The processor starts working in [real mode](https://en.wikipedia.org/wiki/Real_mode). Let's back up a little to try and understand memory segmentation in this mode. Real mode is supported on all x86-compatible processors, from the [8086](https://en.wikipedia.org/wiki/Intel_8086) all the way to the modern Intel 64-bit CPUs. The 8086 processor had a 20-bit address bus, which means that it could work with 0-2^20 bytes address space (1 megabyte). But it only has 16-bit registers, and with 16-bit registers the maximum address is 2^16 or 0xffff (64 kilobytes). [Memory segmentation](http://en.wikipedia.org/wiki/Memory_segmentation) is used to make use of all of the address space available. All memory is divided into small, fixed-size segments of 65535 bytes, or 64 KB. Since we cannot address memory below 64 KB with 16 bit registers, an alternate method was devised. An address consists of two parts: the beginning address of the segment and an offset from this address. To get a physical address in memory, we need to multiply the segment part by 16 and add the offset part:
The processor starts working in [real mode](https://en.wikipedia.org/wiki/Real_mode). Let's back up a little to try and understand memory segmentation in this mode. Real mode is supported on all x86-compatible processors, from the [8086](https://en.wikipedia.org/wiki/Intel_8086) all the way to the modern Intel 64-bit CPUs. The 8086 processor had a 20-bit address bus, which means that it could work with 0-2^20 bytes address space (1 megabyte). But it only has 16-bit registers, and with 16-bit registers the maximum address is 2^16 or 0xffff (64 kilobytes). [Memory segmentation](http://en.wikipedia.org/wiki/Memory_segmentation) is used to make use of all of the address space available. All memory is divided into small, fixed-size segments of 65535 bytes, or 64 KB. Since we cannot address memory above 64 KB with 16 bit registers, an alternate method was devised. An address consists of two parts: the beginning address of the segment and an offset from this address. To get a physical address in memory, we need to multiply the segment part by 16 and add the offset part:
0xAX
9 years ago