[转]About TEXTADDR, ZTEXTADDR, PAGE_OFFSET etc

Initial definitions
-------------------

The following symbol definitions rely on you knowing the translation that
__virt_to_phys() does for your machine.  This macro converts the passed
virtual address to a physical address.  Normally, it is simply:

phys = virt - PAGE_OFFSET + PHYS_OFFSET


Decompressor Symbols
--------------------

ZTEXTADDR
Start address of decompressor.  There's no point in talking about
virtual or physical addresses here, since the MMU will be off at
the time when you call the decompressor code.  You normally call
the kernel at this address to start it booting.  This doesn't have
to be located in RAM, it can be in flash or other read-only or
read-write addressable medium.

ZBSSADDR
Start address of zero-initialised work area for the decompressor.
This must be pointing at RAM.  The decompressor will zero initialise
this for you.  Again, the MMU will be off.

ZRELADDR
This is the address where the decompressed kernel will be written,
and eventually executed.  The following constraint must be valid:

__virt_to_phys(TEXTADDR) == ZRELADDR

The initial part of the kernel is carefully coded to be position
independent.

INITRD_PHYS
Physical address to place the initial RAM disk.  Only relevant if
you are using the bootpImage stuff (which only works on the old
struct param_struct).

INITRD_VIRT
Virtual address of the initial RAM disk.  The following  constraint
must be valid:

__virt_to_phys(INITRD_VIRT) == INITRD_PHYS

PARAMS_PHYS
Physical address of the struct param_struct or tag list, giving the
kernel various parameters about its execution environment.


Kernel Symbols
--------------

PHYS_OFFSET
Physical start address of the first bank of RAM.

PAGE_OFFSET
Virtual start address of the first bank of RAM.  During the kernel
boot phase, virtual address PAGE_OFFSET will be mapped to physical
address PHYS_OFFSET, along with any other mappings you supply.
This should be the same value as TASK_SIZE.

TASK_SIZE
The maximum size of a user process in bytes.  Since user space
always starts at zero, this is the maximum address that a user
process can access+1.  The user space stack grows down from this
address.

Any virtual address below TASK_SIZE is deemed to be user process
area, and therefore managed dynamically on a process by process
basis by the kernel.  I'll call this the user segment.

Anything above TASK_SIZE is common to all processes.  I'll call
this the kernel segment.

(In other words, you can't put IO mappings below TASK_SIZE, and
hence PAGE_OFFSET).

TEXTADDR
Virtual start address of kernel, normally PAGE_OFFSET + 0x8000.
This is where the kernel image ends up.  With the latest kernels,
it must be located at 32768 bytes into a 128MB region.  Previous
kernels placed a restriction of 256MB here.

DATAADDR
Virtual address for the kernel data segment.  Must not be defined
when using the decompressor.

VMALLOC_START
VMALLOC_END
Virtual addresses bounding the vmalloc() area.  There must not be
any static mappings in this area; vmalloc will overwrite them.
The addresses must also be in the kernel segment (see above).
Normally, the vmalloc() area starts VMALLOC_OFFSET bytes above the
last virtual RAM address (found using variable high_memory).

VMALLOC_OFFSET
Offset normally incorporated into VMALLOC_START to provide a hole
between virtual RAM and the vmalloc area.  We do this to allow
out of bounds memory accesses (eg, something writing off the end
of the mapped memory map) to be caught.  Normally set to 8MB.

Architecture Specific Macros
----------------------------

BOOT_MEM(pram,pio,vio)
`pram' specifies the physical start address of RAM.  Must always
be present, and should be the same as PHYS_OFFSET.

`pio' is the physical address of an 8MB region containing IO for
use with the debugging macros in arch/arm/kernel/debug-armv.S.

`vio' is the virtual address of the 8MB debugging region.

It is expected that the debugging region will be re-initialised
by the architecture specific code later in the code (via the
MAPIO function).

BOOT_PARAMS
Same as, and see PARAMS_PHYS.

FIXUP(func)
Machine specific fixups, run before memory subsystems have been
initialised.

MAPIO(func)
Machine specific function to map IO areas (including the debug
region above).

INITIRQ(func)
Machine specific function to initialise interrupts.