1MB ChipRAM expansion for A500+ with disable switch

[a.mi.goun]

I am looking for an 1MB trapdoor chipRAM expansion for A500 plus.
But I need it to have disable switch on it.

 

[SuperFatAgnus]

i have one ELBOX 2 MB CHIP with switch off/on

 

[a.mi.goun]

Can you give me more details? I need it for A500plus and compatible with ACA500plus.

 

[a.mi.goun]

No news from superfatagnus, so still looking.

 

[SuperFatAgnus]

Sorry my Elbox 2 MB CHIP A500plus - without swith on/off - my mistake sorry

http://www.aukcjoner.pl/gallery/021185830-2.html#I2

 

[a.mi.goun]

Thanks, ok, I definitely need with on/off switch, so still looking.

 

[BLTCON0]

Thanks, ok, I definitely need with on/off switch, so still looking.

I'm not sure these exist with on/off option.


The 512 KB ones for the A500 usually had this option, but this was for compatibility reasons with badly written software which only expected the default 512 KB of chip RAM to be present.


[ The default 512 KB of chip RAM can be addressed using only address bits A0 - A18, so the upper five bits A19-A23 of a pointer register could be used to store something else, as they'd effectively be ignored during read/writes on a 512 KB-only system.
The extra 512 KB however mapped to $C00000 as ranger RAM, this uses the full 24 bits.
So with such badly written software, the five "other usage" bits became part of a valid address and data could be randomly written to the wrong place resulting in crashes, so one can see the need for a disable switch ]


On the A500+ though, the system memory map has no $C00000-mapped RAM, the default 1 MB of chip RAM is already linearly mapped with no option to disable the upper 512 KB and, actually, any badly written software would already break at this point.
There's practically no software that would correctly work with 1 MB of chip RAM but breakdown with 2 MB of chip RAM, hence no real need for a disable switch on these A500+ expansions.


If you must have a disable switch, you could probably hack an existing board by cutting the external RAS/CAS lines on the RAM board and wiring them inline with a switch (see S.5-8 in the schematics). With the lines disconnected the RAM would not respond to reads/writes on the upper 1 MB of chip RAM address space and would therefore vanish. Doing this just for the RAS lines is probably enough, too (simpler switch).

 

[a.mi.goun]

Thanks for your explaining. So there probably isnt any - that means I have to live with it or do some hackery on existing one. Thanks again.

 

[BLTCON0]

You could avoid modifying the RAM board by crafting a short pass-through adapter and do the disconnections there.
Most A500+ 1 MB chip-RAM expansions aren't full length anyway so the extra length wouldn't present any problems.

 

[a.mi.goun]

Yes, passthrough is really a nice idea. I could try to make one. I already have 1MB A500plus mem board, that is very small sized. Hopefuly these RAS/CAS disconnecting will make the memory expansion invisible. And I will try to find that schematics (for trapdoor pinout).