How retiring segmentation in AMD64 long mode broke VMware

UNIX, Windows NT, and all the operating systems in their class rely on virtual memory, or paging, in order to provide every process on the system a complete address space of its own. An easier way to protect processes from each other is segmentation: The 4 GB address space of a 32 bit CPU is divided into segments (consisting of a physical base address and a limit), one for each process, and every process may only access their own segment. This is what the 286 did. read more

How to divide fast by immediates

In almost all assembly books you’ll find some nice tricks to do fast multiplications. E.g. instead of “imul eax, ebx, 3” you can do “lea eax, [ebx+ebx*2]” (ignoring flag effects). It’s pretty clear how this works. But how can we speed up, say, a division by 3? This is quite important since division is still a really slow operation. If you never thought or heart about this problem before, get pen and paper and try a little bit. It’s an interesting problem. read more

Shift oddities

Most of the x86 instructions will automatically alter the flags depending on the result. Sometimes this is rather frustrating because you actually what to preserve the flags as long as possible, and sometimes you miss a “mov eax, ecx” which alters the flags. But at least it’s guaranteed that an instruction either sets the flags or it doesn’t touch them, independent of the actual operation… Or is it? read more

Redundant SSE instructions

As we all know the x86-ISA has a lot of redundant instructions (ie. instructions with the same semantic but different opcodes). Sometimes this is unavoidable, sometimes it looks like bad design. But with SSE it gets really weird. Let’s say we want to perform xmm0 <- xmm0 & xmm1 (ie. bitwise and). Not an uncommon operation; but we have 3 different ways do archive this:

  • andps xmm0, xmm1 (0f 54 c1)
  • andpd xmm0, xmm1 (66 0f 54 c1)
  • pand xmm0, xmm1 (66 0f db c1)

(Note that andpd/pand are SSE2 instructions)
Regarding the result in xmm0 these are really the same instructions. Now, why did Intel do this? First we’re going to inspect andps/andpd. Looking at the optimization manuals we get a hint: The ps/pd mark the target register to contain singles or doubles, so they should match the actual data you are operating on. read more

Virtualization: The elegant way and the x86 way

Virtualization means running one or more complete operating systems (at the same time) on one machine, possibly on top of another operating system. VMware, VirtualPC, Parallels etc. support, for example, running a complete GNU/Linux OS on top of Windows. For virtualization, the Virtual Machine Monitor (VMM) must be more powerful than kernel mode code of the guest: The guest’s kernel mode code must not be allowed to change the global state of the machine, but may not notice that its attempts fail, as it was designed for kernel mode. The VMM as the arbiter must be able to control the guest completely. read more

The real reason for driver signing in Vista x64

In Windows Vista x64, drivers are required to be signed by someone holding a VeriSign code certificate or they won’t load. There is no way to (permanently) disable this signing even if you are Administrator. The F8 startup menu has an option to disable it, but you must select it every time you boot up. Microsoft’s claimed reason for this is that it prevents Trojans from installing kernel-mode rootkits. That is a load of crap. read more

Asking the kernel how to make a syscall

Imagine you’re an i386 user mode application on a modern operating system, and you want to make a syscall, for example to request some memory or create a new thread. But syscalls can be made in various ways on the i386 family of CPUs (int, call gates, sysenter, syscall), and CPUs tend to support only a subset of them. But hardcoding “int” into the kernel is a waste of resources on modern CPUs, because sysenter is a lot faster. read more