[Coco] Re: 64K Mode

[Brad Grier]

jdaggett at gate.net wrote:
> Brad
> 
> In the SAM chip if you switch from memory map type 1 to type 0, by 
> a write to $FFDE. you will now map the roms into memeory map. 
> What info you have in the upper 32K can only be accessed in map 
> type 0 by setting the page # byte, $FFD4 to clear the page register 
> and $FFD5 to set the page register.. That will switch the 64K ram in 
> two 32K pages. You code should be position independant. When in 
> map type 1, all ram the page byte does nothing and the roms are 
> not mapped. 
> 
> If you want to switch pages of graphics then you need to have you 
> code running in rom at $C000 and use the lower 32K for grahics 
> storage and program storage. You will have to switch the lower 32K 
> bytes of ram in two pages. You now effectively have 96K machine. 
> 
> james

Hi James,

I'm confused by this. Are you saying the P1 register at 0xffd4 and 
0xffd5 allows two banks of lower 32k ram? None of the 64k programs I 
have write to this register. Are there any programs that do this?

According to my color computer reference manual, "Register P1 controls a 
feature which is not used in the Color Computer. Therefore it should be 
cleared at all times." Is that pre-64k?

By the way, thanks to everybody for all the input. I was able to get 
Mocha to handle graphics beyond 0x8000 in 64k mode (Gold Runner and 
CyberTank) with your help.

Brad