By Fabrice Bellard - May 14, 2011
32 bit x86 compatible CPU
8259 Programmble Interrupt Controller
8254 Programmble Interrupt Timer
Real Time Clock.
Some of the code is inspired from my x86 dynamic translator present in QEMU, but there are important differences because here it is an interpreter. The CPU is close to a 486 compatible x86 without FPU. The lack of FPU is not a problem when running Linux as Operating System because it contains a FPU emulator. In order to be able to run Linux, a complete MMU is implemented. The exact restrictions of the emulated CPU are:
No segment limit and right checks when accessing memory (Linux does not rely on them for memory protection, so it is not an issue. The x86 emulator of QEMU has the same restriction).
No CS/DS/ES/SS segment overrides. FS/GS overrides are implemented because they are needed for Thread Local Storage in Linux.
A few seldom used instructions are missing (BCD operations, BOUND, ...).
No real mode
No 16 bit protected mode (although most 16 bit instructions are present because they are needed to run 32 bit programs).
Most of these restrictions are easy to remove, but I decided to implement the strict minimum to be able to use a recent Linux kernel and its user applications.
I added some tricks which are not present in QEMU to be more precise when emulating unaligned load/stores at page boundaries. The condition code emulation is also more efficient than the one in QEMU.
Currently there is no synchronization between the PIT frequency and the real time, so there is a variable drift between the time returned by Linux (try the "date" command) and the real time.
The UART (serial port) does not support FIFO mode. Perhaps it could help to improve the display speed.
There is no network emulation at this point.
A clipboard device (seen as /dev/clipboard in the emulator) was added to allow exchange of data between the emulator and the outside world.
Although I could have reused the excellent termlib, I decided to write my own because I was curious to see how it could be done. The main problem is the key handling which is different among browsers and OSes, as described here.
I compiled a 2.6.20 Linux kernel (I guess any other version would work provided there is still an FPU emulator). The configuration is here and there is a small optional patchto avoid outputting warnings due to the slow serial port. An uncompressed kernel image is used instead of a compressed one to have a faster boot. It is generated with "objcopy -O binary vmlinux vmlinux.bin".
The disk image is just a ram disk image loaded at boot time. It contains a filesystem generated with Buildroot containing BusyBox. I added my toy C compiler TinyCC and my unfinished but usable emacs clone QEmacs.
A troubling thing is that the PC emulator is about 2 times slower using V8 than Jaeger Monkey (I used the 32 bit version for both). I have no precise explanation yet because I only looked at the Jeager Monkey code so far.
What's the use ?
Learning to use command line Unix tools without leaving the browser.
A more advanced version would allow to use old DOS PC software such as games.
[Back to the PC emulator]