Note: Before I begin, I realize this program isn't really C or C++. If 
it's anything, it's assembler. I posted it to the C++ section because it's 
closer to what I want to accomplish. Personally, I think an ASM section on PSC 
is long, long overdue. So please, no complaining about this not being C or C++.
With having said that, let us begin.
THE 5-BYTE EXECUTABLE - A PRIMER ON ASSEMBLY LANGUAGE AND 
MACHINE CODE
For starters, I'd like to create a program which is only 5 bytes in size and 
does more than the 7 byte EXE that Vbmew shows how to create.
At a DOS prompt, type "copy con echochar.com"
The listing of the program is as follows (with the assembler code 
explaination)

 
Press Alt-180

 Code B4, "mov ah, ??". 'mov' is a symbol used to tell the processor to copy 
 a value from somewhere into somwhere else. 'ah' is a CPU register. ah is 
 commonly used with input and output routines. '??' is the value we want to put 
 in to the ah register. We fill in the value of '??' in the next line.

 
 
Press Alt-1

 ASCII character 1. This makes the first line look 
 like "mov ah,1".

 
 
Press Alt-205

 Code CD, "int ??". 'int' simply calls an interrupt. 
 An interrupt is an instruction built in to the CPU.

 
 
Press Alt-33

 ASCII character 33. In hex, 33 is 21. This makes the 
 previous line look like "int 21h". Interrupt 21h is a commonly used IO 
 interrupt. By setting the ah register to 1 and calling interrupt 33 (or 21h), 
 we're telling the computer to stop and wait for input from the keyboard. Since 
 ah is set to 1, once a key is pressed, it is echoed to the screen. If, 
 however, ah was set to 8, the character pressed would *not* be echoed.

 
 
Press Alt-195

 Code C3, "ret". 'ret' basically tells the computer to 
 return to the previous environment.

 
 
Press Ctrl-Z to mark the end of the file 
 and the press Enter to write the file.

Like Vbmew's program, this one displays a character to the screen.  
Unlike his program, however, this one lets you choose which character is 
displayed. =)
While this program basically does nothing, it's a great primer (for me, at 
least).  It gives an introduction as to what basic assembler commands do 
what, and what their machine code representation is.
Here's the program again, this time, in all assembly.

 
mov ah,1

 int 21h

 ret

To make this even more low-level, we could 
eliminate the automatic display of the character to the screen and display it 
with code instead. As stated earlier, to eliminate the character echo, the ah 
register needs to be set to 8 instead of 1.  After a key is pressed, is put 
into the 'al' register, which is simply another register in the CPU which you 
need not concern yourself with at the moment. Just know that it holds the ASCII 
value of the key that was pressed and we need to put that value into the 'dl' 
register, which is commonly used for output. To do this, we need to use totally 
different symbols specific for moving registers to registers. One of these new 
symbols are 88 and C2 combined. In fact, the 88-C2 command specifically copies 
the value in the al register to the dl register. After performing this 
operation, we need to tell the computer that we want to display the output. This 
is done by setting the ah register to 2 and once again calling interrupt 21h.
I also recommend a unicode hex editor for this 
as the DOS prompt will not suffice (because Alt-8 translates into backspace).

 


Keyboard command

ASM Code

Machine Code
 
 

Alt-180, Alt-8 (can't be 
done at DOS prompt)
mov ah,8
┤◘
 
 

Alt-205, Alt-33
int 21h
═!
 
 

Alt-136, Alt-194
mov dl,al    
(this is the 88-C2 command)
ê┬
 
 

Alt-180, Alt-2
mov ah,2
┤☻
 
 

Alt-205, Alt-33
int 21h
═!
 
 

Alt-195
ret
├
 

There you have it. An even more low-level program that does practically the 
same thing as the first. And it's only 11 bytes in size, still. =)
I know this article might not be the hardcore "we-don't-need-no-stinkin'-programming-language-we've-got-pure-machine-code" 
tutorial, but at least it's a start. I would recommend taking a look at

http://www.theteacher.freeserve.co.uk/alevel/assem/assemix.htm for more 
information on assembly language and CPU architecture, and I'd also recommend 
W32Dasm and NASM for writing programs in assembly and finding out what the 
symbol codes are.
Also, don't forget to vote for an ASM section for PSC!