In fact, it is so similar to Server 2003 (because it literally IS the same code) that even the theming service is disabled by default. (3790.1232)
Given the build date of 3790.1232 (19 August 2004), the common understanding is that "Longhorn" post-reset is based on Server 2003 Service Pack 1 codebase, particularly Server 2003 SP1 build 3790.1232.As such, this build is no different from XP, or at least a client compile of Server 2003. (5001)
However, according to Dave Cutler, in an interview by Dave's Garage (Dave Plummer) in October 2003 (previously shared on BA here):
Timestamps are from this excerpt:Cutler: And in the meantime, in the middle of this, in the middle of this, sort of, somewhere in here, I start this project that's uh, it's uh, off on the side, that is the x64 stuff.
Cutler: So AMD comes in and talks to us and talks to Rob Short and I and says, you know, we have some ideas on how to build 64-bit extensions to the x86 that will be very less intrusive and will allow us to run 32-bit applications at speed on the 64-bit machine. I said, "Geez, that's pretty interesting."
"What have you got?"
"Well, we got just a few ideas."
And we said, "well, you know, go away and come back and tell us what you want to do."
So they come back, we look at it, and say, "This is great, this is great, it's like unbelievable." So I said, "hey, we're going to (gonna) do this". So we just start to do this, this this is not a company-anointed project at all. And I say, we're using the server code base, and we're going to produce a 64-bit workstation, and we're going to produce a 64-bit server. Meantime, we got Longhorn over here still going, and its name is now morphed into Doesn't Matter-horn.
Cutler: So it becomes Does-Not-Matter Horn. And Allchin, of course, he's my boss, and he's running Longhorn. I mean, he's responsible for that. And I'm running this x64 thing. And the server group is, you know, they're off, they're shipping a system. And we're just using their codebase well... We build, um, we get the x64 system running, well enough, we get everything ported and we have a simulator that allows us to simulate setup. And we simulate setup. It takes 24 hours, 24 hours. The simulator is called SimNow. And AMD finally ships us a system. It arrives at 12 noon. And they say, there it is. And we got a CD. And we walk over to that machine, plug the CD in and boot the 64-bit system. The first time it's ever been tried, and it boots and runs, and it runs, and it doesn't fall over and bug check. So this sort of piqued the interest a little bit at that point. And we get reliable enough that we actually convinced Microsoft.com, that website, to switch over to our 64-bit server. (2:57-5:59)
Cutler: So we got to a point where the guys just couldn't get Longhorn or Doesn't Matter-horn out of the build lab. And I tell Allchin and I said, "This is just baloney. You guys should switch the code base to the x64 code base. And eventually that's what we did." (6:37-6:57)
Cutler seems to be talking about Windows XP Professional x64 Edition, with his anecdote seemingly consistent with the exchange of emails posted on the BetaWiki article on Build 3599.
I understand that XP Professional x64 is also based on Server 2003:
Therefore, what is post-reset "Longhorn" based on: Server 2003 SP1 codebase or XP Professional x64 codebase? Distinction is necessary, considering how releases get their own dedicated branches (see Longhorn.MS and Paul Thurrott's Windows Server 2003: The Road to Gold Part 2).The x64-based versions of Windows Server 2003 and Microsoft Windows XP Professional x64 Edition are based on the Windows Server 2003 code tree. Service and support activities for Windows XP Professional x64 Edition use the Windows Server 2003 tree and don't use the Windows XP client tree. (Microsoft)