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Speedup for the Model 4

kb2syd

Veteran Member
Joined
Mar 7, 2005
Messages
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Location
Wantage, NJ
Reading over some old archives I found. Does anyone remember this tip for the model 4?

SIMPLE SPEED MOD
==================

Here's a simple modification for your Model 4. It requires no
additional hardware and it will bring your computer to its true
4Mhz speed. I should add that I have used my machine for about
5 years after making this simple change with no ill effects.

One thing you must consider: This modification has worked on my
machine, but I cannot guarantee the results on yours. If my
memory serves me right, credit for this tip should be given to
Larry Paine.

Whith that out of the way, open your machine; if you need me to
tell you how to do it, you better get someone else to do it.
Remove the shield that covers the computer's main board in the
read of the machine.

Locate the Z-80 chip (CPU). Pull it out carefully. Bend back
pin 27. Reinsert the chip making sure that pin 27 does not make
contact with anything.

Replace the shield and the machine's cover. That's it!

What the modification does is to remove the wait states in the
Z-80, which are not really needed in your Model 4. To test the
difference it makes, run a program such as CLKSPD/CMD before
and after the modification.

Luis M. Garcia-Barrio
Sysop of 8/N/1 #4
555-555-5555
 
I don't remember it, but it's a cute idea. Note that pin 27 is "/M1" which I believe is only used to insert wait states on earlier Model 4's when they read instructions from RAM or ROM. So as long as your RAM and ROM are up to it this will work fine. I suspect in most cases it is fine as the RAM and ROM will very likely perform much better than their minimum specifications.

I also note that pin 24 is "/WAIT" so you'll still get wait states when accessing video RAM or any other devices which impose wait states. That's good because that could definitely cause problems.

This won't always speed up your Model 4. The earliest models imposed 2 T-State waits on instruction read. This was then reduced down to a single wait per instruction and finally is was removed entirely. It may be that all gate-array Model 4's have no instruction waits but that's only my guess.
 
Thanks. I found a part of the MUSIE archive I must have downloaded a decade ago. This was a tip in the first "issue" of trslink there. Files are dated 1987. TRSLINK was started when 80-micro dropped the TRS-80 line from its content.
 
I did a similar modification to my non-gate array Model 4, which I displayed at VCF East in 2015. Unfortunately, I didn't run any benchmarks before and after, but the system seems to be stable. I also replaced the 200-ns RAM with 100-ns RAM.
 
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