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INT 13 - Definition

INT 13 is shorthand for interrupt 0x13, the 20th interrupt vector in an x86 based computer system. (Interrupts start at zero, and are labeled with hexadecimal values.) In short, it is a DOS interrupt used to activate disk functions, such as seek, read, write and format. Continued reliance on the unchanged INT 13 routine inherently creates certain hard drive limiting factors, including a problem with more than two hard drives and BIOS/OS limits to recognizing more than 1024 cylinders, 16 heads, and 63 sectors per track. This created the so called 504 MB barrier in the mid 1990s since a sector holds only 512 bytes. Therefore, 1024 (Cylinders) X 16 (Heads) X 63 (Sectors/Track) X 512 (Bytes/Sector) = 528,482,304 Bytes. Later, the Int 13h interface supported disk drives containing up to approximately 16.5 million sectors, which at 512 bytes per sector, yielded a maximum drive space of 8.46 GB (or 7.88 GB). Then the Int 13h interface finally came to the end and has been replaced with a newer interface called Int 13h extensions.


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