TRS-80 computers: why such slow video?

[MHaensel]

Why are TRS-80 computers (relatively) slow updating the screen?

 

I'm especially watching the TRS-80 model 16 in this video. It's got great text quality, but is painfully slow drawing boxes or filling in lines of reversed characters. The 4D next to it is also sluggish despite being several years newer.

 

Compare it to the Tandy 1000SX at 3:15. The 1000 drives a color screen and pops menus in and out.

 

The model 16 is based on the model II which has "separate keyboard and video processors, direct memory access and vectored interrupts for faster throughput." So on paper the Model II should be a beast. What's holding it back?

[George Phillips]

At the hardware level their graphics are certainly as fast as the Tandy 1000SX.  Both have memory mapped displays.

 

There are a million ways for a program to be slow but I speculate that the programs on the Model 16 and 4D are using the the operating system to display characters.  They may even be using the system call that display as single character.  The overhead per character ends up being very high.  And even then the OS provided routines to display a string aren't very fast.

 

I think this is rather like early PC programs which used DOS or the BIOS for character display.  Their update rates were much slower than programs which accessed the screen memory directly.

[jhd]

On 6/14/2020 at 1:24 PM, George Phillips said:

I think this is rather like early PC programs which used DOS or the BIOS for character display.  Their update rates were much slower than programs which accessed the screen memory directly.

 

I have no experience with a Model II, but I remember how excited I was when I discovered (via a magazine article) that it was possible to access video RAM directly on the Coco. Not only did the resulting code run a bit faster, but it was possible to do things not contemplated by BASIC! 

[MHaensel]

For Deskmate specifically, I think I found the answer!

 

Deskmate for the Model 16/Tandy 6000 runs under XENIX, which is a multi-user operating system. Among other things, that means virtualized access to screens and keyboards, which slows things down.

[Omega-TI]

On 6/14/2020 at 9:26 AM, MHaensel said:

Compare it to the Tandy 1000SX at 3:15. The 1000 drives a color screen and pops menus in and out.

 

If I remember correctly, I sold 4 Tandy 1000's simply because the wives the guys had with them saw the Kings Quest game running on the screen!  

[MHaensel]

On 8/11/2020 at 1:55 PM, MHaensel said:

For Deskmate specifically, I think I found the answer!

 

Deskmate for the Model 16/Tandy 6000 runs under XENIX, which is a multi-user operating system. Among other things, that means virtualized access to screens and keyboards, which slows things down.

Despite my enthusiasm, I no longer think this is correct. Other XENIX software updates the screen quickly. It's not just a virtualization thing. @GeorgePhillips likely has the right answer. Thanks all!