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”.

In this episode of “Computer Archeology“, we will not only examine this story, but also track down the history of Microsoft BASIC on various computers, and see see how Microsoft added a second easter egg to the TSR-80 Color Computer – because they had forgotten about the first one.

Stolen From Apple?

This whole story sounds similar to Apple embedding a “Stolen From Apple” icon into the Macintosh firmware in 1983, so that in case a cloner copies the ROM, in court, Steve Jobs could hit a few keys on the clone, revealing the icon and proving that not just a “functional mechanism” was copied but instead the whole software was copied verbatim.

Altair BASIC

Let’s dig into the history of Microsoft’s BASIC interpreters. In 1975, Microsoft (back then still spelled “Micro-soft”) released Altair BASIC, a 4 KB BASIC interpreter for the Intel 8080-based MITS Altair 8800, which, despite all its other limitations, included a 32 bit floating point library.

An extended version (BASIC-80) that consisted of 8 KB of code contained extra instructions and functions, and, most importantly, support for strings.

Microsoft BASIC for the 6502

In 1976, MOS Technology launched the KIM-1, an evaluation board based around the new 6502 CPU from the same company. Microsoft converted their BASIC for the Intel 8080 to run on the 6502, keeping both the architecture of the interpreter and its data structures the same, and created two versions: an 8 KB version with a 32 bit floating point library (6 digits), and a 9 KB system with 40 bit floating point support (9 digits).

Some sources claim that, while BASIC for the 8080 was 8 KB in size, Microsoft just couldn’t fit BASIC 6502 into 8 KB, while other sources claim there was an 8KB version for the 6502. The truth is somewhere in the middle. The BASIC ROMs of the Ohio Scientific Model 500/600 (KIM-like microcomputer kits from 1977/1978) and the Compukit UK101 were indeed 8 KB in size, but unlike the 8080 version, it didn’t leave enough room for the machine-specific I/O code that had to be added by the OEM, so these machines required an extra ROM chip containing this I/O code.

In 1977, Microsoft changed the 6 digit floating point code to support 9 digits and included actual error stings instead of two-character codes, while leaving everything else unchanged. A 6502 machine with BASIC in ROM needed more than 8 KB anyway, why not make it a little bigger to add extra features. The 6 digit math code was still an assembly time option; the 1981 Atari Microsoft BASIC used that code.

In 1977, Ohio Scientific introduced the “Model 500”, which was the first machine to contain (6 digit) Microsoft BASIC 1.0 in ROM. Upon startup, it printed:

OSI 6502 BASIC VERSION 1.0 REV 3.2 COPYRIGHT 1977 BY MICROSOFT CO. OK

In the same year, MOS started selling a tape version of 9 digit Microsoft BASIC 1.1 for the KIM-1. Its start message was:

MOS TECH 6502 BASIC V1.1 COPYRIGHT 1977 BY MICROSOFT CO. OK

Woz Integer BASIC

The 1976 Apple I was the first system besides the KIM to use the MOS 6502 CPU, but Steve Wozniak wrote his own 4KB BASIC interpreter instead of licensing Microsoft’s. An enhanced version of Woz’ “Integer BASIC” came in the ROM of the Apple II in 1977; Microsoft BASIC (called “AppleSoft”) was available as an option on tape. On the Apple II Plus (1978), AppleSoft II replaced Integer BASIC.

Commodore PET

Commodore had bought MOS in October 1976 and worked on converting the KIM platform into a complete computer system. They licensed Microsoft BASIC for 6502 (also October 1976), renamed it to Commodore BASIC, replaced the “OK” prompt with “READY.”, stripped out the copyright string and shipped it in the ROMs of the first Commodore PET in 1977.

The Easter Egg

In 1979, Commodore started shipping update ROMs with a version 2 of Commodore BASIC for existing PETs. Apart from updates in array handling, it also contained the WAIT 6502 easter egg.

This is what the easter egg code looks like:

.,D710 20 C6 D6 JSR $D6C6 fetch address and value .,D713 86 46 STX $46 save second parameter .,D715 A2 00 LDX #$00 default for third parameter .,D717 20 76 00 JSR $76 CHRGOT get last character .,D71A F0 29 BEQ $D745 no third parameter .,D71C 20 CC D6 JSR $D6CC check for comma and fetch parameter .,D71F 86 47 STX $47 save 3rd parameter .,D721 A0 00 LDY #$00 .,D723 B1 11 LDA ($11),Y read from WAIT address .,D725 45 47 EOR $47 second parameter .,D727 25 46 AND $46 first parameter .,D729 F0 F8 BEQ $D723 keep waiting .,D72B 60 RTS back to interpreter loop

On pre-V2 BASIC, the branch at $D71A just skipped the next line: If there is no third parameter, don’t fetch it. On V2, the line is subtly changed to make the two-parameter case branch to a small patch routine:

.,D745 A5 11 LDA $11 low byte of address .,D747 C9 66 CMP #$66 = low of $1966 (=6502) .,D749 D0 D4 BNE $D71F no, back to original code .,D74B A5 12 LDA $12 high byte of address .,D74D E9 19 SBC #$19 = high of $1966 (=6502) .,D74F D0 CE BNE $D71F no, back to original code .,D751 85 11 STA $11 low byte of screen buffer = 0 .,D753 A8 TAY index = 0 .,D754 A9 80 LDA #$80 high byte of screen buffer .,D756 85 12 STA $12 screen buffer := $8000 .,D758 A2 0A LDX #$0A 10 characters .,D75A BD 81 E0 LDA $E081,X read character .,D75D 29 3F AND #$3F throw away upper bits .,D75F 91 11 STA ($11),Y store into screen RAM .,D761 C8 INY .,D762 D0 02 BNE $D766 no carry .,D764 E6 12 INC $12 increment screen buffer high address .,D766 CA DEX .,D767 D0 F1 BNE $D75A next character .,D769 C6 46 DEC $46 .,D76B D0 EB BNE $D758 repeat n times .,D76D 60 RTS back to interpreter loop

The text “MICROSOFT!” is stored in 10 consecutive bytes at $E082, cleverly hidden after a table of coefficients that is used for the SIN() function:

.;E063 05 6 coefficients for SIN() .;E064 84 E6 1A 2D 1B -((2*PI)**11)/11! = -14.3813907 .;E069 86 28 07 FB F8 ((2*PI)**9)/9! = 42.0077971 .;E06E 87 99 68 89 01 -((2*PI)**7)/7! = -76.7041703 .;E073 87 23 35 DF E1 ((2*PI)**5)/5! = 81.6052237 .;E078 86 A5 5D E7 28 -((2*PI)**3)/3! = -41.3147021 .;E07D 83 49 0F DA A2 2*PI = 6.28318531 .;E082 A1 54 46 8F 13 "SOFT!" | backwards and with .;E087 8F 52 43 89 CD "MICRO" | random upper bits

If we reverse the bytes, we get

CD 89 43 52 8F 13 8F 46 54 A1

The easter egg code clears the upper 2 bits, resulting in

0D 09 03 12 0F 13 0F 06 14 21

The easter egg code does not print the characters through library routines, but instead writes the values directly into screen RAM. While BASIC used the ASCII character encoding, the Commodore character set had its own encoding, with “A” starting at $01, but leaving digits and special characters at the same positions as in ASCII. Thus, the 10 hidden and obfuscated bytes decode into:

MICROSOFT!

Microsoft’s Code?

Commodore engineers are known for putting easter eggs into ROM, but there would be no reason for them to encode the string “MICROSOFT!” and hide it so well. The “WAIT 6502” easter egg did not show up in Commodore BASIC until version 2, which is in contrast to almost all sources claiming Commodore licensed Microsoft BASIC for a flat fee and never returned to Microsoft for updates, but continued improving BASIC internally.

Commodore had indeed updated its source with Microsoft’s changes since V1. 6502 guru Jim Butterfield states:



Commodore paid Microsoft an additional fee to write a revision to the original BASIC that they had bought. Among other things, spaces-in-keywords were changed, zero page shifted around, and (unknown to Commodore) the WAIT 6502,x joke was inserted.



Targeting Commodore?

While all of Microsoft BASIC only depends on the CPU, makes no other assumptions on the hardware it runs on (be it Commodore, Apple, Atari, …), and does all its input and output by calling into ROM functions external to BASIC, the easter egg writes directly to screen RAM at a fixed address of $8000, and uses the PET character encoding for it: The easter egg has clearly been written specifically for the PET.

We can only speculate on the reasons why Microsoft and possibly Bill Gates himself added the easter egg. A possible reason is that Microsoft wanted to make sure Commodore cannot take credit for “Commodore BASIC” – similar to the “Stolen From Apple” case.

Or it was only about showing the world who really wrote it. Jim Butterfield: As an afterthought, Microsoft would have liked to see their name come up on the screen. But it wasn’t in the contract.

Commodore’s Reaction

The easter egg only exists in BASIC version 2 on the PET. All later Commodore computers didn’t contain it: The branch was restored and the extra code as well as the 10 bytes hidden after the SIN() coefficients were removed.

Jim Butterfield: Shortly after that implementation, I show this to Len Tramiel [of Commodore engineering] at the Commodore booth of a CES show. He was enraged: “We have a machine that’s short of memory space, and the #$#!* [Gates] put that kind of stuff in!!”

Commodore employee Andy Finkel states that the “Gates” (!) easter egg had to be removed for space reasons. It had occupied 51 extra bytes.

Interestingly, starting with the BASIC V7 on the C128 six years later, Commodore started crediting Microsoft, like this:

COMMODORE BASIC V7.0 122365 BYTES FREE (C)1985 COMMODORE ELECTRONICS, LTD. (C)1977 MICROSOFT CORP. ALL RIGHTS RESERVED

According to Jim Butterfield, this is probably due to negotiations concerning Microsoft BASIC for the Amiga.

The Easter Egg before the PET

But Microsoft did not encode its company name specifically for Commodore: The 9 digit BASIC 6502 version 1.1 for the KIM-1 contained the 10 hidden bytes:

.;3FAA 05 6 coefficients for SIN() .;3FAB 84 E6 1A 2D 1B -((2*PI)^11)/11! = -14.3813907 .;3FB0 86 28 07 FB F8 ((2*PI)^9)/9! = 42.0077971 .;3FB5 87 99 68 89 01 -((2*PI)^7)/7! = -76.7041703 .;3FBA 87 23 35 DF E1 ((2*PI)^5)/5! = 81.6052237 .;3FBF 86 A5 5D E7 28 -((2*PI)^3)/3! = -41.3147021 .;3FC4 83 49 0F DA A2 2*PI = 6.28318531 .;3FC9 A6 D3 C1 C8 D4 "!TFOS" .;3FCE C8 D5 C4 CE CA "ORCIM"

The extra bytes here are:

A6 D3 C1 C8 D4 C8 D5 C4 CE CA

If we XOR every byte with 0x87, we get:

21 54 46 4f 53 4f 52 43 49 4d

which, again, is “MICROSOFT!” backwards, but this time in the ASCII encoding. (Note that no XOR or add/sub can be found for the 10 bytes in Commodore BASIC that would convert them into ASCII instead of PETSCII. Also, thanks to Tom for his help here.)

The version of Microsoft BASIC for the 6502-based Apple II, called “AppleSoft“, contains the same 10 bytes after the coefficients in all tape and ROM versions. On AppleSoft II, for example, they are located at address $F075.

KIM-1 BASIC was released in 1977, AppleSoft II in spring 1978, and the V2 ROM of the PET in spring 1979. So Microsoft didn’t “target” Commodore with this at first, but probably put the data in for all their customers – possibly right after they had shipped the easteregg free V1 to Commodore. And when Commodore came back to them, they changed their codebase to encode string differently and added the easter egg code to show the string.

The Easter Egg after the PET

After the second source drop to Commodore, they removed the “WAIT6502” code again, but kept the 10 encoded bytes in their master codebase: Every non-Commodore post-1978 6502 Microsoft BASIC with the 40 bit floating point library contains the 10 encoded bytes after the SIN() coefficients – still in PET encoding:

Tangerine Microtan 65

Tangerine Oric-1 and Oric-Atmos

Pravetz 8D

This is a snippet from microtan/tanex_h2.rom:

0000fd8: 0f da a2 a1 54 46 8f 13 ....TF.. 0000fe0: 8f 52 43 89 cd a5 d5 48 .RC....H

The ROM of the Ohio Scientific Superboard II (and its clone, the Compukit UK101) as well as the Atari Microsoft BASIC tape are based on the 32 bit floating point version and don’t contain the easter egg data.

“MICROSOFT!” on the 6800 and the 6809

It doesn’t stop there: Even the BASIC versions on the TRS-80 Color Computer and the TRS-80 MC-10, which were versions for the 6809 and 6800 CPU architectures, respectively (BASIC-69 and BASIC-68), had the encoded “MICROSOFT!” string after the SIN() coefficients. Here is a snippet of Spectral Associates’ disassembly of the CoCo ROM in his book “Color Basic Unravelled II“

* MODIFIED TAYLOR SERIES SIN COEFFICIENTS BFC7 05 LBFC7 FCB 6-1 SIX COEFFICIENTS BFC8 84 E6 1A 2D 1B LBFC8 FCB $84,$E6,$1A,$2D,$1B * -((2*PI)**11)/11! BFCD 86 28 07 FB F8 LBFC8 FCB $86,$28,$07,$FB,$F8 * ((2*PI)**9)/9! BFD2 87 99 68 89 01 LBFD2 FCB $87,$99,$68,$89,$01 * -((2*PI)**7)/7! BFD7 87 23 35 DF E1 LBFD7 FCB $87,$23,$35,$DF,$E1 * ((2*PI)**5)/5! BFDC 86 A5 5D E7 28 LBFDC FCB $86,$A5,$5D,$E7,$28 * -((2*PI)**3)/3! BFE1 83 49 0F DA A2 LBFE1 FCB $83,$49,$0F,$DA,$A2 * 2*PI BFE6 A1 54 46 8F 13 8F LBFE6 FCB $A1,$54,$46,$8F,$13 UNUSED GARBAGE BYTES BFEC 52 43 89 CD FCB $8F,$52,$43,$89,$CD UNUSED GARBAGE BYTES

You can tell that Microsoft didn’t reimplement BASIC for the remaining 8 bit architectures, but practically converted the 6502 code, copying all constants verbatim, even the ones they did not understand, since these are still the obfuscated bytes in PET-encoding.

A Second Easter Egg on the Color Computer

The TSR-80 Color Computer (1980) also has an easter egg in BASIC: If you type “CLS9” (or any higher number), it will clear the screen and print “MICROSOFT”.

Let’s see how it is done:

* CLS A910 BD 01 A0 CLS JSR RVEC22 HOOK INTO RAM A913 27 13 BEQ LA928 BRANCH IF NO ARGUMENT A915 BD B7 0B JSR LB70B CALCULATE ARGUMENT, RETURN VALUE IN ACCB A918 C1 08 CMPB #8 VALID ARGUMENT? A91A 22 1B BHI LA937 IF ARGUMENT >8, GO PRINT âMICROSOFTâ [...] A937 8D EF LA937 BSR LA928 CLEAR SCREEN A939 8E A1 65 LDX #LA166-1 * A93C 7E B9 9C JMP LB99C * PRINT âMICROSOFTâ

The string to be printed is stored here:

A166 4D 49 43 52 4F 53 LA166 FCC 'MICROSOFT' A16C 4F 46 54 A16F 0D 00 LA16F FCB CR,$00

That’s right, Microsoft added a different easter egg, and included the string “MICROSOFT” again, this time in cleartext. They seem to have forgotten about the obfuscated 10 bytes intended for the PET that had been copied from the 6502 version to the 6800 during conversion, and had still been present in the Color Computer ROM.

The same easter egg exists on the 6800-based TRS-80 MC-10 (also 1980), which also had the 10 PET bytes in ROM:

FBBF 27 13 BEQ $FBD4 ; branch if no argument FBC1 BD EF 0D JSR $EF0D ; get argument FBC4 C1 08 CMPB #$08 ; easter egg? FBC6 22 1D BHI $FBE5 ; yes [...] FBE5 8D ED BSR $FBD4 ; clear screen FBE7 CE F8 33 LDX #$F834-1 FBEA 7E E7 A8 JMP $E7A8 ; print "MICROSOFT" [...] F834 4D 49 43 52 4F FCC "MICROSOFT" F834 53 4F 46 54 0D FCB $0D F834 00 FCB $00 [...] F724 A1 54 46 8F 13 FCB $A1,$54,$46,$8F,$13 ; "!TFOS" F729 8F 52 43 89 CD FCB $8F,$52,$43,$89,$CD ; "ORCIM"

Microsoft BASIC 6502 Timeline

Version 1.0 (in the 6 digit version) is used on the Ohio Scientific, and contains a major bug in the garbage collection code.

Version 1.0 (in the 9 digit version) is also used in the first Commodore PET as Commodore BASIC V1. It is the oldest known Microsoft BASIC to support 9 digit floating point.

Version 1.1, which contained bug fixes, is used on the KIM-1. It is the oldest version to contain the “MICROSOFT!” string (in ASCII).

AppleSoft BASIC I is forked from Microsoft BASIC 1.1. It contains the ASCII string.

Microsoft BASIC version 2 changes the ASCII string to PET screencode, adds the easter egg code, and is given to Commodore.

The code is removed again after the source drop to Commodore. The Tangerine Microtan is based on this.

Apple, Commodore and Tangerine continue development of their respective forks without the involvement of Microsoft.

The BASIC V2 used on the VIC-20 and the C64 is actually a stripped-down version of PET BASIC 4.0 and not a ported version of PET BASIC V2.

So did Bill Gates write it himself?

Altair BASIC was written by Bill Gates, Paul Allen (the founders of Microsoft) and Monte Davidoff (a contractor), as comments in the original source printout show:

00560 PAUL ALLEN WROTE THE NON-RUNTIME STUFF. 00580 BILL GATES WROTE THE RUNTIME STUFF. 00600 MONTE DAVIDOFF WROTE THE MATH PACKAGE.

Bill Gates wrote “the runtime stuff” (which probably means the implementation of the instructions), as opposed to “the non-runtime stuff” (probably meaning tokenization, memory management) and “the math package”. Consequently, the implementation of the WAIT command would have been his work – on the 8080, at least.

Now who wrote the 6502 version? The KIM-1 BASIC manual credits Gates, Allen and Davidoff, the original authors of the 8080 version, but it might only be left over from the manual for the 8080 version. Davidoff, who worked for Microsoft in the summers of 1975 and 1977, had not been there when BASIC 6502 was written in the summer of 1976, but he probably changed the 6 digit floating point code into the 9 digit version that is first found in BASIC 6502 1.1 (KIM-1, 1977).

The ROM of the 1977/1978 Ohio Superboard II Model 500/600 (6 digit BASIC 1.0) credits RICHARD W. WEILAND, and the 1977 9 digit KIM-1 BASIC 1.1 as well as the 1981 Atari Microsoft BASIC 2.7 credit “WEILAND & GATES”. Ric Weiland was the second Microsoft employee. These credits, again, were easter eggs: While they were clearly visible when looking at the ROM dump, they were only printed when the user entered “A” when BASIC asked for the memory size.

According to apple2history.org, Marc McDonald (employee number 1) wrote the 6502 version, but it is more likely that McDonald wrote the 6800 simulator and Weiland ported 8080 BASIC to the 6800 and then McDonald adapted the 6800 simulator to the 6502 and Weiland wrote the 6502 BASIC.

This and the hidden credits in version 1.0 of 6502 BASIC suggest that Weiland was the main author of 6502 BASIC. Gates is added to the hidden credits in the 1.1 version, so Gates probably contributed to the 1.1 update..

So it is very possible that Gates wrote the easter egg code himself, given that he was responsible for the implementation of WAIT on the 8080, he is credited in BASIC 6502 1.1+, Finkel and Butterfield refer to WAIT6502 as “Gates'” easter egg – and after all, he can write code.

Open Questions

What was Atari’s version based on? What versions were there? Atari Microsoft BASIC images are very hard to find.

Why did Atari use the 6 digit version, if they extended it with lots of commands (so size couldn’t have been an issue)?

Annotated Disassembly of Different Versions