PDP-12 at the RICM

Guy Sotomayor ggs at shiresoft.com
Wed Jul 15 00:15:08 CDT 2015

On 7/14/15 9:53 PM, Fred Cisin wrote:
>>>>  The 8086 had four segment registers:
>>>>     CS    - Code segment, used with IP register
>>>>     DS    - Data segment
>>>>     SS    - Stack segment, used with SP and BP registers
>>>>     ES    - Extra segment, used with DI for string instructions as
>>>>           destination (DS:SI as source)
>>>>  You could override instructions dealing with memory with any of the
>>>> segment registers:
>>> If you were writing an emulator, what would you have it do if an 
>>> interrupt occurred while in a REP operation with a segment override?
>>> Just how accurately should an emulator match the behavior of the 
>>> emulated processor?
> On Tue, 14 Jul 2015, Guy Sotomayor wrote:
>> It's actually described in the Intel SDM (Software Developer's 
>> Manual).  ;-)
>> The start of the instruction is actually the segment override (one of 
>> the things that makes decoding x86 instructions hard). Since the REP 
>> XXX instructions are interruptible, the IP does not move to the next 
>> instruction until after it completes (and the state of the REP XXX 
>> instruction is kept in various registers that are updated on each 
>> iteration).
>> So, when an interrupt occurs, the execution is stopped (after the 
>> current
>> iteration completes) and is restarted when the interrupt returns.
> That certainly sounds reasonable, but,
> have you noticed the difference in behavior of 8086/8088 V 80386?

Haven't.  The SDM covers Pentium forward (and even then it attempts to
document the differences between the different models).  I think 
anything prior to
that has been relegated to "incompatible" prior products if their 
behavior is
different from what is described.

The 8086/8088 were done in a similar vein to the 8080 and 8008.  Not too
much attention was paid to forward looking architectural decisions. The 
and follow on CPUs *tried* to look forward a bit but it wasn't really 
serious until
the Pentium when features such as the TSC and APIC were introduced.

TTFN - Guy

More information about the cctalk mailing list