Contents
- 1 "Low" memory (< 1 MiB)
- 2 "Upper" Memory (> 1 MiB)
- 3 See Also
-
3.1 External Links
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，包含一个可用的BIOS。在CPU处于实时模式期间，IRＱ０，时钟中断会重复的引发，用于启动的盘比如软盘，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，因为它一直有用。
当中断发生时，会跳到中断向量表中调用ＢＩＯＳ的相应程序去处理这个中断。ｂｏｏｔｌｏａｄｅｒ也可以访问ＢＩＯＳ的程序。这就意味着在ｂｏｏｔｌｏａｄｅｒ运行期间要好好保存ＢＩＯＳ的两块内存空间，ＢＤＡ和ＥＢＤＡ。而且每次ＢＩＯＳ发生时钟中断的时候，ＢＩＯＳ会更新ＢＤＡ的一些数据，不要试图去存储任何数据到BDA。
在所有的BIOS的功能已经被调用，你的操作系统已经被加载到内存里面，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。如果内核不再使用实时模式，开始的0x500字节可以被重复使用，但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。如果它存在。它的地址低于0xa000。它保证小于128K个字节。一般是1K个字节。最大的一个事实上才8K。你可以通过BIOS的12H的中断确定EBDA的大小，或者通过测试0x40e。这两个方法都可以获得EBDA的最低的位置。
你的bootloader的代码最好加载到0x7c00-0x7dff这512b中。这段区域不可以被占用，知道执行进入了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
2，“高内存”（>1M）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
>1M的内存区域并没有规范的，很好的定义，或者说不连续。区域包括内存映射硬件，设备驱动可以访问的； ACPI表，初始化代码很可能会读取的，然后可以被重新利用；32位计算机的硬件可以被扩展为4G。使用BIOS的中断INT 15h, EAX=0xE820可以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.可以随意使用包括了，你的bootloader如GRUB，很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.