Switching modes with Style
pushl $(0xcb<<24)|0x08 call .-1
What does this instruction sequence do? (This was a collaborative effort by Chuck Gray, Myria and Michael.)
Michael Steil
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.
Arithmetic mean of two signed integers
It’s time for a puzzle again! (submitted by sheepmaster)
How Itanium messed up Intel’s CPUID family IDs
Assigning internal version/family/model IDs to products is a non-trivial task, especially if there are several different families/architectures on your roadmap, and if the marketing names and target markets have no real correlation to the internal architecture.
FFREEP – the assembly instruction that never existed
Due to simplified instruction decoding of the Intel 80287, this CPU had opcode aliases for instructions like FXCH, FSTP, i.e. there were some additional encodings that did the same as the originals as defined by the 8087. As a side effect of this, a new instruction, FFREEP appeared, although not intented by Intel.
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.
The funny page table terminology on AMD64
What’s the next word in this sequence: PT, PD, PDP, …?
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.
Why does PUSHA also push the stack pointer?
This puzzle is actually a quite easy one – but when I asked it in a university course, it kept some people busy for some time to find out the answer, so I thought it might be a good idea to ask you nevertheless:
How NOP nearly became a non-NOP on AMD64
AMD64 is a quite clean extension of the i386 instruction set, it obsoletes many rarely used features of the i386 and introduces new registers, making the instruction set a lot more… logical.