I used to own IBM PS/2 Model 30 8530 8086s and with and without NEC V30 they were pretty slow for most MCGA/VGA games like Civilization / Wings of Fury. To be fair the V30 did speed things up a lot but compared to a 80286 games were very slow nonetheless.

To replace an 8086 you need a V30, a V20 is the replacement for an 8088.
A V20 at 4.77 MHz will run somewhat faster than an 8088, a V30 will run faster still.
I don't fully understand what you are going for...
I suppose the higher speed is somewhat better for earlier Sierra games, but there will be a lot of games that will struggle at all these speeds.

Moved to Marvin

Guys let me explain a little bit better..

My laser turbo XT 8086/3 have a siemens 10mhz processor i think
My custom build XT 8088 have the NEC V20 processor already and works on 8 mhz..

My actually question here is.. Which turbo speed do i have more benefit of??

The 8 mhz one or the 10mhz.. I know that faster normally would always be better.. But i really dont like when software is running to fast then its design for..

NEC V20's should work ok at 10MHz. I actually have a JUKO ST-12 motherboard back in Portugal. Had no time to test it has i didn't have XT-IDE and an XT keyboard.

The utmost importante thing for these systems is to be able to function at 4.77MHz. A NEC V20 will still be faster than a normal 8088 at that speed. But i don't know if speed sensitive games will be unplayable, my guess is that they will feel just a little faster.

Obviously the faster the better, as long as the original 4.77MHz is not compromised. I don't think no one builds these systems for their speed, but to run software that otherwise would run too fast on anything faster than the original 4.77 speed. If i'm not mistaken these systems get their 4.77MHz speed by diving a 14.318MHz crystal by 3. I believe some of these boards might also have a separate crystal to keep the ISA bus speed at no more than 8MHz. Certainly the ISA bus running at 10MHz is gonna create problems.

Now you have a 8086 at 10MHz and a NEC V20 at 8MHz. What do you want? Speed or compatibility with these systems? If you want speed, a 10MHz 8086 will be faster than a 8MHz NEC V20. If you want even more speed you could probably get a NEC V30 to replace the 8086. I guess a NEC V30 could maybe run at 12MHz without hige problems. But keep the ISA bus speed at bay.

Again it really depends on what you wanna do.

So the 24.000 mhz crystal is to keep the ISA bus to 8Mhz? Can you tell me why the divider is set to 3 ?

The laser turbo XT 8086 at 10Mhz is just fine..

But iam think of raising the 8 mhz NEC V20 to 10Mhz as well..

Robin4 wrote:

So the 24.000 mhz crystal is to keep the ISA bus to 8Mhz? Can you tell me why the divider is set to 3 ?

The divider acts as a straight division operation on the frequency: 24/3 = 8.

But why is it 3 ?

Why not? IBM had to choose a 14.318MHz crystal because it would keep the oard design simpler and cheaper. One would thing that the ideal would be a 15MHz crystal to make the 8088 tick at 5MHz. Aferall that's the speed they were rated.

But if they did that they would need to include other crystals to make other stuff work correctly. This way by choosing a single 14.318MHz osci they killed 3 birds with one stone.

CPU 8088: 14.318MHz/3 = 4.77MHz
NTSC: 14.318MHZ/4= 3.58MHz needed for the vídeo modulation work
Intel 8253: 14.318MHz/12= 1.19MHz needed for other stuff like time of day clock or memory refresh

The NTSC frequency was the key to the base frequency. There was room to play with everything else, such as the CPU and PIT clocks, but no 3.58MHz signal, no NTSC color. Without NTSC color, IBM would have been hard pressed to have been able to release a Color/Graphics Monitor Adapter. Many people made do with NTSC-based color monitors in the early days, not RGB.

Nobody cant tell me where that 3 is comming from?

I don't quite get what you are not understanding. The ISA slot needs 8mhz. The crystal is 24mhz. They just picked the divider that, from that crystal, would give the desired clock speed.