Re: [cc65-devel] HuC6280 zero-page addressing problem.

[Greg K. <gr...@us...>]

On 2016-06-23 2:29 PM, John Brandwood wrote:
> Re: [cc65-devel] HuC6280 zero-page addressing problem.
> From: groepaz -- 2016-06-22 23:11:53
>>
>> I still think it'd be weird if the assembler assembles "lda $2000" into a
>> two-byte instruction, however -- you should be writing "lda $00" instead; and,
>> treat the first RAM page as actual zeropage; what speaks against it anyway?
>
> So, you think that I should write $0000 when I really mean $2000???
> That's a very strange suggestion, IMHO.
>
> Here's an example of the problem in the real-world.
> There are specific locations that are used by Hudson's CD System Card BIOS.
>
> _ax = $20F8
> _al = $20F8
> _ah = $20F9
>
> Those are meant to be used with direct-page/zero-page addressing.
> So, let's say that I do decide to lie, just to make CA65 happy, and change
> those to:
>
> _ax = $00F8
> _al = $00F8
> _ah = $00F9
>
> What happens when I put the address of one of those variables into a pointer;
> or, if I use an instruction that doesn't support 1-byte addressing?
> They'll end up pointing to the wrong locations!  That's madness!
>
> Similarly, let's say that I want to write to location $0000, which is where
> the VDC chip normally is mapped.
>
> VDC_CTRL    = $0000
> VDC_DATA_LO = $0002
> VDC_DATA_HI = $0003
>
> Suddenly, CA65 is generating a 1-byte addressing instruction; and, the write
> actually is going to $2000.  Again, madness!
>
> The current solution for them is to add "<" or "a:" address-overrides to
> every instruction, in order to generate the proper code.
> That's ugly, and shouldn't be needed IMHO; but, you guys might disagree.

The video chip problem is easy to fix:  Use I/O mirror locations.
VDC_CTRL := $0200
VDC_DATA := $0202

>
> Isn't the point of computers that they actually should do things right?
> Don't you want CA65 to work in a way that matches the reality of the
> actual processor?
> If you expand your thinking of the concept of 1-byte addressing into
> something that's referring to the direct-page rather than just the
> zero-page, then you'll have no problem matching your current notions
> of what's correct to the actual reality of the situation.

That issue is important for the Commodore 128 and the 65816 CPU also. My 
thought is a directive, similar to .ORG, that tells ca65 where the 
direct page currently is.  When ca65 sees an address constant operand, 
it checks the mnemonic.  If that instruction allows the direct-page 
addressing mode, and the constant is in the 256-byte range of the 
directive, then it is changed into a "zero-page" address.