Real Mode
In the 8086 processor, the IDT resides at a fixed location in memory from address 0x0000 to 0x03ff, and consists of 256 four-byte real mode pointers (256 × 4 = 1024 bytes of memory). In the 80286 and later, the size and locations of the IDT can be changed in the same way as it is done in protected mode, though it does not change the format of it. A real mode pointer is defined as a 16-bit segment address and a 16-bit offset into that segment. A segment address is expanded internally by the processor to 20 bits thus limiting real mode interrupt handlers to the first 1 megabyte of addressable memory. The first 32 vectors are reserved for the processor's internal exceptions, and hardware interrupts may be mapped to any of the vectors by way of a programmable interrupt controller.
A commonly used x86 real mode interrupt is INT 10, the Video BIOS code to handle primitive screen drawing functions such as pixel drawing and changing the screen resolution.
Read more about this topic: Interrupt Descriptor Table
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