Model 4 hires board compatibility?

[mykrowyre]

I know there are some minor differences between the hires boards, I've read that the Radio Shack version requires a different method to display graphics over text.

Are most games compatible with both boards?

Asking because I have a Radio Shack hires board, and some games such as pentagram and alien 8 don't display correctly.

Most of the hires test programs do work correctly.

 

[gp2000]

Some programs auto detect the board type. Their main problem is that the detection fails with Ian Mavric's Grafyx clone boards. The Tandy board has a full 32K while the Grafyx has only 20K. But the Grafyx clones have 32K and confuse the auto-detect.

Technically its the clone boards that have a different way of mixing text and graphics since the Tandy came first. On the Tandy, bit 0 of port $8E must be set to mix text and graphics. On the Grayx boards bit 1 of port $83 must be clear to mix text and graphics (i.e., it is a disable text bit). But on the Tandy that bit controls whether or not the Z/80 is told to wait for HBLANK or VBLANK when reading/writing graphics data.

In that picture it looks like a lot of screen hashing going on. Seems likely it was meant for the Grafyx with text mixing which means a 0 in bit 1 of port $83 which means no I/O waits on the Tandy hence the screen hash.

Incidentally, the Tandy also has X,Y scroll registers at $8C and $8D. Almost nothing uses them.

 

[mykrowyre]

In that picture it looks like a lot of screen hashing going on. Seems likely it was meant for the Grafyx with text mixing which means a 0 in bit 1 of port $83 which means no I/O waits on the Tandy hence the screen hash.

Incidentally, the Tandy also has X,Y scroll registers at $8C and $8D. Almost nothing uses them.

Ok that makes sense. So were there patches to make these Grafyx compatible programs work with the older Tandy board?

 

[gp2000]

For those particular games I'll ask Mark about fixing them. Though it looks like it would be pretty easy to patch.

 

[mykrowyre]

For those particular games I'll ask Mark about fixing them. Though it looks like it would be pretty easy to patch.

Any idea why the image doesn't fill the screen as it does on the Grafyx board? Its sort of 3/4 size and at the upper left.

Is there a screen resolution mode which is different between the two?

Coincidentally or not, 24K / 32K = .75 (75%)

 

[gp2000]

Any idea why the image doesn't fill the screen as it does on the Grafyx board? Its sort of 3/4 size and at the upper left.

It's because the text mixing is enabled on the Grafyx and not on the Tandy.

When text mixing is on and the text display is in 64 x 16 mode the hires resolution is 512 x 192. If text mixing is off or the text display is 80 x 24 the hires resolution is 640 x 240.

The programs you mention don't set bit 1 of port $83. This means text mixing is on for the Grafyx. But not for the Tandy board because it wants bit 0 of port $8E set to enable text mixing. So it remains in 640 x 240 mode.

Both boards follow the same rules when determining the hires resolution but you can end up with different results because they use a different I/O port bit to handle text mixing.

 

[mykrowyre]

IThe programs you mention don't set bit 1 of port $83. This means text mixing is on for the Grafyx. But not for the Tandy board because it wants bit 0 of port $8E set to enable text mixing. So it remains in 640 x 240 mode.

Thanks, seems like an easy patch

 

[mykrowyre]

Is there a schematic available for this board? I think mine has a bad ram chip, the the part numbers appear to be tandy part numbers. I think they are 8k motorolla static ram chips, but the pin count appears different.

 

[gp2000]

I've created some patches to have the programs properly detect Tandy or Grafyx hires boards and initialize properly. Works for Tandy, Grafyx and Grafyx clone boards. Here are the patches and I've applied them to the originals.

 

[mykrowyre]

thanks!!