asm problems [FIXED]

GLneo · Post by **GLneo** » Tue Feb 20, 2007 12:31 pm

hi all, why does

EXTERN bss
EXTERN bsslength
mov edi, bss
mov ecx, bsslength
xor eax, eax
rep stosd

( should clear bss ) crashes my kernel, when "rep stosd" is run my kernel breaks, with out it it boots but i don't think the bss is cleared. this above code works on qemu but not on bochs or a real pc, anyone know why?

my link script is here: http://lemonos.cvs.sourceforge.net/lemo ... iew=markup
( it's where bss and bsslength are defined )

o and bochs says this:

Code: Select all

00004441948i[CPU  ] write_virtual_checks(): write beyond limit, r/w

thx

os64dev · Post by **os64dev** » Tue Feb 20, 2007 2:02 pm

did you print the values of bss and bsslength this to verify the correctness of the values. I had a similar problem that the divide didn't work so i manuallly did the divide in code and worked perfectly. also make sure that the direction is ok cld statement and the ds or es is properly setup.

proxy · Post by **proxy** » Tue Feb 20, 2007 3:07 pm

could be be because length is in bytes and your are storing dwords...thus you are clearing 4 times the amount of data as needed.

just add a "shr ecx, 2" before the rep movsd (effectively divides by 4) and i think you will be fine.

**EDIT: scratch that, your length is in dwords, dunno what's wrong

**EDIT2: perhaps you need a "cld" to make sure addresses are incrementing and not decrementing?

proxy

Candy · Post by **Candy** » Tue Feb 20, 2007 3:23 pm

If bsslength + base of segment > limit then you get an error. Since you have the same item...

Is this in rmode, pmode or lmode?

As said before, it could be the direction bit but I doubt it since I think bochs cleans it as well...

Combuster · Post by **Combuster** » Tue Feb 20, 2007 4:16 pm

Do you have paging setup properly and enabled? otherwise you'll be clearing miles beyond end of memory at 0xC01xxxxx ...

GLneo · Post by **GLneo** » Tue Feb 20, 2007 4:49 pm

ok, i set e,f,g(s):

Code: Select all

mov ax, 10h
mov ds, ax
mov ss, ax
mov es, ax  ; added
mov fs, ax  ; added
mov gs, ax  ; added

now it boots... but i still have other problems, i'll report them if i cant figure them out

thx all!

p.s. what is this: "cld" ?

m · Post by m » Tue Feb 20, 2007 10:09 pm

CLD:Clear the direction flag(DF) in the (E)FLAG.(i.e. Set Bit 10 to 0.)

If this flag is set,the SI and DI will be decremented when executing most of the string instructions(e.g. MOVS family,CMPS family,STOS and LODS family etc.).When the operands are 8-bit ones,they will be decremented by 1 at every loop,and 16-bit for 2,and 32-bit for 4,etc.,respectively.If the DF is cleared,they will be incremented with an appropriate value.

GLneo · Post by **GLneo** » Wed Feb 21, 2007 10:26 am

O, thx!