Jose (1.delete@this.2.com) on January 13, 2020 9:04 pm wrote:
> gallier2 (gallier2.delete@this.gmx.de) on January 13, 2020 12:42 am wrote:
> > Jose (1.delete@this.2.com) on January 13, 2020 12:23 am wrote:
> > > Anon (no.delete@this.spam.com) on January 12, 2020 10:34 pm wrote:
> > > > Ricardo B (ricardo.b.delete@this.xxxxx.xx) on January 12, 2020 11:15 am wrote:
> > > > > Ronald Maas (ronaldjmaas.delete@this.gmail.com) on January 12, 2020 9:47 am wrote:
> > > > >
> > > > > > Other reason nobody realized how important IBM PC was going to
> > > > > > be. Even IBM estimated sales of IBMP PC was going to be 100K.
> > > > >
> > > > > Again, not really.
> > > > >
> > > > > IBM went from thinking how to enter the personal computer market to
> > > > > shipping in a blistering fast 12 months, give or take a month.
> > > > > And IBM was in a hurry.
> > > > >
> > > > > There was simply no time to develop a 68008 in the time between IBM saying
> > > > > "we like the 68000 but we want a 68008 for this project" to shipping.
> > > > >
> > > >
> > > > The 8 bit requirement was there because of the low budget of the project,
> > > > and the low budget was there because of low sales projection.
> > > >
> > > > If they had imagined the importance of the PC they wouldn't care about the 8 bit bus, probably
> > > > they wouldn't care about Motorola either and instead go with an in house design.
> > > I believe 16 bit peripheral chips were quite rare at the time, you had the 8080/Z80/6800 series with
> > > a full family of 8 bit peripherals, this could have been also an incentive to go for the 8088.
> > >
> > No, the peripherals work absolutely fine and in the same way with a 8086.
> > The component that the choice between 8088 and 8086 would change is the memory bus. So requiring 2 ROM
> > chips instead of 1 and twice as much RAM chips. This was already too much for the initial IBM project.
> > The proof that the peripheral chips were never the problem are the first clones with
> > 8086 which had no problems accommodating the 8259 PIC, 8237 DMA, 8253 PIT (which
> > are afaict the 3 chips required besides BIOS code for IBM-PC compatibility).
> >
> >
> I think you needed some glue to make the 8 bit peripherals work seamlessly with the 8086.


It depends on what you mean by seemlessly.

The 8086 bus was similar enough that pretty much all the 8-bit 8080 peripherals could be trivially connected to it. The one gotcha is that the I/O ports (or memory mapped locations), would be on alternate bytes as viewed from the CPU.

If you wanted them on consecutive bytes, as you would if you were attempting to emulate the IBM PC, you need some logic to generate that 8-bit bus from the 16-bit bus. It's not hard, and you really only need to do it once in the system.

You can see the required circuit on page 192 of the follow (page 5-8, figure 5-6). This happens to be the Intel recommendation for the 286, but doing this for the 8086 would have been essentially identical.

http://bitsavers.trailing-edge.com/components/intel/80286/210760-002_80286_Hardware_Reference_Manual_1987.pdf