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TI-99/2 questions


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#201 Stuart OFFLINE  

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Posted Thu May 17, 2018 10:30 AM

So why 10.7 MHz? That's also the frequency for the VDP, so are they driving both the CPU and VDP from the same crystal? Is running the CPU at this slower speed just a cost-cutting measure to save a separate 12 MHz crystal, or is there some benefit somewhere?



#202 mizapf OFFLINE  

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Posted Thu May 17, 2018 2:54 PM

The same situation can be found in the 99/8. Also, the 99/2 has no VDP, just a simple controller. Yes, they share the same clock.

#203 fabrice montupet OFFLINE  

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Posted Thu May 17, 2018 3:43 PM

The TI-99/5 has two crystals: One for the CPU 9500 and one for the VDP, as the 99/4 &4A.



#204 kl99 OFFLINE  

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Posted Thu May 17, 2018 7:31 PM

Meanwhile, some numbers.

 

CPU clock 10738635 Hz internally divided by 4 is 2684658.75 Hz. This yields a cycle time of 372.487 ns (a bit longer than the 99/4A with 333 ns).

 

The outer loop is 400, the inner loop is 65536 iterations, which is a total of 26214400 iterations, each with one memory access.

 

If there is a wait state of one cycle for each memory access, this means 9.76 seconds, which is roughly that what I found as the time difference on the MAME emulation.

 

If it was a late decision change and you are saying that having this correct is key for Tape loading/saving to work it could mean that the real machine is actually running in a wrong speed for it to work. I mean that the routines were not adapted yet in Rom. At least we have to consider this option.



#205 mizapf OFFLINE  

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Posted Fri May 18, 2018 12:32 AM

The good thing about emulation is that you can play "what if...". But I already tried it, and this did not work for me either, so my hope was to get a reference point with the real machines to figure out what goes wrong.

#206 fabrice montupet OFFLINE  

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Posted Fri May 18, 2018 2:11 PM

The TI-99/4B has also two crystals.



#207 Casey OFFLINE  

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Posted Fri May 25, 2018 7:05 PM

Not to hijack the technical discussion, but did TI ever produce any prototype cartridges for the 99/2 to go along with it when it was planned for release?  How about the "demonstration cassette" that the 99/2 Getting Started manual makes reference to?  

 

Just kind of curious to know how close TI was to having a fully released product when the 99/2 was cancelled.



#208 fabrice montupet OFFLINE  

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Posted Sat May 26, 2018 4:39 AM

There is at least one cartridge known to exist. The one presented  at the 1983 CES. Here is a picture:

ti992hcm.jpg

Its name is "Learn to Program".  A second cartridge has been introduced: "Learn  to "Learn to program Basic". I have never see it.

 

For the expansion devices, many were already available thanks to the Hexbus port.


Edited by fabrice montupet, Sat May 26, 2018 4:45 AM.





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