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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.
 
=="Low" memoryReal mode address space (< 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)
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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 (PC)#BIOS functions|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.
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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 [[How_do_I_set_a_graphics_mode|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
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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 lessat thanmost 128 KiB in size. ItOlder iscomputers oftentypically uses 1 KiB. Thefrom biggest0x9FC00 ones- ever0x9FFFF, actuallymodern seenfirmware arecan 8be KiBfound using significantly more.
You can determine the size of the EBDA by using BIOS function [[Detecting Memory (x86)#Detecting Low Memory|INT 12h]], or (often) by examining the word at 0x40E0x413 in the BDA (see below).
Both of those methods will tell you thehow locationmuch ofconventional thememory bottomis ofusable before 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)
 
{| {{wikitable}}
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! end
! size
! type
! description
! colspan=2 | type
|-
! colspan=56 | LowReal Memorymode address space (the first MiB)
|-
| 0x00000000
| 00000000
| 0x000003FF
| 000003FF
| 400 (1 KiB)
| RAM - partially unusable (see above)
| Real Mode IVT (Interrupt Vector Table)
| rowspan=2 | unusable in real mode
| rowspan=6 | 640 KiB RAM ("Low memory")
|-
| 0x00000400
| 00000400
| 0x000004FF
| 000004FF
| 256 bytes
| 100
| RAM - partially unusable (see above)
| BDA (BIOS data area)
|-
| 0x00000500
| 00000500
| 0x00007BFF
| 00007BFF
| 7700 (almost 3029.75 KiB)
| RAM (guaranteed free for use)
| Conventional memory
| rowspan=3 | usable memory
|-
| 0x00007C00
| 00007C00 (typical location)
| 0x00007DFF
| 00007DFF
| 512 bytes
| 200
| RAM - partially unusable (see above)
| Your OS BootSector
|-
| 0x00007E00
| 00007E00
| 0x0007FFFF
| 0007FFFF
| 7FB00 (481480.5 KiB)
| RAM (guaranteed free for use)
| Conventional memory
|-
| 0x00080000
| 00080000
| 0x0009FFFF
| 0009FBFF
| 128 KiB
| 1FC00 (approximately 120 KiB)
| RAM (free for use, <b>if it exists</b>)
| Conventional memory
|-
| 0009FC00 (typical location)
| 0009FFFF
| 400
| RAM (unusable)
| EBDA (Extended BIOS Data Area)
| partially used by the EBDA
|-
| 0x000A0000
| 0x000BFFFF
| 128 KiB
| Video display memory
| hardware mapped
| rowspan=4 | 384 KiB System / Reserved ("Upper Memory")
|-
| 0x000C0000
| 0x000C7FFF
| 32 KiB (typically)
| Video BIOS
| rowspan=3 | ROM and hardware mapped / Shadow RAM
|-
| 0x000C8000
| 0x000EFFFF
| 160 KiB (typically)
| BIOS Expansions
|-
| 0x000F0000
| 000A0000
| 0x000FFFFF
| 000FFFFF
| 6000064 KiB
| Motherboard BIOS
| various (unusable)
| ROM Area (384 KiB)
|}
 
 
===BIOS Data Area (BDA)===
 
The BDA is only partially standardized, and almostmostly allrelevant thefor valuesreal storedmode thereBIOS are completely obsolete and uninterestingoperations.
The following is a partial list. See the External Links references below for more detail.
 
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! description
|-
| 4000x0400 (word4 words)
| IO portports for COM1-COM4 serial (each address is 1 word, zero if none)
|-
| 4080x0408 (word3 words)
| IO portports for LPT1-LPT3 parallel (each address is 1 word, zero if none)
|-
| 40E0x040E (word)
| EBDA base address >> 4 (<b>usually!</b>)
|-
| 4100x0410 (word)
| packed bit flags for detected hardware
|-
| 4490x0413 (byteword)
| Number of kilobytes before EBDA / unusable memory
|-
| 0x0417 (word)
| keyboard state flags
|-
| 0x041E (32 bytes)
| keyboard buffer
|-
| 0x0449 (byte)
| Display Mode
|-
| 4630x044A (2 bytes, taken as a word)
| number of columns in text mode
|-
| 0x0463 (2 bytes, taken as a word)
| base IO port for video
|-
| 46C0x046C (word)
| # of IRQ0 timer ticks since boot
|-
| 4750x0475 (byte)
| # of hard disk drives detected
|-
| 4970x0480 (byteword)
| 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. TheyCurrent doBIOS might use a different map. The EBDA area is not standardized.
apply to any current EBDA, used by any current BIOS. The EBDA area is not standardized. It <b>does</b> contain data that your
OS will need, but you must do a bytewise pattern search to find those tables. (See [[PlugNPlay]].)
 
==Extended Memory (> 1 MiB)==
 
===ROM Area===
 
{| {{wikitable}}
|-
! start
! end
! size
! region/exception
! description
|-
! colspan=5 | 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
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Some ACPI areas cannot be "reclaimed" this way. Some of the computer's RAM may extend above 4 GiB.
 
Use the BIOS function [[Detecting Memory (x86)#BIOS Function: INT 0x15, EAX = 0xE820|INT 15h, EAX=0xE820]] to get a reliable map of UpperExtended Memory.
 
 
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! colspan=5 | High Memory
|-
| 0x00100000
| 00100000
| 0x00EFFFFF
| 003FFFFF
| 0x00E00000 (14 MiB)
| 00300000
| RAM -- guaranteed free for use<sup>1</sup> (if it exists)
| Extended memory <sup>1, 2</sup>
|-
| 0x00F00000
| 00400000
| 0x00FFFFFF
| 00EFFFFF
| 0x00100000 (1 MiB)
| 00B00000 (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?)<sup>3</sup>
|-
| 0x01000000
| 01000000
| ????????
| ???????? (whatever exists)
| RAM -- free for use
| More Extended memory <sup>1</sup>
|-
| C0000000xC0000000 (sometimes, depends on motherboard and devices)
| 0xFFFFFFFF
| FFFFFFF
| 0x40000000 (1 GiB)
| 4000000
| various (unusabletypically exceptreserved byfor driversmemory mapped devices)
| Memory mapped PCI devices, PnP NVRAM?, LAPICIO APIC/s, local APIC/s, BIOS, ...
|-
| 100000000x0000000100000000 (possible memmemory above 4 GiB)
| ????????????????
| ???????????????? (whatever exists)
| RAM -- free for use (PAE/64bit)
| More Extended memory <sup>1</sup>
|-
| ????????????????
| ????????????????
| ????????????????
| Possible memory mapped hardware
| Potentially usable for memory mapped PCI devices in modern hardware (but typically not, due to backward compatibility)
|}
 
<sup>1</sup>: 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".
<sup>1</sup>: Free for use except that your bootloader (ie. GRUB) may have loaded your "modules" here, and you don't want to overwrite those.
 
<sup>2</sup>: Free for use except that your bootloader (ie. GRUB) may have loaded your "modules" here, and you don't want to overwrite those.
 
<sup>3</sup>: 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.
==Comments==
 
 
==See Also==
* [[Detecting Memory (x86)]]
 
===External Links===
* https://web.archive.org/web/20120130052813/http://www.nondot.org/sabre/os/files/Booting/BIOS_SEG.txt -- detailed BIOS Data Area map
* httphttps://web.archive.org/web/20221127214951/https://www.bioscentral.com/misc/bda.htm -- another detailed BIOS Data Area map
* [http://myfiles.execpcosdev.comorg/mirrors/~geezer/osd/ram/index.htm#layout Geezer's memory layout description]
* http://stanislavs.org/helppc/bios_data_area.html
 
[[Category:X86]]
[[Category:Physical Memory]]
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