🚀 go-pugleaf

> I'd appreciate someone providing a definition of real mode and
> protected mode.  I thought I understood the difference - until I
> needed to explain it someone.  I looked in the MS-DOS 5.0 manual and
> found nothing.  Is this something that usually involves Windows only?
> The question came to me from someone who just bought a new compiler
> that said he could compile either for real or protected mode.

The number of address lines the microprocessor can use depends on the mode the 
PC is running in.  The 8086 only had one mode, and in it all 20 address lines 
are active (giving it 2x10e20, or 1Mb RAM access).  The '286 (24 address 
lines=16Mb RAM), '386 & '486 (32 address lines=4Gb RAM) can use all of their 
address lines, but only when they are running in *protected mode*.  DOS does 
not run in protected mode; DOS will only run in these processors' *real* mode,
which emulates an 8086.  In real mode, only the first 20 address lines are 
active.  A 286, 386 or 486 running in real mode (running DOS) is limited to 
1Mb of memory, just like an 8086.  The 386 and 486 support a third mode called 
*virtual 86 mode*, which is similar to real mode, except that it is possible 
for DOS to gain access to more than 1Mb of memory because of the quirks of 
virtual 86 mode.  When run in protected mode, each program can be allocated a 
certain section of memory, which other programs cannot touch, hence the name
"protected mode."  PM also allows the use of extended memory, virtual memory 
and performs multitasking of programs.  DOS does not run in protected mode, so
does not allow any of these features either, but an O/S such as OS/2, Unix or 
the to be released Windows NT, which are true 32 bit operating systems, take 
full advantage of the uP power.  Windows 3.1 is *not* an O/S...it still needs 
DOS to do most of the work.