[Coco] CoCo 3's CoCo 1/2 mode

[William Astle]

If VCC is implemented correctly, most stuff in FF9x will return junk 
when read. Those registers are write-only on the real hardware.

Regardless, setting FF90 to 0 will likely crash the system. That turns 
off the MMU and the "constant page" at FExx.

On 2021-08-23 3:47 p.m., Joel Rees wrote:
> What happens if you set it to zero, then read it?
> 
> 2021年8月23日(月) 12:38 M. David Johnson <mdj at bds-soft.com>:
> 
>> I read that when Bit 7 of &HFF90 (cf.
>> http://www.cococommunity.net/socks-gime-register-reference/ ) is set, the
>> CoCo 3 is then in CoCo 1/2 mode.
>>
>>
>>
>> I don't have a physical CoCo 3 to check that on (I only have a CoCo 2).
>>
>>
>>
>> But in VCC 2.1.0d I notice that, immediately on startup, &HFF90 contains
>> &HCC (1100 1100 binary).
>>
>>
>>
>> So it's already in CoCo 1/2 mode? Am I missing something?
>>
>>
>>
>> --
>>
>> M. David Johnson
>>
>> mdj at bds-soft.com
>>
>>
>>
>>
>>
>>
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>>
>