PC DOS 1.0, but Not Quite | Hacker News Reader

PC DOS 1.0, but Not Quite

os2museum.com

94 pointsskissane17 comments

I follow the demoscene so I'm pretty familiar with the tiny-yet-featureful productions its produced, and yet the sizes on some of those binaries still somewhat surprises me --- an entire assembler is merely 6389 bytes! A "Hello World" compiled with GCC is roughly the same size ( http://timelessname.com/elfbin/ ). The DEBUG.COM debugger is 5450B, and the shell, COMMAND.COM, which contains 7 builtin commands, is merely 2576 bytes.

More amazing is the fact that the original developers probably weren't exerting great effort in size-optimising them either, unlike demoscene productions, and yet they're still extremely tiny.

I believe the main size optimization here comes from having only teletype and batch processing interfaces. When you do that, the surface area of the UI, and hence the amount of supporting code, drops tremendously. These early tools also skimped on error messages and checks, so the user experience was generally one of confusion and catastrophic error, despite being small and simple. You really needed the documentation to have any hope of understanding an old system.

Now we have operating systems that go out of their way to automate away everything and present it in real-time with multitasking and custom graphical elements everywhere, while also supporting many more protocols and hardware interfaces - wireless networking, GPU APIs, etc. The biggest growing pains seem to be past - things went from simple and stable to complex and unstable in the 1990's and then to complex and stable but insecure now. There's a lot of room to mature all of these features, but there aren't as many novel ones.

One reason is that .COM programs were copied to memory as they were, in a single segment, and therefore could access a max of 64 kilobytes of RAM, and they itself could be max 64 kb. They are a tiny memory dump of the program.

.EXE programs have loaders that handled the segmented memory and could access much more, up to the 1MB of memory space. That loader of course made .EXE programs larger than .COM.

> could access a max of 64 kilobytes of RAM

This is incorrect. A COM program had to fit its code and static data into 64K, but it could access as much RAM as existed on the machine.

A related misconception is that the segment registers in COM programs had to have all the same value. This is incorrect as well, as the program once loaded could reset them to any value.

Indeed! First thing the executables generated by my early DOS compilers did was to set up the es: and ds: segments in order to get more space.

Eventually I overcame laziness though, and started to study the EXE file format, which, AFAIR, does not have a "loader", (as Shorel suggested) but just a header describing the layout of the file. I think EXE files tend to be larger, just because they can be larger.

MZ (DOS .exe) requires a loader - smarter than just "copy this whole thing into memory and jump to start", anyway - because it has a relocation table. But, of course, the loader is not part of the executable itself, so it doesn't contribute to its size. I suspect that the reference was to the relocation tables themselves.

> and they itself could be max 64 kb

Minus 256 bytes at the beginning for the Program Segment Prefix (hence why every .com assembly listing usually starts with ORG 100h).

I'm pleasantly surprised that people 1. care about this corner of computer history, and 2. have so much knowledge. I know there's a lot less to look through, but things like having found a message on the disk for the released version of PC DOS, even though it apparently was just un-scrubbed data outside the filesystem, seems crazy. I suppose the closed nature of the releases has been offset by the relatively open platform and the fact that they've had decades to dig through it.

Oh man... The screen took me back to the days on creating .COM file from Debug.com..

Is anyone getting 403 Forbidden on this whole domain? I had to read the article through Google cache.

edit: I can load it over 4G on my phone though. Wonder why they would ban a random Japanese ISP?

Unsure why you're being downvoted. I'm in Australia and cannot access the site either :(

In Australia, and can see it fine here... (Telstra network).

No 403 here. Sometimes sites with overzealous blocklists ban Japanese IPs.

I was underwhelmed by the 'power' of the original IBM PC because I came from a BBC micro background. DOS was a bit 'meh' compared to ADFS on the BBC Micro with BASIC where you could drop down to the assembler and bang out some 6502. Who needs more than three registers anyway?

I found DOS to be incomplete and demanding where it need not be. A lot of time was wasted on config.sys and autoexec.bat with the install disks treating you as stupid.

What an exciting find! I wonder where it came from?

>But there’s also BAS18.COM and BAS18A.COM which expect the BASIC “ROM” at segment 1800h (at 96K, clearly assuming a 128K machine) in RAM rather than at segment F600h in ROM.

Is int 18h the [intended] coincidence then?

Seeing that directory listing (and especially seeing BASICA.COM) brought back a flood of memories of my first days of programming, playing with the IBM PC-1 and PC/XT.

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