. 

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The second position corresponds to the two bytes at b8002, and so on.

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Within the 2 bytes of memory space, the first byte is for ascii code and the second byte is 

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attribute of the character.  

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The lower half are foreground color and the higher half are background color. The color index is listed here.

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For example, we print text mode is set.  

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We change the message to text mode is set.

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we move

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si message

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and di

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0xb8000. What we do here is save the address of characters to regiser si and text mode address 

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b8000 to di.  

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Note that the value b8000 is too large for 16-bit register di to store. 

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So we save b800 to 

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es register 

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and zero di register. 

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When we reference the memory address b8000, we use es di which points to the same memory location

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.

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We also save the number of characters to cx. 

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Then we print characters one at a time. 

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move al si 

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and move es di, 

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al.

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We copy the data in memory pointed to by si which is the first character of message at this point. 

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and copy the data to the memory addressed by di which is b8000. Next we specify 

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the attribute of character in the next byte, 

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we use color index a which is bright green. 

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Ok now we have printed the first character in green. 

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We continue the same process until all the characters have been printed. 

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So we add di,2 and si 1.  Because the character takes up two bytes and 

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the character stored in message takes up 1 byte. 

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By adjusting di and si register, we just move to the next character.  The last instruction is loop 

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which use cx register as a counter so that we can continue the process.

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As you see, we have initialized the cx with the number of characters. So loop instruction will execute 

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this block of code the same number of times.

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OK, let's build the project.

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and the run the bochs.

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As you see, we printed a message without using bios service.

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This is a message we have printed in the real machine and in the next video, we will switch to protected mode

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.

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See you in the next video.