Understanding the Video Interrupt
TheINT 10h interrupt is the standard interface for video and display services in x86 architecture. When running in real mode, which is the state of the CPU immediately after power-on reset, the BIOS provides these services. To output a character or string, you typically use function 0Eh (Write Character in Teletype Mode) or function 13h (Write String). The original problem described a scenario where only a single character 'H' appeared because the loop logic was missing. Without a loop, the code executes the write instruction once and halts, leaving the rest of the buffer unread.
To set parameters effectively, you must understand the registers involved. For function 0Eh, the AL register holds the ASCII character, BH specifies the page number, and BL can hold the color attribute in some modes. For function 13h, which writes a string, AL contains flags indicating whether to update the cursor position and whether the string includes attributes. The BX register holds the page number, CX holds the length of the string, DX holds the initial row and column, and ES:BP points to the string data.
Implementing the Loop for String Output
A common mistake in bare metal development is assuming that loading a string into memory automatically displays it. You must explicitly iterate through the string or use the block-write function. If you choose to write characters one by one usingINT 10h function 0Eh, you need a loop that decrements a counter until it reaches zero. This ensures every character in your message is processed.
Consider the following corrected approach. First, load the address of your string into SI. Then, initialize a counter in CX with the length of the string. Inside the loop, load the byte at [SI] into AL, call the interrupt, increment SI, decrement CX, and jump back if CX is not zero. This structure guarantees that the entire string is rendered on the screen rather than just the first byte.
mov si, msg ; Load address of string
mov cx, len ; Load length of string
.loop:
lodsb ; Load byte from [SI] into AL, increment SI
mov ah, 0Eh ; BIOS teletype output
int 10h ; Call video interrupt
dec cx ; Decrement counter
jnz .loop ; Jump if not zero
lodsb and a conditional jump based on CX, we ensure all characters are handled. Note that lodsb implicitly uses DS:SI, so ensure your data segment is correctly initialized if you are not using the default segment values provided by the BIOS.
Compiling and Writing to Boot Sector
Once the assembly code is correct, you must compile it into a raw binary file. Tools likenasm are commonly used for this purpose. You should assemble the code with the origin set to 0x7C00, which is the traditional memory location where the BIOS loads the boot sector. It is crucial to pad the output to exactly 512 bytes, as the boot sector size is fixed. Add a magic number 0xAA55 at the end of the file to mark it as a valid bootable sector.
After generating the binary file, you can write it to a floppy disk image or a physical USB drive using tools like dd on Linux or similar utilities on Windows. Ensure you target the correct device to avoid overwriting important data. For testing purposes, using a virtual machine like QEMU or VirtualBox allows you to debug the boot process without risking hardware damage.
For those interested in avoiding BIOS dependencies entirely, consider exploring Using 8086 BUS Compatibility Instead of BIOS Interrupts for a 8086 Game, which discusses direct hardware access methods.
Debugging Common Issues
If the screen remains blank, verify that your code is actually executing. Insert a simple infinite loop or a break point if your debugger supports it. Check if the cursor is positioned off-screen; you may need to clear the screen first using function06h (Scroll Window Up). Also, ensure that the stack pointer (SP) is initialized, as some BIOS interrupts may rely on stack operations. Although INT 10h functions generally do not require a deep stack, initializing SS:SP to a safe location like 0x9000:0xFFFE is good practice.
Avoid using non-standard functions like AH=0Bh for palette settings unless you are certain your specific BIOS supports it, as this can lead to unpredictable behavior across different hardware. Stick to the documented functions for text output to ensure compatibility.
Frequently Asked Questions
Why does my string appear garbled or cut off?
This usually happens if the length counter in CX is incorrect or if the string lacks a null terminator when using functions that expect one. Ensure your loop condition matches the actual length of the data in memory.
Can I use INT 10h in protected mode?
No, INT 10h is a real-mode BIOS service. In protected mode or long mode, you must use direct VGA memory writes or GPU drivers. Switching back to real mode is required to use these interrupts again.
How do I change the text color?
For function 0Eh, set the BL register to the desired color attribute before calling the interrupt. For function 13h, include the color attribute in the string data itself if using the extended format.
What if the screen is black after boot?
The BIOS might have left the display in a graphics mode or cleared the screen. Try calling function 00h to set video mode to text mode 03h before attempting to print any text.
Is it necessary to save registers before calling INT 10h?
It depends on the function. Some functions preserve registers, while others modify them. It is safest to push registers onto the stack before the call and pop them afterward if you need their values later.