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Please mail me (Jon Diekema, or
if you have questions, comments or corrections.
* EST SBC8260 Linux memory mapping rules
Initial conditions:
Tasks that need to be perform by the boot ROM before control is
transferred to zImage (compressed Linux kernel):
- Define the IMMR to 0xf0000000
- Initialize the memory controller so that RAM is available at
physical address 0x00000000. On the SBC8260 is this 16M (64M)
- The boot ROM should only clear the RAM that it is using.
The reason for doing this is to enhances the chances of a
successful post mortem on a Linux panic. One of the first
items to examine is the 16k (LOG_BUF_LEN) circular console
buffer called log_buf which is defined in kernel/printk.c.
- To enhance boot ROM performance, the I-cache can be enabled.
Date: Mon, 22 May 2000 14:21:10 -0700
From: Neil Russell <>
LiMon (LInux MONitor) runs with and starts Linux with MMU
off, I-cache enabled, D-cache disabled. The I-cache doesn't
need hints from the MMU to work correctly as the D-cache
does. No D-cache means no special code to handle devices in
the presence of cache (no snooping, etc). The use of the
I-cache means that the monitor can run acceptably fast
directly from ROM, rather than having to copy it to RAM.
- Build the board information structure (see
include/asm-ppc/est8260.h for its definition)
- The compressed Linux kernel (zImage) contains a bootstrap loader
that is position independent; you can load it into any RAM,
ROM or FLASH memory address >= 0x00500000 (above 5 MB), or
at its link address of 0x00400000 (4 MB).
Note: If zImage is loaded at its link address of 0x00400000 (4 MB),
then zImage will skip the step of moving itself to
its link address.
- Load R3 with the address of the board information structure
- Transfer control to zImage
- The Linux console port is SMC1, and the baud rate is controlled
from the bi_baudrate field of the board information structure.
On thing to keep in mind when picking the baud rate, is that
there is no flow control on the SMC ports. I would stick
with something safe and standard like 19200.
On the EST SBC8260, the SMC1 port is on the COM1 connector of
the board.
EST SBC8260 defaults:
Memory Sel Bus Use
--------------------- --- --- ----------------------------------
0x00000000-0x03FFFFFF CS2 60x (16M or 64M)/64M SDRAM
0x04000000-0x04FFFFFF CS4 local 4M/16M SDRAM (soldered to the board)
0x21000000-0x21000000 CS7 60x 1B/64K Flash present detect (from the flash SIMM)
0x21000001-0x21000001 CS7 60x 1B/64K Switches (read) and LEDs (write)
0x22000000-0x2200FFFF CS5 60x 8K/64K EEPROM
0xFC000000-0xFCFFFFFF CS6 60x 2M/16M flash (8 bits wide, soldered to the board)
0xFE000000-0xFFFFFFFF CS0 60x 4M/16M flash (SIMM)
- The chip selects can map 32K blocks and up (powers of 2)
- The SDRAM machine can handled up to 128Mbytes per chip select
- Linux uses the 60x bus memory (the SDRAM DIMM) for the
communications buffers.
- BATs can map 128K-256Mbytes each. There are four data BATs and
four instruction BATs. Generally the data and instruction BATs
are mapped the same.
- The IMMR must be set above the kernel virtual memory addresses,
which start at 0xC0000000. Otherwise, the kernel may crash as
soon as you start any threads or processes due to VM collisions
in the kernel or user process space.
Details from Dan Malek <> on 10/29/1999:
The user application virtual space consumes the first 2 Gbytes
(0x00000000 to 0x7FFFFFFF). The kernel virtual text starts at
0xC0000000, with data following. There is a "protection hole"
between the end of kernel data and the start of the kernel
dynamically allocated space, but this space is still within
Obviously the kernel can't map any physical addresses 1:1 in
these ranges.
Details from Dan Malek <> on 5/19/2000:
During the early kernel initialization, the kernel virtual
memory allocator is not operational. Prior to this KVM
initialization, we choose to map virtual to physical addresses
1:1. That is, the kernel virtual address exactly matches the
physical address on the bus. These mappings are typically done
in arch/ppc/kernel/head.S, or arch/ppc/mm/init.c. Only
absolutely necessary mappings should be done at this time, for
example board control registers or a serial uart. Normal device
driver initialization should map resources later when necessary.
Although platform dependent, and certainly the case for embedded
8xx, traditionally memory is mapped at physical address zero,
and I/O devices above physical address 0x80000000. The lowest
and highest (above 0xf0000000) I/O addresses are traditionally
used for devices or registers we need to map during kernel
initialization and prior to KVM operation. For this reason,
and since it followed prior PowerPC platform examples, I chose
to map the embedded 8xx kernel to the 0xc0000000 virtual address.
This way, we can enable the MMU to map the kernel for proper
operation, and still map a few windows before the KVM is operational.
On some systems, you could possibly run the kernel at the
0x80000000 or any other virtual address. It just depends upon
mapping that must be done prior to KVM operational. You can never
map devices or kernel spaces that overlap with the user virtual
space. This is why default IMMR mapping used by most BDM tools
won't work. They put the IMMR at something like 0x10000000 or
0x02000000 for example. You simply can't map these addresses early
in the kernel, and continue proper system operation.
The embedded 8xx/82xx kernel is mature enough that all you should
need to do is map the IMMR someplace at or above 0xf0000000 and it
should boot far enough to get serial console messages and KGDB
connected on any platform. There are lots of other subtle memory
management design features that you simply don't need to worry
about. If you are changing functions related to MMU initialization,
you are likely breaking things that are known to work and are
heading down a path of disaster and frustration. Your changes
should be to make the flexibility of the processor fit Linux,
not force arbitrary and non-workable memory mappings into Linux.
- You don't want to change KERNELLOAD or KERNELBASE, otherwise the
virtual memory and MMU code will get confused.
arch/ppc/Makefile:KERNELLOAD = 0xc0000000
include/asm-ppc/page.h:#define PAGE_OFFSET 0xc0000000
include/asm-ppc/page.h:#define KERNELBASE PAGE_OFFSET
- RAM is at physical address 0x00000000, and gets mapped to
virtual address 0xC0000000 for the kernel.
Physical addresses used by the Linux kernel:
0x00000000-0x3FFFFFFF 1GB reserved for RAM
0xF0000000-0xF001FFFF 128K IMMR 64K used for dual port memory,
64K for 8260 registers
Logical addresses used by the Linux kernel:
0xF0000000-0xFFFFFFFF 256M BAT0 (IMMR: dual port RAM, registers)
0xE0000000-0xEFFFFFFF 256M BAT1 (I/O space for custom boards)
0xC0000000-0xCFFFFFFF 256M BAT2 (RAM)
0xD0000000-0xDFFFFFFF 256M BAT3 (if RAM > 256MByte)
EST SBC8260 Linux mapping:
DBAT0, IBAT0, cache inhibited:
Memory Sel Use
--------------------- --- ---------------------------------
0xF0000000-0xF001FFFF n/a IMMR: dual port RAM, registers
DBAT1, IBAT1, cache inhibited: