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The Cabin >> 8 Bits and Beyond >> Commodore Vic 20 Upgrades and Comparisons
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Message started by Fernando on Aug 9^th, 2024, 1:13pm

Title: Commodore Vic 20 Upgrades and Comparisons
Post by Fernando on Aug 9^th, 2024, 1:13pm

As far as the Vic 20 is concerned, though it was the first Home computer by Commodore and very popular for the short time is had before it’s younger brother, the C64, came out, was considered as more of a Toy Computer than a true computer. This is because of the size of the screen being 23 X 22 instead of a 40 or 80 by 25. Despite the screen limitation, the Vic 20 was faster than a C64. Another issue, especially for the older Vic 20s tends to get hot that you can feel the keyboard getting warm after a couple hours of use.

But as a computer, it is comparable to anything out in the market at the time for the price. But can it be made better? This upgrade is something I did for my Vic20 back around ’84.

The reason why the Vic20 was faster than the C64 was because of the use of Static RAM on the Vic20 vs. Dynamic RAM on the C64. Dynamic RAM requires that the RAM gets refreshed every clock cycle, which was supported by the Vic II (video) Chip on the C64. Thus the C64 was forced to wait for the RAM to be refreshed before it can act on what the RAM has. With Static RAM, there is no refreshing of the RAM and can be immediately accessed by the CPU and Video Vic chips as needed. Since there is no waiting for RAM to Refresh, the system is that much more faster than a C64. This exact same happened with Commodore PETs; where the older 2001 8K & 16K PETs using the Static RAM was faster than the newer 4016 and 4032 PETs of the time which used Dynamic RAM.

But can the Vic20 be made faster without too much work or money invested into it? In a way, yes. And it will run cooler too. If the chips on the motherboard are on sockets, then 90% of the work is done for you. All you have to do is to replace the chips on the board. But which chips?

You will need to get the following: 65C02 and two 65C22 VIAs (Versatile Interface Adapter). Being CMOS Chips they run slightly faster than the original MOS variant of the chips in question, and they run a lot cooler. Speed increase can be as high as 15% or as low as 5%, it depends on what the computer is doing at the time.
http://www.hondosackett.com/yabb/VintageComputers/Vic20-CMOS-Mods.png

Furthermore you can increase the speed of your Vic 20 by replacing the RAM is has with faster RAM. When systems like the Apple II, TRS 80 and PET/Vic20 were out, they were using RAM that was 250ns to 200ns speed. Replacing the RAM with 150ns to 120ns will increase the speed of the system dramatically. But this is true for any computer out there, replace the slow RAM with fast RAM, and the system will improve greatly. The faster RAM also means that the system will run cooler as well.

The Vic20 could go a bit more faster if you replace the ROMs with faster EPROMs. But in doing so, the speed of the EPROMS must be as fast as the speed of the RAM you put in. Doing all this will get you 10% to 20% increase, depending on what the system is doing. For example, running a repeated loop in BASIC will run faster than stock. I/O will remain the same as they are programmed to be at a specific speed. Accessing Video RAM, which is part of the System RAM, will go faster in displaying that changes you make to the screen, including animation. Sound will remain the same as that is ran through various registers inside the Vic Chip and not in RAM though values can be loaded from RAM and into the Vic Chip.

Can the same be done with the C64? Within reason, no. You can replace the RAM with faster RAM and CIAs with CMOS versions of them but there is no CMOS version of the 6510 the C64 uses. At best you could get a 5% to 10% speed increase from such a Chip Swap. There are “Hacks” to put in a 65C02 in place of the 6510, there are incompatibility issues to deal with.

What about the Plus4 / C16? That system is already running at 1.75MHz, the chips inside are faster chips, similar to the Atari 8-bit system, which also run at 1.75MHz. You could get some speed increase by replacing the RAM with a faster version (from 150ns to 120ns or 100ns RAM). The 7501/8501 is already a CMOS Variant of the 6502/6510.

What about other 6502 Systems? Like the Plus 4/C16, the Atari is running at 1.75MHz. The XL and XE Series Ataris already have the 65C02. The older 800 and 400 have a fast 6502B, replacing it with a 65C02 would give it a slight 5% speed increase making it equal in speed to the XL and XE Series. The same with the Apple II/II+ series, swapping out the 6502 with a 65C02 will give it a slight speed boost, again about 5%. Most IIe’s were enhanced with the 65C02 and Enhanced ROMs though there might be a tiny few that are in the original 6502 form factor. The issue with the Apple II/II+ is Slow RAM. Most would have 4116 RAM chips running at 200ns. One can upgrade to 150ns or 120ns RAM, but they would have to be the same speed. If you mix fast RAM with slow RAM, the system will run at the speed of the slowest RAM. This is true for any and all systems. Upgrading the CPU and RAM on an Apple II/II+ will bring the speed to a comparable Enhanced Apple IIe system. But without Enhanced ROMS such an upgraded Apple II/II+ will not be able to run ProDOS, though there is a modified version of ProDOS that would on an older Apple II/II+.

Some would state that 5% speed increase is not much of an advantage or improvement. For most things, including games, it may not be. But for complex computations, data handling and hires graphics creation, shaving off seconds off every minute the system original worked is a major improvement. 10% improvement is a bit more noticeable, with 6 seconds shaved off every minute an original system can do. Most importantly, the system will run a lot cooler from using less power to run, which was an issue with original equipment in their day many years ago.

Before making such upgrades, you have to examine the system to see how much work needs to be done. At worst you would have desolder the old chips off the logic board, solder in chip sockets and put in the new replacement chips into the chip sockets. At best, sockets were already installed at the factory and you have to remove the old chips from the sockets, spray in some electronics cleaner like “De-Oxit” into the chip sockets and then install the new replacement chips into the sockets. Once all this is done, you can enjoy the benefits of an upgraded and improved system.