Testing Bits

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me239

Joined: 06 Jan 2011
Posts: 200

me239

Hello everyone! I just got directory support and file execution on my OS and now I want to add some form of protection from running the kernel again. I want to make it so that hidden files can neither be run or seen. I know that the attribute byte on fat12 is at 11d and that bit 2 is hidden. My question is how do I find whether bit 2 is enabled or not?

16 Jun 2011, 06:23

typedef

Joined: 25 Jul 2010
Posts: 2913
Location: 0x77760000

typedef

or it ?

idk, wait for the os gurus who know about fat12

16 Jun 2011, 07:02

JohnFound

Joined: 16 Jun 2003
Posts: 3500
Location: Bulgaria

JohnFound

It is strange you write your own OS, but miss so basic knowledge as testing bits... Shocked

Anyway, read "bt" instruction and "test" instruction reference.

16 Jun 2011, 07:37

me239

Joined: 06 Jan 2011
Posts: 200

me239

JohnFound wrote:

It is strange you write your own OS, but miss so basic knowledge as testing bits...
Anyway, read "bt" instruction and "test" instruction reference.

LOL, I know how to test for things like odd numbers. I think I figured it out now. I and it with 2 and test for a zero. Is this correct?
P.S it's 4am where I live and am very tired so I'm probably not thinking too well.

16 Jun 2011, 07:52

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4633
Location: Argentina

LocoDelAssembly

Code:

BIT_TO_TEST = 2 ; Change at will
test byte[mem], 1 shl BIT_TO_TEST
jnz .itIsSet

; If you want to create a BOOL in AL
mov al, [mem]
shr al, BIT_TO_TEST
and al, 1
; Now AL is either zero or one

;Alternate method (386+):
test byte[mem], 1 shl BIT_TO_TEST
setnz al

16 Jun 2011, 17:17

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