Re: [cc65] simple assembly interrupt handler

Ullrich von Bassewitz wrote:

>Hi!
>
>On Mon, Jan 02, 2006 at 04:24:51PM -0600, PH wrote:
>  
>
>>   As I understand it, $FFFA is the memory address where the interrupt
>>looks for the handler to jump to.  In this case, I am mostly interested
>>in the vblank isr, as I need to update the video ram buffer during the
>>video blank interval.  In the original code, several things are updated
>>in the handler by calling the 3 subroutines, but I think if I could just
>>set a vblank flag variable in the handler and do the updates in my main
>>code loop, that would be fine.  So, if I want to do this, I think I need
>>to use a linker config file and a .segment command to place the handler
>>address at $FFFA, but I'm not sure how to do that, since I don't know
>>how I specify where I want to jump to.  Can someone help me out with a
>>bit of example code or something?  Any help is appreciated.
>>    
>>
>
>You may want to look at groepaz NES config which is part of cc65. It does
>exactly what you need (placing the hardware vectors at a specific address). If
>I remember correctly, a copy is in the doc archive. If not, you may need to
>download the cc65 sources and look into src/ld65/cfg/nes.cfg. The memory area
>named ROMV contains the ROM vectors. The segment named VECTORS is placed into
>this memory area. Using "fill=yes" on the memory areas will fill unused areas
>in the file, so the contents of the file become an image dump of the real
>memory.
>
>Since you should not write interrupt handlers in C for reasons stated in the
>FAQ, you need to add a small assembler module:
>
>-----------------------------------------------------------------------------
>        .export         _vblank_flag
>        .import         reset
>
>.bss
>
>_vblank_flag:   .res    1
>
>.code
>
>vblank: pha
>        lda     _vblank_flag
>        eor     #$FF                    ; Toggle flag
>        sta     _vblank_flag
>        pla
>irq:    rti
>
>; The following segment is mapped to $FFFA
>.segment        "VECTORS"
>
>        .addr   vblank
>        .addr   reset
>        .addr   irq
>-----------------------------------------------------------------------------
>
>Using this declaration:
>
>extern volatile unsigned char vblank_flag;
>
>your main loop can then poll the flag like a normal C variable:
>
>        unsigned char flag = vblank_flag;
>
>        /* Wait for the flag to toggle */
>        while (flag == vblank_flag) ;
>
>        /* Go ahead */
>
>Regards
>
>
>        Uz
>
>
>  
>
That makes sense to me, but I've got two problems with it.  One is that 
the VECTORS segment in the nes.cfg file is as follows:

# Hardware Vectors at End of 2nd 8K ROM
    ROMV: start = $fff6, size = $c, file = %O, fill = yes;

Which means that he started the vectors at $fff6 instead of $fffa (I 
think).  I can't find any documents anywhere that say $fff6-$fffa is 
used as interrupt vectors.  So, I assume this is a mistake?  (I hope 
groepaz is listening and can enlighten me)

Also, when I try running the linker on the code, it says:

ld65.exe: Warning: Memory area overflow in `ROMV', segment `VECTORS' (6 
bytes)
ld65.exe: Error: Cannot generate output due to memory area overflow

I'm using your code as your wrote it.  Any ideas why it is complaining 
about an overflow?

Thanks,
Paul
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