I've been attemping to get Linux to boot up out of FLASH on a SC400 eval
board and I've run into a little snag that someone may be able to help me
with.
Since I already had a fairly full featured bootloader running on the board
which sets up the board and switches to protected mode, etc, I took the
approach of skipping the initial code in bootsect.S and setup.S and loading
the 32-bit code directly into memory and just jumping in. I figured that
any of the stuff I skipped I could just add into my bootloader or to some
place early the the 32-bit code. This seemed to work fairly well. The
decompression code seems to run just fine, and after decompression it jumps
in to the kernel proper at abosolute address 0x100000 (1 Meg). Once in the
kernel I'm executing fine for several instructions, but then the board
reboots. I've narrowed it down to one instruction (set ATW: comment below)
that is causing the problem, but I can't see why. I've dumped memory (after
the decompression) and everything seems to be in the correct place.
Something about turning on paging is causing things to fail, but I can't see
why. There must be something which is either set up by the BIOS or by the
code in setup.S (which I skipped) but I'm not enough of an x86 person to
know. This same kernel/code will run fine if I make a boot disk out of it
and use one of the BIOS's included with the board.
from arch/i386/kernel/head.S
/*
* swapper_pg_dir is the main page directory, address 0x00101000
*/
ENTRY(stext)
ENTRY(_stext)
startup_32:
/*
* Set segments to known values
*/
cld
movl $(__KERNEL_DS),%eax
movl %ax,%ds
movl %ax,%es
movl %ax,%fs
movl %ax,%gs
#ifdef __SMP__
orw %bx,%bx
jz 1f
/*
* New page tables may be in 4Mbyte page mode and may
* be using the global pages.
*
* NOTE! We have to correct for the fact that we're
* not yet offset PAGE_OFFSET..
*/
#define cr4_bits mmu_cr4_features-__PAGE_OFFSET
movl %cr4,%eax # Turn on 4Mb pages
orl cr4_bits,%eax
movl %eax,%cr4
#endif
/*
* Setup paging (the tables are already set up, just switch them on)
*/
1:
movl $0x101000,%eax
movl %eax,%cr3 /* set the page table pointer.. */
movl %cr0,%eax
orl $0x80000000,%eax
movl %eax,%cr0 /* ..and set paging (PG) bit */
jmp 1f /* flush the prefetch-queue */
1:
movl $1f,%eax
jmp *%eax /* make sure eip is relocated */
1:
/* ATW: my Elan board reboots on this instruction!!! (lss) */
/* Set up the stack pointer */
lss stack_start,%esp
#ifdef __SMP__
orw %bx,%bx
jz 1f /* Initial CPU cleans BSS */
pushl $0
popfl
jmp checkCPUtype
1:
#endif __SMP__
/*
* Clear BSS first so that there are no surprises...
*/
xorl %eax,%eax
movl $ SYMBOL_NAME(__bss_start),%edi
movl $ SYMBOL_NAME(_end),%ecx
subl %edi,%ecx
cld
rep
stosb
--andy
