V9958 documentation unclear | MSX Resource Center (Page 3/3)

Champion (361)

01-04-2023, 20:27

gdx wrote:

I explained it this way because the logical AND doesn't seem right. I don't see how an AND could cause the effects I observed.

I agree that I’m wrong in my characterisation.

By Accumulator

Champion (330)

02-04-2023, 08:06

This is also applicable for V9938 :
Why can the VDP be accessed by standard port 98H to 9BH and 9CH to 9FH?

I have not tested yet, but does it have any negative or additional effect if you address the VDP interchangeable by those addresses.?

By gdx

Enlighted (6208)

02-04-2023, 11:10

Accumulator wrote:

Why can the VDP be accessed by standard port 98H to 9BH and 9CH to 9FH?

On which machine did you see this? I have never observed this.

By Grauw

Ascended (10767)

02-04-2023, 12:02

To simplify address decoding. It is not standard so may vary per machine. Other system I/O may also mirror.

By Accumulator

Champion (330)

02-04-2023, 18:26

Sony Hbf700. Among.
If exchange & h98 for & h9c as base VDP port,
Everything runs perfectly....

I run some code now executing some VDP instructions first using first 4 next last 4 io ports..
Fast Hmmm not yet tested...

It should be standard, reading the documentation of S1985 it mentions for VDP
98-9F W write
98-9F R Read

By gdx

Enlighted (6208)

03-04-2023, 02:42

This is not standard but a specificity of the S1985 because ports 9Dh-9Fh are classified as reserved in all official documentations of the standard. I tried on my turbo R, there is no such mirror.
My FS-A1 has this mirror. It uses the S1985. So all MSXs with an S1985 have without a doubt this mirror. It remains to be determined if there are other MSX engines or architectures that cause this mirror effect.

I tried on OpenMSX with an emulated FS-A1, it does not emulate this mirror.

By Manuel

Ascended (19462)

03-04-2023, 08:04

We had confirmation that the S3527 mirrors and adjusted the configurations to that. But not for the S1985.

By gdx

Enlighted (6208)

03-04-2023, 17:22

Now you have it.

By Accumulator

Champion (330)

03-04-2023, 21:52

So, All S1985 mirrors the 98-9B port to 9C-9F?
It is not additional ports to address the VDP which results in simultaneous executing commands of VDP?

What I understand it does not matter which of the 4 io you take.
And what does it really mean when something is specified by documentation of S1985?
@gdx, If it is specified in documentation, what is the difference between standard for this CHIP or specified for this Chip?
As for S3527, only VDP ports 98 and 99 are specified, not 9c and 9D.
Does this mean S1985 has 2 VDP 'busses' to V99x8 and S3527 only 1?
What is the use of mirroring VDP io ports?

I noticed that OpenMSX was capable of using the upper ports of VDP. Therefore I thought they knew it was possible to address them..