Hey x86-asm-experts,
On 31-7-2012 0:36, Erwin Rol wrote:
Hallo,
after I had successfully "ported" barebox to about 5 different embedded
x86 (mostly N270 and Z510 Atom boards) I ran into problems with a
Celeron SU2300 CPU based board. After a lot of trail and error, which
included the implementation of a BIOS based console cause the board does
not have a RS232 port, it looked like the global variables were not what
I expected them to be, namely zero.
So in arch/x86/boot/main_entry.c i changed to following;
/**
* Called plainly from assembler that switches from real to flat mode
*
* @note The C environment isn't initialized yet
*/
void uboot_entry(void)
{
#if 0
/* clear the BSS first */
memset(__bss_start, 0x00, __bss_stop - __bss_start);
#else
unsigned char* ptr;
for (ptr = __bss_start; ptr < __bss_stop; ptr++)
*ptr = 0x00;
#endif
start_barebox();
}
After doing that it works. I have not yet looked into the resulting asm
to find the differences, but could someone have an explanation for why
the memset doesn't work and the simple loop does work?
So when I use the memset code the asm looks like this below. As you can
see the memset gets its 3 parameters from the stack (see comments below
in the asm src). But when you look at the location when memset is called
it looks like the caller tries to pass the parameters in registers.
After looking into the Makefile I noticed the arch/x86/boot/* sources
get compiled with -mregparm=3, but the other sources use normal calling
convention. This means code from arch/x86/boot/* can not call any other
barebox functions when they are not of type void f(void). The file
arch/x86/boot/regs.c also calls memset but it seems the compiler notices
that regs.c is 16bit code and so it inlines a 16bit memset version (I at
least don't see a call function).
Is there any GCC or LD magic we can use to prevent error like this?
Wouldn't it be better to _not_ change the calling convention in the
Makefile, but mark the functions that need it with __attribute__ ?
00023e98 <in_pm32>:
23e98: 8e d9 mov %ecx,%ds
23e9a: 8e c1 mov %ecx,%es
23e9c: 8e e1 mov %ecx,%fs
23e9e: 8e e9 mov %ecx,%gs
23ea0: 8e d1 mov %ecx,%ss
23ea2: bc c8 f8 03 00 mov $0x3f8c8,%esp
23ea7: 0f 00 df ltr %di
23eaa: 31 c9 xor %ecx,%ecx
23eac: 31 d2 xor %edx,%edx
23eae: 31 db xor %ebx,%ebx
23eb0: 31 ed xor %ebp,%ebp
23eb2: 31 ff xor %edi,%edi
23eb4: 0f 00 d1 lldt %cx
23eb7: e9 00 00 00 00 jmp 23ebc <uboot_entry>
00023ebc <uboot_entry>:
// calculate (3rd parameter) size and put it in %ecx
23ebc: b9 c8 78 03 00 mov $0x378c8,%ecx
23ec1: 81 e9 70 a2 02 00 sub $0x2a270,%ecx
// clear %edx (2nd paremeter)
23ec7: 31 d2 xor %edx,%edx
// start address in %eax (1st parameter)
23ec9: b8 70 a2 02 00 mov $0x2a270,%eax
23ece: e8 e6 0b ff ff call 14ab9 <memset>
23ed3: e9 4c a4 fe ff jmp e324 <start_barebox>
00014ab9 <memset>:
14ab9: 55 push %ebp
14aba: 89 e5 mov %esp,%ebp
14abc: 53 push %ebx
// get parameter 1 (dst address)
14abd: 8b 45 08 mov 0x8(%ebp),%eax
// get parameter 2 (src byte to fill)
14ac0: 8b 5d 0c mov 0xc(%ebp),%ebx
// get parameter 3 (size)
14ac3: 8b 4d 10 mov 0x10(%ebp),%ecx
14ac6: 31 d2 xor %edx,%edx
14ac8: eb 04 jmp 14ace <memset+0x15>
14aca: 88 1c 10 mov %bl,(%eax,%edx,1)
14acd: 42 inc %edx
14ace: 39 ca cmp %ecx,%edx
14ad0: 75 f8 jne 14aca <memset+0x11>
14ad2: 5b pop %ebx
14ad3: 5d pop %ebp
14ad4: c3 ret
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