Re: [cc65] How to force a read from a memory mapped register w/o volatile

On Mon, Dec 6, 2010 at 7:57 AM, Dave Dribin <ddribin@gmail.com> wrote:
> I'm trying to replicate this bit of assembly code (used on an NES) in C:
>
> lda $2002 ; Read PPU status to reset the high/low latch
> lda #$20 ; Write the high byte of $2000
> sta $2006 ;  .
> lda #$00 ; Write the low byte of $2000
> sta $2006 ;  .
>
> The addresses $2002 and $2006 correspond to memory mapped registers.
> THus, even though the result of the first read is ignore, it still
> needs to be performed to put the device in the correct state.  Here's
> my attempt at the C version:
>
> Typically the 'volatile' keyword tells the compiler not to perform
> this optimization.  Since the 'volatile' keyword is parsed, but has no
> effect, how do I go about doing this with C in cc65?

Read the value to a dummy variable. If there's a better way, I'd like
to know also.

Btw, Uz, there's a bug in CC65 optimizer which occurs when reading the
controller in NES. Like this (_M is a macro to access a byte in
memory):

    _M(0x4016) = 1;
    _M(0x4016) = 0; // capture the controller state

    {
        byte btn_a = _M(0x4016) & 1; // this read is optimized away,
the optimizer assumes the value is 1
        byte btn_b = _M(0x4016) & 1;
        byte btn_select = _M(0x4016) & 1;
        // etc
    }

-thefox
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