Re: Help on understanding mbr.S
On Fri, Sep 23, 2011 at 11:24 AM, Daniel Dickman didick...@gmail.comwrote: What are you trying to do though? Working with x86 in real mode and dealing with ancient PC conventions is probably not the easiest place to start. I'm trying to learn how kernel (or OS) works. I went through a couple of books on OS design and implementation and think I know some general rules on OS, now I'm reading the source code to learn the details. Thanks Daniel for the explanation. I went over the links you posted, and that gives me a better understanding of what the 1: is and local labels in general. A follow up question, though: So ljmp $BOOTSEC, $1f is used to set seg:offset to 07C0:. While $BOOTSEC is defined to be 07C0, why $1f is guaranteed to be , isn't it something volatile?
Re: Help on understanding mbr.S
Thanks Bryan. Your explanation makes things a lot clearer to me. As mentioned in my reply to Daniel, I not cannot figure out why $1f will be . On Fri, Sep 23, 2011 at 12:05 PM, Brynet bry...@gmail.com wrote: One of the first things an MBR does is do a long jump from where the BIOS loaded it. The thing is, often you can't trust the BIOS to do the right thing, the x86 in 16-bit real mode uses segmented memory, so you may be at :07C0 or 7C00: depending on the implementation. If you read the comment higher up you'll see they perform a long jump to normalize the Code Segment to 07C0, offset 0. :07C0 and 7C00: technically resolve to the same address, but enforcing segment:offset (cs:ip) just makes things consistent. The references to :1 is a local label, used for relative addressing, 'f' meaning forward and 'b' meaning backward. http://sourceware.org/binutils/docs/as/Symbol-Names.html Most MBR's are OS-independent, they relocate, parse partition table, load the PBR/VBR to 7C00 and perform a ljmp to it. Hope that helps, -Bryan.
Help on understanding mbr.S
Hello, Not sure if this is the right place to request help for this, but I'm reading mbr.S file (i386 arch), but could not figure out what the function is for the line that reads 1:. The code below that line is setting up statck, but why do we need this line? and there are more than one line that reads 1: in the mbr.S file, which confues me even more. Could someone help expain it a little a bit, or point me to some links that helps? thanks. ... ... .text .code16 .globl start start: /* Adjust %cs to be right */ ljmp $BOOTSEG, $1f 1: /* what is this, a label? */ /* Set up stack */ movw %cs, %ax /* * We don't need to disable and re-enable interrupts around the * the load of ss and sp. ... ... Complete mbr.S file I'm refering to: http://www.openbsd.org/cgi-bin/cvsweb/src/sys/arch/i386/stand/mbr/mbr.S?rev=1.21;content-type=text%2Fx-cvsweb-markup I'm still working on my ATT assembly learning, googled around but could not find anything related. Thanks, Alan
Re: Help on understanding mbr.S
The 1: is the target for the preceding ljmp instruction. This is a local label. Reference here: http://sourceware.org/binutils/docs/as/Symbol-Names.html#Symbol-Names The reason the ljmp is needed in the first place is because In real mode there are multiple ways to refer to the same memory address. The mbr does a ljmp early on to set the real mode segment:offset to known values. See: http://wiki.osdev.org/MBR_%28x86%29#Initial_Environment What are you trying to do though? Working with x86 in real mode and dealing with ancient PC conventions is probably not the easiest place to start.
Re: Help on understanding mbr.S
One of the first things an MBR does is do a long jump from where the BIOS loaded it. The thing is, often you can't trust the BIOS to do the right thing, the x86 in 16-bit real mode uses segmented memory, so you may be at :07C0 or 7C00: depending on the implementation. If you read the comment higher up you'll see they perform a long jump to normalize the Code Segment to 07C0, offset 0. :07C0 and 7C00: technically resolve to the same address, but enforcing segment:offset (cs:ip) just makes things consistent. The references to :1 is a local label, used for relative addressing, 'f' meaning forward and 'b' meaning backward. http://sourceware.org/binutils/docs/as/Symbol-Names.html Most MBR's are OS-independent, they relocate, parse partition table, load the PBR/VBR to 7C00 and perform a ljmp to it. Hope that helps, -Bryan.
