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Code Issues Releases Wiki Activity

Better description to how cpu starts

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Leandro Moreira 2015-01-20 23:32:26 -02:00
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linux-bootstrap-1.md
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@ -34,7 +34,7 @@ CS selector 0xf000
CS base 0xffff0000
```
The processor works in [real mode](http://en.wikipedia.org/wiki/Real_mode) now and we need to make a little retreat for understanding memory segmentation in this mode. Real mode is supported in all x86 compatible processors, from [8086](http://en.wikipedia.org/wiki/Intel_8086) to modern Intel 64bit 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 had 16 bit registers, and with 16 bit registers the maximum address is 2^16 or 0xffff (64 kilobytes). Memory segmentation was used to make use of all of the address space. All memory was divided into small, fixed-size segments of 65535 bytes, or 64 KB. Since we can not address memory behind 64 KB with 16 bit registers, another method to do it was devised. An address consists of two parts: the beginning address of the segment and the offset from the beginning of this segment. 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](http://en.wikipedia.org/wiki/Real_mode) now and we need to make a little retreat for understanding memory segmentation in this mode. Real mode is supported in all x86 compatible processors, from [8086](http://en.wikipedia.org/wiki/Intel_8086) to modern Intel 64bit 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 had 16 bit registers, and with 16 bit registers the maximum address is 2^16 or 0xffff (64 kilobytes). Memory segmentation was used to make use of all of the address space. All memory was divided into small, fixed-size segments of 65535 bytes, or 64 KB. Since we can not address memory behind 64 KB with 16 bit registers, another method to do it was devised. An address consists of two parts: the beginning address of the segment and the offset from the beginning of this segment. To get a physical address in memory, we need to multiply the segment part by 16 and add the offset part:
```
PhysicalAddress = Segment * 16 + Offset