pagetable.com – Page 14 – Some Assembly Required

How Itanium messed up Intel’s CPUID family IDs

2019-05-072006-07-26 by Michael Steil

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. read more

Win32's MulDiv

2006-07-02 by Myria

In Win32, there is an API call called “MulDiv”: read more

FFREEP – the assembly instruction that never existed

2019-05-072006-06-26 by Michael Steil

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. read more

Virtualization: The elegant way and the x86 way

2019-05-072006-06-16 by Michael Steil

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 funny page table terminology on AMD64

2019-05-072006-06-12 by Michael Steil

What’s the next word in this sequence: PT, PD, PDP, …? read more

The C ! operator

2006-06-07 by Myria

In C, the ! (“logical NOT”) operator used on a value x evaluates to 0 when x is not 0, and 1 when x is 0. In other words, it’s equivalent to the following C: read more

The real reason for driver signing in Vista x64

2006-06-04 by Myria

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

Microsoft changes CS value in Win64

2019-05-072006-05-21 by Myria

I just found out the hard way that in 32 bit programs under Win64, the value of CS changed. In Win32, the value of CS is 0x001B. In 32 bit programs under Win64, it’s 0x0023. This will probably break some programs, especially debuggers. read more

Simple compiler optimization

2019-05-072006-05-20 by Myria

I thought of an optimization that compilers for most CPUs could do that I think should be implemented. Let’s say you have C code like this: read more

Asking the kernel how to make a syscall

2019-05-072006-05-10 by Michael Steil

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