Bill Gates’ Personal Easter Eggs in 8 Bit BASIC

If you type “WAIT6502,1” into a Commodore PET with BASIC V2 (1979), it will show the string “MICROSOFT!” at the top left corner of the screen. Legend has it Bill Gates himself inserted this easter egg “after he had had an argument with Commodore founder Jack Tramiel”, “just in case Commodore ever tried to claim that the code wasn’t from Microsoft”. read more

64 bit is a lot!

When people talk about porting their applications to 64 bit, I sometimes hear them wonder how long it will be until they have to port everything to 128 bit – after all, the swiches from 8 to 16 bit (e.g. CP/M to DOS), 16 to 32 bit (DOS/Windows 3 to Windows 95/NT) and 32 to 64 have all happened in the last 25 years. read more

Game Development Archeology: Zelda on Game Boy comes with source

Imagine you’re writing a Game Boy game, and the resulting ROM with all the code and data is just a little over one megabyte in size. No big deal, just pad the game to two megabytes, and use a 2 MB ROM in the cartridge. Just tell the linker to allocate 2 MB or RAM, put the actual data at the beginning, and then write a 2 MB “.gb” image to disk, which will then be sent to the ROM chip factory. read more

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