First of all, hello ...
I would like to know what is the advantage to go to Ring0 mode ... what kind of thing can you do while in Ring0 mode ?
I would also like to know what is the use of shr shl ( shif left, shift right )
And finally what does db means before a string
like db ญญ'some thing'
Thank you
Well,
I've not so many time... I'll explain quickly what db is:
db means that you define byte(s), it can be a string (many bytes) or just 1 byte or many bytes. For example:
Name db "abcdf",0
is equal to
Name db 61h, 62h, 63h, 64h, 65h, 0
You can define also 1 byte (to make a counter i.e):
MyCount db 0
Then you can turn it to 0 to 255 (0FFh in hexadec)
Hope you understand
PS:there is also 'dd', 'w', ...
The shl/shr instructions are used to quickly multiply/divide by two (unsigned). Also in graphics/encryption/compression algorithms. To start with, it's just good to know what it does; and then in time you'll get a feel for where it's used. Just think of the bits lined up in a row (a parade, food line, etc.). One zero bit comes in from the left or right side, and a bit is lost out of the other side - not really lost - it gets put into the carry flag.
It's important to compare these instructions to the other shift instructions to know the differences. For example, to do signed multiply/divide by two use sal/sar - the high bit is presevered with sar - there is no difference between shl and sal.
I chickened out and took the other easy question. :D You should probable browse the message board and web on the ring0 question - there is much information.
You want me,
Ring 0 is an access right to certain instructions in the
processor, it is usually used for device drivers that must
supply services to the operating system. Major source of info
is in the platform specific device development kits from
Microsoft. If you look around the net, you will find some info
on device drivers and Iczelion has a set of tutorials on the
win95 form of device drivers, VxD.
DB literally means BYTE size data and it is used in assembler
for things like string data which is usually a sequence of bytes.
In the section of a 32 bit PE file called the .DATA section, you
can use the notation,
varname DB "This is a zero terminated string",0
SHR eax, 2
shl ax, 1
shl ax, 1
shl ax, 1
shl ax, 1
and
shl ax, 4
which one is faster? some book i read said that if you shl n time
where n is less than 4, then you should use shl reg, 1
n time. instead of shl ax, n