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Blurry Memories of DOS

Blurry Memories of DOS Programming

So, for whatever reason, you want to program something in DOS — that is to say, MS-DOS or one of its clones. I totally support this sort of endeavour! It is a totally worthwhile thing to try your hand at. It can give you a sense of historical perspective, or simply be a change from the everyday.

And it might even be a reasonable (in some sense) platform for building and distributing certain kinds of software, like games, since you have lots of options for emulating DOS on modern devices: DOSBox, or FreeDOS under QEMU, or v86, to name just three.

Now, I don't have tons of experience programming for DOS — mainly writing a few weird programming languages. I also tried writing a few games (all such attempts abandoned, alas).

But I do recall, pretty clearly, that there are several parts of DOS programming that can be a real PITA.

So this document attempts to collate my blurry memories of my scant experience programming things in DOS, to shed light on those parts.

Development Tools

Borland tools are highly recommended; Borland C++, Turbo Pascal, and Turbo Assembler specifically. DJGPP and nasm (or yasm) are also highly recommended, and more modern.

You don't need to know x86 machine code, but because DOS isn't much of an operating system, it helps to be comfortable with thinking about low-level behaviour of the machine.

Since you're almost certainly going to be running the resulting program in an emulator, you could even consider writing in BASIC. You can crank the emulator up to an unreasonable speed on a modern machine to compensate for any performance problems that come from BASIC being an interpreted language.

Or if you're doing this for laughs, you could write your code in ILLGOL.

Or if you prefer a minimalist approach, you could use Shelta. (Or colorForth!)


OK, so when you write a program in DOS, you actually have a choice: you can use DOS for what you want to do, or you can totally bypass DOS and use the underlying BIOS instead.

(This is what I was getting at when I said "DOS isn't much of an operating system", above.)

If you go straight to the BIOS, DOS won't try to stop you; at worst, you might leave DOS a little confused about the state of things. You will lose the advantages that DOS gives you, like being able to redirect output to a file, talking to hardware in a more abstract way via a driver, etc. But those advantages aren't terribly great, especially for games, and especially in the modern era, where your code will almost certainly be running on an emulator instead of on a real machine where you need to worry about things like what kind of soundcard the user has.

And if you only use the BIOS and never DOS itself, your program can run without DOS. That's right, you can just put your program on a boot disk and boot right into it and not worry about DOS at all. Some actual "DOS" games did this.

But if you want to do things like load and save files, then using DOS will be much easier than e.g. writing your own filesystem routines. And if you're using a language other than assembler, your compiler might insert code which uses DOS anyway, and it might be more painful to work around that than to just live with it.

In either case, under the hood, everything you do with the system will be done by invoking an interrupt. This is, if you ask me, backwards. A program should respond to interrupts, not cause them. But this is how they built this architecture, so who am I to complain. I think of them as system calls.

int 21h is a "system call" to DOS. The "system calls" to the BIOS include int 10h and int 16h, but there are also other numbers. (The trailing h is x86 assembly language convention for "hexadecimal", although you may be more used to a preceding 0x.)

If you want to get serious about this, consult a document called Ralf Brown's Interrupt List. It lists probably every interrupt you will ever care about, and describes each of them briefly (although not always in much detail).

Interrupts are something that, if you are programming in assembler, you will need to know, but if you're using some higher-level programming language, chances are there will be standard or 3rd-party libraries that will wrap calling these interrupts.

One such library is the conio.h library which came with Borland C++, and which has been cloned several times, which exposes functions like clrscr().

Another such 3rd party library is Allegro, which, until version 4.2, supported DOS.

Addressing Memory

For addressing memory, the 80286 and earlier models support only a mode called "real mode", which is this terrible thing where 32 bits are used to address a byte location in memory, but it's actually a pair of 16-bit numbers call the "segment" and the "offset", but all but the topmost 4 bits of the segment overlap bits in the offset, so it's really only 20 bits of address space.

There is a reason for this, and I bet it dates back to CP/M days, and I bet it's that this allows you to relocate a program in memory in a fine-grained(ish) way simply by changing the segment (but leaving the offsets, hardcoded in the program, the same).

But it's just horrible for normal use, and leads to things like the Borland languages supporting near and far pointers to memory which you can't interchange without various machinations.

If you must stay in "real mode" my recommendation would be to stick with the "tiny" memory model, where the data and code segment are the same and they never change and your program is a .COM file and you have the extra fun of trying to fit everything you need into 64K. If you must have more, you can upgrade to the "small" model where the segments still don't change, but now they are disjoint, so you have 64K for your code and 64K for your data.

But that's as far as it should go. Once you get into the "large" and "huge" models, you will lose hope, and instead you should look into getting out of "real mode".

The sanest alternative to "real mode" is "protected mode", which gives you a nice and simple flat 32-bit address space, at the small price of having to run on a 80386 or later, and having to start something called a DPMI (DOS Protected Mode Interface) driver first. DJGPP targets this mode and comes with a DPMI driver called CWSDPMI.

But the sanest alternative isn't necessarily the most entertaining. There is also something called "unreal mode" that I've wanted to try for a long time, which is sort of a "glitch" mode in between "real" and "protected" modes.

There are also EMS and XMS memory, but those are possibly best left to the imagination at this point. (I tried working with them once, in BefOS, but I gave up, because IIRC you need to page data into and out of them, and for that you need a paging system, and memory management systems are often a bit subtle, by which I mean annoying, to design and implement.)

Then there's the A20 Line. We don't talk about the A20 Line.

By the way, 20 bits of address space is 1024K, which was split up into 640K of main memory plus 384K for ROM and I/O (BIOS and video memory, basically). The 640K there is the same 640K that was made famous by the phrase "640K ought to be enough for anybody."

Text and Graphics

At this point in history you can safely assume the machine has VGA — i.e. that the emulator knows how to support it. You don't have to fiddle with CGA and EGA unless you really, really want to.

VGA provides various modes, both text and graphic.

The "standard" text mode is probably 80-column-by-25-row 16-colour text. 16 colours doesn't always mean 16 colours; sometimes the "brightness" bit is interpreted to mean "blinking" instead. There's a Wikipedia article about it: VGA-compatible text mode.

This is the mode that the computer generally already is in, but to switch to it (from e.g. a graphics mode), set AH=0h and AL=03h and call int 10h.

If you want to output text via DOS, the controls codes will cause things to happen, but if you write directly to the screen memory, you get the entire IBM OEM font, including the happy faces and whatnot.

The screen memory for this mode starts at B8000h (the uppermost- leftmost character) and extends 80 * 25 = 2000 bytes beyond that. It's probably easiest to set the extended segment pointer es to B800h and address individual bytes with es:si or es:di.

There are two bytes for every character; the high byte is the attribute byte, which contains the foreground and background colours, and the low byte is the character code, which matches ASCII but of course goes beyond just ASCII.

There are other text configurations, like black-and-white, 40 (or 132?) columns, 50 rows... but these are left as exercises for the interested reader.

The simplest graphic mode by far is 320x200x256 because the video RAM is just a 2D array of bytes and every pixel is one byte.

To switch into this mode, set AH=0h and AL=13h and call int 10h.

Screen memory starts at A0000h (the uppermost-leftmost pixel) and extends 320 * 200 = 64000 bytes beyond that. It's probably easiest to set the extended segment pointer es to A000h and address individual bytes with es:si or es:di.

There is also the fact that if you want smooth animation, you should wait for the vertical retrace period (a.k.a. the vertical blanking interval) before making changes to screen memory, as discussed in A Basic Theory of Video Games.

I don't actually remember how to do this on an IBM PC, if I ever knew, but I do remember that Allegro has a function for it called vsync(). If this function was all you wanted from Allegro, you would still be justified in using it, IMO.

Keyboard Input

If you're writing a game, you could take the player's input from a joystick. But traditionally, PC games would assume that you might not own a joystick and would let you use the keyboard for control instead.

DOS will let you read text from "standard input", even when that input is coming from a console, but that will be line-buffered, i.e. the user will need to press Enter before the program sees any of it, which is fine for text adventures and the like, but not really sufficient for video games.

I don't think DOS has a way to check if input is available and only consume it if it is available, but I might be misremembering.

But the BIOS certainly can do this.

To wait for a key to be pressed, set AH=0h and int 16h. To check if a key is pressed or not, set AH=01h instead, then check the Z flag afterward. For both calls, the ASCII value will be available in AL afterward, and the raw scan code will be available in AH.

Here is a really comprehensive list of keyboard scancodes.

If you also want to see if any of the modifier keys (Shift, Ctrl, etc,) are being pressed, set AH=02h and int 16h. I'd describe the result but this is getting outside the scope of this article; consult Ralf Brown's Interrupt List, or simply do a web search for int 16h, for more information.


I never played much with sound on the IBM PC architecture. It's probably reasonable to assume that any given emulator supports at least the SoundBlaster 16. It's probably even OK to assume that the SB16 lives at a certain interrupt and DMA address, since those can be configured on the emulator's side now (instead of asking you with one of those horrible SETUP.EXE menus!)

But I don't actually know anything about talking to an SB16. But I would be really surprised if there wasn't information about it somewhere on the internet.

Failing the SB16, you could, if your really wanted to, drive the internal speaker, and the emulator, if it is worth its salt, should support that. However, I'm not sure there are many listeners who would appreciate it greatly. In fact, many would probably appreciate if you didn't make the internal speaker make any sounds...