From cbf5b150ec2beab0b915041723fc88b2e271a01e Mon Sep 17 00:00:00 2001 From: xinqiu Date: Wed, 16 Sep 2015 22:25:41 +0800 Subject: [PATCH] =?UTF-8?q?=E7=BF=BB=E8=AF=91=E5=87=A0=E5=8F=A5?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- Booting/linux-bootstrap-1.md | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/Booting/linux-bootstrap-1.md b/Booting/linux-bootstrap-1.md index 436f15e..5d32381 100644 --- a/Booting/linux-bootstrap-1.md +++ b/Booting/linux-bootstrap-1.md @@ -35,29 +35,29 @@ CS selector 0xf000 CS base 0xffff0000 ``` -The processor starts working in [real mode](https://en.wikipedia.org/wiki/Real_mode) and we need to back up a little to understand memory segmentation in this mode. Real mode is supported in all x86-compatible processors, from [8086](https://en.wikipedia.org/wiki/Intel_8086) to 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 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: +处理器开始在[实模式](https://en.wikipedia.org/wiki/Real_mode)工作，我们需要退回一点去理解在这种模式下的内存分割。所有 x86兼容处理器都支持实模式，从[8086](https://en.wikipedia.org/wiki/Intel_8086)到现在的Intel 64位 CPU。8086处理器有一个20位寻址总线，这意味着它可以对0到2^20 位地址空间进行操作(1Mb).不过它只有16位的寄存器，通过这个16位寄存器最大寻址是2^16即 0xffff(64 Kb)。[内存分配](http://en.wikipedia.org/wiki/Memory_segmentation) 被用来充分利用所有空闲地址空间。所有内存被分成固定的65535 字节或64 KB大小的小块。由于我们不能用16位寄存器寻址小于64KB的内存，一种替代的方法被设计出来了。一个地址包括两个部分：数据段起始地址和从该数据段起的偏移量。为了得到内存中的物理地址，我们要让数据段乘16并加上偏移量： ``` PhysicalAddress = Segment * 16 + Offset ``` -For example if `CS:IP` is `0x2000:0x0010`, the corresponding physical address will be: +举个例子，如果 `CS:IP` 是 `0x2000:0x0010`, 相关的物理地址将会是： ```python >>> hex((0x2000 << 4) + 0x0010) '0x20010' ``` -But if we take the biggest segment part and offset: `0xffff:0xffff`, it will be: +不过如果我们让最大端进行偏移：`0xffff:0xffff`，将会是： ```python >>> hex((0xffff << 4) + 0xffff) '0x10ffef' ``` -which is 65519 bytes over first megabyte. Since only one megabyte is accessible in real mode, `0x10ffef` becomes `0x00ffef` with disabled [A20](https://en.wikipedia.org/wiki/A20_line). +这超出1MB65519字节。既然只有1MB在实模式中可以访问，`0x10ffef` 变成有[A20](https://en.wikipedia.org/wiki/A20_line)缺陷的 `0x00ffef`。 -Ok, now we know about real mode and memory addressing. Let's get back to register values after reset. +我们知道实模式和内存地址。回到复位后的寄存器值。 `CS` register consists of two parts: the visible segment selector and hidden base address. We know predefined `CS` base and `IP` value, logical address will be: