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tezza
November 3rd, 2007, 02:01 AM
Hi Guys,

I was playing around with my Amiga 500 tonight, and I noticed there is a slide switch on the cover of the side expansion port on the left side of the unit? At one end of the switch is the letter C, and the other is an F.

Does anyone know what this does? I just noticed it tonight. The switch was set to C when I received the unit. I tried to prise open the cover but it resisted my efforts and I didn't want to break it.

The next step I guess is to open up the machine, but I'll see if it's a common addition first. I'm assuming it is some kind of addition, as all of the pictures I've seen of Amiga 500s just show a plain slot cover?

I did throw the switch from C to F when the machine was going and it seemed to crash it?

Any ideas?

curtis
November 4th, 2007, 02:20 PM
Might be some sort of boot ROM switcher.

tezza
November 5th, 2007, 02:30 PM
Hmm..could be.

Next time I drag it out I'll try flipping the switch to F first before booting and see what happens.

In a google search I did come across an 512k expansion called "Fast-RAM". Could this be what the F is for? Thing is even on "C" it still boots up with 1MB so the expanded RAM is obviously functioning in that position.

curtis
November 5th, 2007, 07:01 PM
That is a thought. Some sort of memory switcher to switch between Fast Ram and Chip Ram.

Don't really see any reason for that unless it was to move memory around for some weird games.

carlsson
November 6th, 2007, 06:29 AM
Ah, yes! Of course it is a switch to enable/disable DMA access to expansion RAM, selecting between Chip and Fast RAM. For those not familiar with the Amiga jargon, Chip RAM is between 512K and 2 MB (depending on Amiga model) that the custom chips can access directly: graphics memory, audio samples etc. Thus the CPU shares the bus with the custom chips, leading to a bit slow memory accesses. Fast RAM on the other hand is outside of what the custom chips can access. Therefore it doesn't help with better graphics or sound, but will be many times faster for the CPU to read and write to. I think there was a third one, Slow RAM or maybe it was another denomer for Chip RAM.

Using a mem program (maybe it was called mem?) in a CLI/shell window, you should be able to see exactly how much memory you have installed, and of which type. Write down the numbers, turn off the computer, flip the switch and try again to see that one type of memory increases and the other decreases.

tezza
November 7th, 2007, 12:38 AM
Ah, yes! Of course it is a switch to enable/disable DMA access to expansion RAM, selecting between Chip and Fast RAM. ........
.................................................. .................................................. ..
Using a mem program (maybe it was called mem?) in a CLI/shell window, you should be able to see exactly how much memory you have installed, and of which type. Write down the numbers, turn off the computer, flip the switch and try again to see that one type of memory increases and the other decreases.

Yes, that was it alright! The CLI command was "avail" (for available memory I guess). Sure enough, when the switch was on C, I had all 1MB of memory available as Chip RAM (and 0 bytes Fast-RAM) but when Booted with F, about 1/2 of the available memory was Fast-RAM.

I'll still not sure what the optimum configuration is though? What's the best default configuration? All of the memory as Chip RAM, or some as Fast-RAM?

carlsson
November 7th, 2007, 04:38 AM
If you play arcade games, you want at least 1 MB Chip. If you do a lot of number crunching and other CPU intensive things, you will get quicker results with 0.5 MB Fast. Ideally you would want more RAM alltogether. Newer Amiga 500's had 1 MB Chip installed, at least the Plus model with Fatter Agnus and the Enhanced Chipset.

gonk23
January 12th, 2015, 08:24 PM
I know this is a very old thread, and it's probable that the participants know a lot more about the topic now, however I stumbled across it in a google search and thought I'd add my 2 cents.


I'll still not sure what the optimum configuration is though? What's the best default configuration? All of the memory as Chip RAM, or some as Fast-RAM?

The best "default" configuration would be to put it in the "F" position (i.e. the 512kB/512kB split). This sets it up in the same way as when it was purchased.

The majority of A500 machines (excluding "A500 Plus" machines) were delivered with 512kB chip RAM. The popular trapdoor memory expansion added another 512kB pseudo fast RAM (often called slow RAM because it shared the slower chipset bus, without the chipset actually being able to use it).

The A500 rev 6A that tezza owns shipped with an Agnus chip that could address 1MB chip RAM, however Commodore configured the systems to only allow 512kB chip RAM. The motherboard was designed to have two jumpers (JP2 and JP7a) to allow the trapdoor RAM to be treated as chip RAM and use the full 1MB chip RAM. Changing these jumpers (which required cutting 2 traces and soldering 2 pads together) was a popular hack to the A500 and is documented at: http://aminet.net/package/docs/hard/a500chip . It appears whoever did the hack to tezza's machine decided to add a switch to allow both configurations.

Most games were designed for the unhacked A500, so 512kB chip RAM (with some needing more slow/fast RAM) works fine. In fact, some poorly designed games assumed there was only 512kB chip RAM and would crash if it was greater. This is why the switch is useful.

The advantage of having the extra chip RAM is mainly when multimedia applications (e.g. DPaint) are used, especially if you're running more than one at the same time. In more modern usage, it's also useful when using WHDLoad to allow some games to run successfully from hard disk.

Of course the "F" label of tezza's switch is a misnomer because it's actually slow RAM. ;)

curtis
January 13th, 2015, 09:39 AM
Fast, Slow. With the Amiga does it matter? Fact is, I seem to remember someone referring to motherboard memory that was hacked to Fast memory as Fast/Slow memory!

So, I guess, that would make it "medium" memory?:confused:

gonk23
January 13th, 2015, 01:06 PM
In this context it doesn't matter. I think the OS only differentiates between chip and fast memory. I should have really put a smiley at the end of my last statement (edited).

tezza
January 18th, 2015, 06:12 AM
Much obliged. I now understand this. Thanks!

gonk23
January 29th, 2015, 02:55 AM
By the way, the switch wiring is documented in section iv-6 of:
http://aminet.net/docs/hard/500hacks.txt

ChrisUnionNJ
July 13th, 2016, 09:11 PM
Also some Demo's and chip tune's need more chip to play..