129 lines
4.1 KiB
Plaintext
129 lines
4.1 KiB
Plaintext
/* This document is intended to provide the developer with information
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* how to integrate a new CPU (MACH) into this part of the barebox tree
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*/
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/** @page dev_x86_mach barebox on x86 at runtime
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@section mach_x86_memory_layout barebox's memory layout (BIOS based)
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@a barebox uses the following memory layout at runtime when it still depends
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on some kind of BIOS function:
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@verbatim
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Addresses
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------------------------
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seg:off flat
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xxxx:xxxx 0x01xxxxxx end of barebox's malloc area
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. .
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xxxx:xxxx 0x01000000 start of barebox's malloc area
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. .
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. . (used while loading a Linux kernel of type 'bzImage')
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. .
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xxxx:xxxx 0x00100000 start of extended memory and malloc area
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. .
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. . (the big hole)
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. .
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9000:ffff 0x0009ffff end of expected real mode memory
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. .
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. . (used while loading a Linux kernel of type 'bzImage')
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. .
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9000:0000 0x00090000 end of used lower real mode memory
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. .
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. .
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. . Flat mode stack (about 32 kiB)
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. . bss
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. . Data
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. . Text
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0000:7e00 0x00007e00 Real and flat mode barebox code
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0000:7c00 0x00007c00 MBR initial boot loader code
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0000:7a00 0x00007a00 location of the indirect sector (while booting only)
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below: real mode stack
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@endverbatim
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@note The start address of 0x0000:7c000 is a fixed one, defined by the BIOS.
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So, for a BIOS based @a barebox this address can't be changed.
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While the @a barebox code is runnung in flat mode, all interrupts are disabled.
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But in the CPU only. All other interrupt settings are still valid. This is
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required to be able to call real mode code from inside @a barebox flat mode
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code. Thats why not the PIC is touched nor the IDT.
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@todo Add some notes about drive numbers used by the BIOS. They may change
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if one change orders in the BIOS setup. Drive orders and numbers may be
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different at BIOS runtime and Linux runtime! But these numbers are required
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at BIOS runtime for booting and the persistant environment storage.
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@attention Currently there is a 4 GiB limit for the disk sizes!
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@section mach_x86_image_layout barebox's image layout
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@a barebox's binary image layout
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@verbatim
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Offset Content
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0x?????
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. 32 bit barebox code
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.
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. 16 bit bootstrap code, BIOS calling code
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0x00400
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0x003ff
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. indirect sector
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0x00200
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0x001ff
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. MBR
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0x00000
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@endverbatim
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The "indirect sector" is a free area in the image where the sector information
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gets stored when this image will be written to a boot media. This information
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is required to load all parts of the image from the boot media at runtime.
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The image gets installed in two ways onto the boot media, depending on the
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need for a persistant storage.
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@subsection mach_x86_drive_layout_wops barebox's boot media layout without persistant storage
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In this case @a barebox's persistant storage is anywhere:
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@verbatim
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Sector Content
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---------------------------
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X start of first partition
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.
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? end of the binary image
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. 32 bit barebox code
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2 16 bit bootstrap code, BIOS calling code
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1 indirect sector
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0 MBR, Partition table, boot code
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@endverbatim
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@subsection mach_x86_drive_layout_wps barebox's boot media layout with persistant storage
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@a barebox's persistant storage is part of the boot media (more
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space required in front of the first partition) and interferes with the
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boot loader image itself:
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@verbatim
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Sector Content
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---------------------------
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X start of first partition
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.
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n+? end of the binary image
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. 32 bit barebox code
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n+2 16 bit bootstrap code, BIOS calling code
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n+1 indirect sector
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n end of persistant environment storage
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.
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1 start of persistant environment storage
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0 MBR, Partition table, boot code
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@endverbatim
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The information where the persistant storage is located is also stored into
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the MBR at specific locations by @p setupmbr. The @a barebox runtime will use
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it to load and store all environment relevant data.
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*/
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