On September 6, 2025, Microsoft quietly released what may be one of the most historically significant pieces of code in personal computing history: the complete source code for Microsoft BASIC for 6502 Version 1.1 under the MIT open-source license. This 1978 interpreter powered some of the most iconic personal computers of the late 1970s and early 1980s, including the Apple II, Commodore 64, and dozens of other systems that defined the home computing revolution.
This isn't just about nostalgia. If you wrote BASIC programs as a kid, this is the actual code that made it possible. This is the actual code that made Gates rich as fuck.
The Code That Built an Empire
The MOS 6502 microprocessor, released in 1975, became the foundation of the personal computer revolution due to its low cost and high performance.
The 6502 processor, designed by MOS Technology in 1975, became the heart of the personal computer revolution due to its low cost and high performance. Unlike Intel's more expensive 8080 processor, the 6502 was affordable enough for hobbyist computer makers, leading to its adoption in the Apple II, Commodore PET, Atari 2600, and dozens of other systems.
Microsoft's BASIC interpreter for the 6502 was co-authored by Bill Gates and Paul Allen in 1976-1977, building on their earlier work with 8080 BASIC for the Altair 8800. The 6502 version needed serious optimization because the processor was different from Intel's 8080, and you had maybe 16KB of RAM total if you were lucky.
Gates and Allen crammed an entire BASIC interpreter into 4KB of RAM - smaller than this paragraph. Meanwhile, today's JavaScript frameworks need 100MB to display "Hello World." What the fuck happened to efficiency? This code includes floating-point arithmetic, string handling, and graphics commands, all hand-optimized in 6502 assembly language. Every byte mattered - one wasted instruction meant less program space.
Technical Significance and Innovation
You can see the actual code on GitHub now, and it shows some clever tricks that became standard. The code includes memory management that would make modern malloc() implementations jealous, tokenization that compressed programs into tiny memory footprints, and error handling that didn't just segfault when you did something stupid.
The really impressive part is how they made the same interpreter work across completely different machines. By abstracting hardware-specific functions into separate modules, the company could port BASIC to new computers quickly, establishing the economic model that would define Microsoft's success throughout the 1980s.
This interpreter had multi-dimensional arrays, string handling, and even graphics commands - all in 1978, when most computers couldn't even display lowercase letters properly. Try running DIM A(100,100,100) on a Commodore 64 and watch it freeze harder than Windows Vista. You'll get "OUT OF MEMORY ERROR IN 10" because you just asked for 4MB of RAM from a machine that has 64KB total. I learned this the hard way when I was 12 and couldn't figure out why my "ultimate RPG" crashed before it started. These capabilities helped establish BASIC as the dominant programming language for early personal computers, with Microsoft's version becoming the de facto standard.
Historical Context and Impact
The Apple II, powered by the 6502 processor and Microsoft BASIC, became one of the first successful mass-market personal computers.
When this code was originally written, the personal computer industry barely existed. Apple had just incorporated, and IBM wouldn't release its PC until 1981. Microsoft was a tiny company with fewer than a dozen employees, operating out of a small office in Albuquerque, New Mexico.
Yet this BASIC interpreter became ubiquitous across the early personal computer industry. Apple licensed it for the Apple II, where it was known as Applesoft BASIC. Commodore used a modified version in the PET and later the Commodore 64, where it introduced millions of users to programming. Radio Shack, Atari, and dozens of smaller computer manufacturers also licensed Microsoft's BASIC.
The economic model was revolutionary for software. Rather than selling complete computers, Microsoft licensed its software to hardware manufacturers, establishing the platform strategy that would make it the world's most valuable technology company. The revenue from BASIC licensing funded Microsoft's growth and provided the foundation for later products like MS-DOS and Windows.
Open Source as Preservation
Microsoft's decision to open-source this historic code shows that Microsoft finally realized they could get PR points for open-sourcing code that's older than most of their employees. Unlike physical artifacts, software can be perfectly preserved and studied, but only if the source code survives. Too many foundational programs from the early computer era have been lost when companies died, tapes degraded, or backup systems failed. Digital preservation efforts are crucial for maintaining computing history.
Computer historians and nerds have been begging tech companies to open-source old code before it disappears forever. Microsoft's move follows Adobe releasing 1990 Photoshop source code and Apple's Lisa office suite.
This is a gold mine for anyone who wants to see how code was written when programmers actually gave a shit about efficiency. You can still compile and run this interpreter on vintage hardware or emulators - it's not just museum pieces.
Modern Relevance
Nobody writes 6502 assembly anymore, but the memory tricks in this code still matter. Modern devs writing Electron apps that eat 500MB of RAM could learn something. You can run this interpreter in a browser emulator and watch it do more with 4KB than most websites do with 50MB.
Retrocomputing nerds are losing their shit over this release. Anyone fixing up old Commodore 64s or Apple IIs now has the actual Microsoft BASIC code that made those machines work. Modern 6502 homebrew projects can use the real deal instead of reverse-engineered clones.
Want to teach students about efficient programming? Show them this interpreter that does more in 4KB than most modern React apps do in 4MB. Slack's desktop app uses 400MB of RAM to display text messages. This entire BASIC interpreter fits in 4KB. I once had a Node.js app that used 2GB of RAM just to serve static HTML files - turns out a memory leak in Express 4.16.3 was keeping every request in memory. Took down prod for 3 hours until we figured out the bug was in our middleware stack. Then watch them cry when they realize their "Hello World" Electron app uses 1000 times more memory than this entire BASIC system that could run spreadsheets, games, and word processors.
Microsoft released this under MIT license, so anyone can study how code was written when every byte counted. Before we had the luxury of throwing more RAM at problems. The source code is available on GitHub along with other historic Microsoft releases.