[Coco] I think you'll like this (especially Allen)

[gene heskett]

On 3/21/25 01:31, William Astle via Coco wrote:
> On 2025-03-20 20:02, Allen Huffman via Coco wrote:
>> That said … does anyone know (Tim Lindner? William Astle?) if the ROM 
>> inside the CoCo 3 contains an UNMODIFIED Color BASIC and Extended 
>> BASIC, with the new code just overlaying on them once loaded into 
>> RAM?  Or is my above statement not true and they WERE allowed to 
>> modify the ROM?
I've had several different coco's over the years. There are minor 
patches in several places corresponding with production runs  I think. I 
found the first one just a few bytes  into the rom when Jake Commander's 
rather handy little utility called chromkey (sp?) stopped working well 
on coco2's. The entry point of the keyboard scan had been changed from a 
direct jump to an indirect jump and allowed the keyboard scan routine to 
be moved by about 4 bytes. Modifying Jakes code to use that address made 
it work on all coco's to the end of coco2's.
>
> There are a couple  of modifications to the CB 1.2 and ECB 1.1 ROMs 
> that are actually part of the ROMs. These look like binary overlay 
> patches rather than reassembly so that tracks with them not having 
> access to rebuild. The modified sections are in the system bootup 
> code, which includes a bit that clobbers DLOAD. There's also a few 
> bits related to text mode handling. Overall, it's minimal. The rest is 
> handled by patching at bootup.
>
> They could have done the bootup patches as binary overlays if they 
> wanted to the same as the modifications actually in the ROM. However, 
> because they needed to maintain compatibility with cartridges ROMs, 
> most of these patches could not have been always active without 
> possibly crashing the system on ROM calls (due to jumping into the 
> E000+ area). By keeping the basically stock CB 1.2 and ECB 1.1 ROMs, 
> maximum compatibility is retained. I have a strong suspicion that that 
> is a major reason for the bootup patching having persisted instead of 
> a burned in solution.
Cheers, Gene Heskett, CET.

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