C integral types [was Re: Semi-OT: Why Kids Can't use Computers] (fwd)

[Lennart Sorensen]

On Tue, Aug 20, 2013 at 05:49:12PM -0400, James Knott wrote:
> Well, I just happen to have handy "The 8086 Family Users Manual"
> published by Intel in 1979.
> 
> On page 2-14:
> 
> "Dedicated and Reserved Memory Locations
> 
> Two areas in extreme low and high memory are dedicatied to specific
> processor functions or reserved by Intel Corporation for use by Intel
> hardware and software products.  As shown in figure 2-21, the location
> are: 0H through 7FH (128 bytes) and FFFF0H through FFFFFH (16 bytes). 
> These areas are used for interrupt and system reset processing.  8086
> and 8088 application systems should not use these areas for any other
> purpose.  Doing so may make these system incompatible with future Intel
> products."
> 
> This would tend to rule out IBM putting stuff at the top 16 bytes.

IBM put the startup code there.  What else is system reset processing?
If there is nothing at that address, the system won't do anything.

And yes the low end has all the IO ports.

It doesn't say "don't put stuff here".  It says it has special purpose.
So you can't put something random at those addresses and expect it to
work, but you can put stuff that does what is expected there, and that
is exactly what IBM did.  They put IO decides in the IO port range,
and they put the ROM in the system reset execution range.

Can you find ANY system ever made with an 8086/8088 that does NOT have
ROM at FFFF0-FFFFF to handle system startup?

> However, I have seen systems that use "ghost" ROM, where ROM memory from
> elsewhere is made to temporarily appear at a specific location in the
> memory space.  The Data General line did that, IIRC.

Many systems do.  Chip selecting ROM to address 0 at reset is common,
and then the system remaps it to somewhere better later when it has
decided what should go where.

> It's been too many years since I looked at this sort of thing in any
> detail to remember.

-- 
Len Sorensen
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