Re: Xi 8088 - The improved version of my XT board

[Sergey <skis...@gmail.com>]

Greg,

Supporting 8086 is much more complicated, at least in IBM PC
compatible machine.
The main problem is mismatch between CPU data bus size (16 bit) and
peripherals data bus (often 8 bit). To work properly it requires data
bus steering logic which does the following:
1. Upon a memory or I/O access checks if device supports 16-bit or 8-
bit operation
2. Splits 16-bit write access into two 8-bit operations for 8-bit
devices. (Essentially emulates two memory or I/O access cycles)
3. Emulates 16-bit read access by combining two 8-bit read accesses
for 8-bit devices.
4. For 8-bit accesses to odd address redirects D8-D15 to D0-7 for
writes or the another way around for reads
This logic is pretty complicated... for example see schematics of the
original AT machine. Other than that, it is not much different from
say 80286 or 80386SX AT, so in this case I'd rather use one of these
CPUs.

While it is possible to eliminate these issues by putting all I/O
devices at even addresses and using only 16-bit memory, it would break
IBM PC compatibility.

Micro ATX - Thanks for the reference, I found it already :-) The
motherboard will have ISA card format, but I have MicroATX format
backplane in works.

Thanks,
Sergey

On Sep 28, 7:27 pm, "G. Beat" <gregor...@gmail.com> wrote:
> Sergey -
>
> Like your earlier version 1 board, you designed this version 2 for the 8088 (or V20) with 8087 support.
> No thoughts of 8086 (10 MHz) support or version?
>
> Greg