x86内存映射

1 "Low"
memory (< 1 MiB)
2 "Upper"
Memory (> 1 MiB)
3 See
Also

这篇文章主要内容是计算机启动时，BIOS跳转到你的bootloader代码后的计算机的物理内存。

1，“低”内存（<1M）

x86计算机启动的时候进入实时模式，包括一个可用的BIOS。在CPU处于实时模式期间。IRＱ０。时钟中断会反复的引发，用于启动的盘比方软盘，硬盘等都会引起中断，这就意味着，在ＣＰＵ启动的过程，中断向量表（ＩＶＴ）要被好好保存，由于它一直实用。

其中断发生时，会跳到中断向量表中调用ＢＩＯＳ的对应程序去处理这个中断。

ｂｏｏｔｌｏａｄｅｒ也能够訪问ＢＩＯＳ的程序。这就意味着在ｂｏｏｔｌｏａｄｅｒ执行期间要好好保存ＢＩＯＳ的两块内存空间，ＢＤＡ和ＥＢＤＡ。并且每次ＢＩＯＳ发生时钟中断的时候。ＢＩＯＳ会更新ＢＤＡ的一些数据。不要试图去存储不论什么数据到BDA。

在全部的BIOS的功能已经被调用，你的操作系统已经被载入到内存里面，bootloader或者内核可能要永久的退出实时模式了。一般进入32位的保护模式。

假设内核不再使用实时模式，開始的0x500字节能够被反复使用。可是常见的回到实时模式操作。就是处理视频显示模式的时候。

当CPU处于保护模式的时候，系统管理模式在无形之中被激活，不能关闭，系统管理模式会使用EBDA的区域，多以EBDA不能被覆盖。

注意：EBDA是变量区内存。对于不同的BIOS。假设它存在。它的地址低于0xa000。它保证小于128K个字节。

通常是1K个字节。最大的一个其实才8K。

你能够通过BIOS的12H的中断确定EBDA的大小，或者通过測试0x40e。这两个方法都能够获得EBDA的最低的位置。

你的bootloader的代码最好载入到0x7c00-0x7dff这512b中。这段区域不能够被占用。知道运行进入了bootloader的第二阶段或者进入了你的内核。

Overview

start	end	size	type	description
Low Memory (the first MiB)
0x00000000	0x000003FF	1 KiB	RAM - partially unusable (see above)	Real Mode IVT (Interrupt Vector Table)
0x00000400	0x000004FF	256 bytes	RAM - partially unusable (see above)	BDA (BIOS data area)
0x00000500	0x00007BFF	almost 30 KiB	RAM (guaranteed free for use)	Conventional memory
0x00007C00 (typical location)	0x00007DFF	512 bytes	RAM - partially unusable (see above)	Your OS BootSector
0x00007E00	0x0007FFFF	480.5 KiB	RAM (guaranteed free for use)	Conventional memory
0x00080000	0x0009FBFF	approximately 120 KiB, depending on EBDA size	RAM (free for use, if it exists)	Conventional memory
0x0009FC00 (typical location)	0x0009FFFF	1 KiB	RAM (unusable)	EBDA (Extended BIOS Data Area)
0x000A0000	0x000FFFFF	384 KiB	various (unusable)	Video memory, ROM Area

BIOS Data Area (BDA)

The BDA is only partially standardized, and almost all the values stored there are completely obsolete and uninteresting. The following is a partial list. See the External Links references below for more detail.

address (size)	description
0x0400 (4 words)	IO ports for COM1-COM4 serial (each address is 1 word, zero if none)
0x0408 (3 words)	IO ports for LPT1-LPT3 parallel (each address is 1 word, zero if none)
0x040E (word)	EBDA base address >> 4 (usually!)
0x0410 (word)	packed bit flags for detected hardware
0x0417 (word)	keyboard state flags
0x041E (32 bytes)	keyboard buffer
0x0449 (byte)	Display Mode
0x044A (word)	number of columns in text mode
0x0463 (2 bytes, taken as a word)	base IO port for video
0x046C (word)	# of IRQ0 timer ticks since boot
0x0475 (byte)	# of hard disk drives detected
0x0480 (word)	keyboard buffer start
0x0482 (word)	keyboard buffer end
0x0497 (byte)	last keyboard LED/Shift key state

Extended BIOS Data Area (EBDA)

You may see "maps" of the EBDA if you search the web. However, those maps are for the original IBM BIOS EBDA. They do not apply to any current EBDA, used by any current BIOS. The EBDA area is not standardized. It does contain
data that your OS will need, but you must do a bytewise pattern search to find those tables. (See Plug-and-Play.)

ROM Area

start	end	size	region/exception	description
Standard usage of the ROM Area
0x000A0000	0x000BFFFF	128 KiB	video RAM	VGA display memory
0x000C0000	0x000C7FFF	32 KiB (typically)	ROM	Video BIOS
0x000C8000	0x000EFFFF	160 KiB (typically)	ROMs and unusable space	Mapped hardware & Misc.
0x000F0000	0x000FFFFF	64 KiB	ROM	Motherboard BIOS

2，“高内存”（>1M）

>1M的内存区域并没有规范的，非常好的定义。或者说不连续。区域包含内存映射硬件，设备驱动能够訪问的； ACPI表。初始化代码非常可能会读取的，然后能够被又一次利用；32位计算机的硬件能够被扩展为4G。

使用BIOS的中断INT
15h, EAX=0xE820能够获得可靠的高内存映射表。

start	end	size	region/exception	description
High Memory
0x00100000	0x00EFFFFF	0x00E00000 (14 MiB)	RAM -- free for use (if it exists)	Extended memory ^{1, 2}
0x00F00000	0x00FFFFFF	0x00100000 (1 MiB)	Possible memory mapped hardware	ISA Memory Hole 15-16MB ³
0x01000000	???? ? ? ??	? ? ?? ? ??? (whatever exists)	RAM -- free for use	More Extended memory ¹
0xC0000000 (sometimes, depends on motherboard and devices)	0xFFFFFFFF	0x40000000 (1 GiB)	various (typically reserved for memory mapped devices)	Memory mapped PCI devices, PnP NVRAM?, IO APIC/s, local APIC/s, BIOS, ...
0x0000000100000000 (possible memory above 4 GiB)	??? ? ? ????? ????? ?	?? ? ?? ?? ? ???? ???? (whatever exists)	RAM -- free for use (PAE/64bit)	More Extended memory ¹
? ???? ????? ???? ??	??? ? ????? ? ?? ????	???? ? ???? ???? ???	Possible memory mapped hardware	Potentially usable for memory mapped PCI devices in modern hardware (but typically not, due to backward compatibility)

¹: Different computers have different amounts of RAM, therefore the amount of extended memory you might find will vary and may be anything from "none" (e.g. an old 80386 system) to "lots".

²: Free for use except that your bootloader (ie. GRUB) may have loaded your "modules" here, and you don't want to overwrite those.能够任意使用包含了。你的bootloader如GRUB，非常可能调入你的模块在这里，所以你不想覆盖它。

³: The "ISA Memory Hole" (from 0x00F00000 to 0x00FFFFFF) was used for memory mapped ISA devices (e.g. video cards). Modern computers have no need for this hole, but some chipsets still support it (as an optional feature)
and some motherboards may still allow it to be enabled with BIOS options, so it may exist in a modern computers with no ISA devices.