On 9/2/2015 9:12 AM, dmccunney wrote:
> On Wed, Sep 2, 2015 at 1:37 AM, Mateusz Viste <mate...@viste.fr> wrote:
>> On 02/09/2015 04:43, Ralf Quint wrote:
>>> You are making a totally wrong assumptionn that 16bit software means you
>>> are limited to 640KB of memory.
>> Absolutely not - my assumption is that running on 8086/80186 means I am
>> limited to 640KB (or let's say < 1M). Hence for software with higher
>> memory requirements there is little or no advantage of being 16bit.
You keep looking at it from the wrong end. There is no disadvantage of
running 16bit code on >80{1}[6,8] type CPU. And for plain DOS software,
like an installer, using a compiler/memory requirements of 386+ CPUs is
rather limiting the number of potential users...
> And getting around the 640K limit was the reason for HIMEM,SYS, EMS,
> XMS, and playing games with the A20 address line. I routinely used
> the capability when I *was* running on an 8086. My old XT clone had
> an expansion card with a MB of additional RAM, split between a
> ramdisk, disk cache, and EMS memory for apps that could use it. This
> all started back *before* the 286 and later 386 CPUs became common.
Well, yes. No. Maybe. All/none of the above...
On an 8086,8088, 80186 type CPU, you can use EMS, if you have
specifically hardware (EMS board) installed. You can not use any XMS,
HMA or similar extensions, as those CPUs have only 20 address bits
(A00-A19), resulting in a hard 1MB memory limit. There aren't any
"games" you can play with the A20 line because there is no A20 on those
CPUs.
That all just became possible with the arrival of the 80286 CPU, which
allowed due to a bug/quirk in the CPU to have a wrap-around of the first
64KB segment, which allowed for an address space of 1MB+64KB-16 bytes.
Ralf
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