Memory Map (x86)

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最新推荐文章于 2024-09-09 17:51:49 发布

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文章标签： x86 video initialization keyboard byte search

本文详细介绍了计算机在BIOS跳转到引导加载程序代码时刻的物理内存内容，包括低内存区域（小于1MiB）、高内存区域（大于1MiB）等关键信息，并讨论了这些区域的具体用途。

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This article describes the contents of the computer's physical memory at the moment that the BIOS jumps to your bootloader code.

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"Low" memory (< 1 MiB)

When a typical x86 PC boots it will be in Real Mode, with an active BIOS. During the time the CPU remains in Real Mode, IRQ0 (the clock) will fire repeatedly, and the hardware that is used to boot the PC (floppy, hard disk, CD, Network card, USB) will also generate IRQs. This means that during the PC boot process, the Real Mode IVT (see below) must be carefully preserved, because it is being used.

When the IVT is activated by an IRQ, it will call a BIOS routine to handle the IRQ. Bootloaders will also access BIOS functions. This means that the two memory workspaces that the BIOS uses (the BDA and the EBDA) must also be carefully preserved during boot. Also, every time the BIOS handles an IRQ0 (18 times a second), several bytes in the BDA get overwritten by the BIOS -- so do not attempt to store anything there while IRQs are active in Real Mode.

After all the BIOS functions have been called, and your kernel is loaded into memory somewhere, the bootloader or kernel may exit Real Mode forever (often by going into 32bit Protected Mode). If the kernel never uses Real Mode again, then the first 0x500 bytes of memory in the PC may be reused and overwritten. (However, it is very common to temporarily return to Real Mode in order to change the Video Display Mode.)

When the CPU is in Protected Mode, System Management Mode (SMM) is still invisibly active, and cannot be shut off. SMM also seems to use the EBDA. So the EBDA memory area should never be overwritten.

Note: the EBDA is a variable-sized memory area (on different BIOSes). If it exists, it is always immediately below 0xA0000 in memory. It is absolutely guaranteed to be less than 128 KiB in size. It is often 1 KiB. The biggest ones ever actually seen are 8 KiB. You can determine the size of the EBDA by using BIOS function INT 12h, or (often) by examining the word at 0x40E in the BDA (see below). Both of those methods will tell you the location of the bottom of the EBDA.

It should also be noted that your bootloader code is probably loaded and running in memory at physical addresses 0x7C00 through 0x7DFF. So that memory area is likely to also be unusable until execution has been transferred to a second stage bootloader, or to your kernel.

Overview

(all values except KiBs are in hex)

start	end	size	type	description
Low Memory (the first MiB)
00000000	000003FF	400 (1 KiB)	RAM - partially unusable (see above)	Real Mode IVT (Interrupt Vector Table)
00000400	000004FF	100	RAM - partially unusable (see above)	BDA (BIOS data area)
00000500	00007BFF	7700 (almost 30 KiB)	RAM (guaranteed free for use)	Conventional memory
00007C00 (typical location)	00007DFF	200	RAM - partially unusable (see above)	Your OS BootSector
00007E00	0007FFFF	7FB00 (481 KiB)	RAM (guaranteed free for use)	Conventional memory
00080000	0009FBFF	1FC00 (approximately 120 KiB)	RAM (free for use, if it exists)	Conventional memory
0009FC00 (typical location)	0009FFFF	400	RAM (unusable)	EBDA (Extended BIOS Data Area)
000A0000	000FFFFF	60000	various (unusable)	ROM Area (384 KiB)

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
400 (word)	IO port for COM1 serial
408 (word)	IO port for LPT1 parallel
40E (word)	EBDA base address >> 4 (usually!)
410 (word)	packed bit flags for detected hardware
449 (byte)	Display Mode
463 (2 bytes, taken as a word)	base IO port for video
46C (word)	# of IRQ0 timer ticks since boot
475 (byte)	# of hard disk drives detected
497 (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 PlugNPlay.)

ROM Area

start	end	size	region/exception	description
Standard usage of the ROM Area
000A0000	000AFFFF	10000	video RAM	VGA framebuffer (64 KiB)
000B0000	000B7FFF	8000	video RAM	VGA text monochrome (32 KiB)
000B8000	000BFFFF	8000	video RAM	VGA text color (32 KiB)
000C0000	000C7FFF	8000	ROM	Video BIOS (32 KiB is typical size)
000C8000	000EFFFF	28000	ROMs and unusable space	Mapped hardware & Misc.
000F0000	000FFFFF	10000	ROM	Motherboard BIOS (64 KiB is typical size)

"Upper" Memory (> 1 MiB)

The region of RAM above 1 MiB is not standardized, well-defined, or contiguous. There are likely to be regions of it that contain memory mapped hardware, that nothing but a device driver should ever access. There are likely to be regions of it that contain ACPI tables which your initialization code will probably want to read, and that then can be overwritten and reused. Some ACPI areas cannot be "reclaimed" this way. Some of the computer's RAM may extend above 4 GiB.

Use the BIOS function INT 15h, EAX=0xE820 to get a reliable map of Upper Memory.

start	end	size	region/exception	description
High Memory
00100000	003FFFFF	00300000	RAM -- guaranteed free for use¹	Extended memory
00400000	00EFFFFF	00900000 (if it all exists)	RAM -- free for use	Extended memory
00F00000	00FFFFFF	100000	Possible memory mapped hardware	ISA Memory Hole 15-16MB (only with ISA bus?)
01000000	????????	???????? (whatever exists)	RAM -- free for use	More Extended memory
C000000 (sometimes)	FFFFFFF	4000000	various (unusable except by drivers)	PnP NVRAM?, LAPIC, BIOS, ...
10000000 (possible mem above 4 GiB)	????????	???????? (whatever exists)	RAM -- free for use (PAE/64bit)	More Extended memory