Happy Birthday VAX 11/780 (influence of)

[Chuck Guzis]

On 28 Oct 2010 at 13:34, Richard wrote:

> Just because a CPU architecture has IO instructions doesn't mean you
> can't do memory-mapped I/O.

Just because a CPU doesn't have any I/O instructions or I/O space, 
doesn't mean that you can't invent them. 

> Memory mapped IO is ubuiqitous in MS-DOS based PCs, as it is generally
> fastest to write directly to the memory-mapped display memory for
> CGA > and VGA displays.   

And there were other PC displays, that didn't use memory-mapped video 
at all.  One liability with the PC was that you weren't able to move 
display space anywhere--software assumed it to be at a fixed 
location.  That pretty much limited real-mode RAM space to 640K, 
which eventually became something of a liability until protected-mode 
CPUs with address remapping came along.

--Chuck