Re: [RFC 05/10] hw/mos6522: Don't clear T1 interrupt flag on latch write

[Finn Thain]

On Wed, 25 Aug 2021, Mark Cave-Ayland wrote:

> On 24/08/2021 11:09, Finn Thain wrote:
> 
> > The Synertek datasheet says, "A write to T1L-H loads an 8-bit count value
> > into the latch. A read of T1L-H transfers the contents of the latch to
> > the data bus. Neither operation has an affect [sic] on the interrupt
> > flag."
> > 
> > Signed-off-by: Finn Thain <fthain@linux-m68k.org>
> > ---
> >   hw/misc/mos6522.c | 1 -
> >   1 file changed, 1 deletion(-)
> > 
> > diff --git a/hw/misc/mos6522.c b/hw/misc/mos6522.c
> > index c0d6bee4cc..ffff8991f4 100644
> > --- a/hw/misc/mos6522.c
> > +++ b/hw/misc/mos6522.c
> > @@ -313,7 +313,6 @@ void mos6522_write(void *opaque, hwaddr addr, uint64_t
> > val, unsigned size)
> >           break;
> >       case VIA_REG_T1LH:
> >           s->timers[0].latch = (s->timers[0].latch & 0xff) | (val << 8);
> > -        s->ifr &= ~T1_INT;
> >           break;
> >       case VIA_REG_T2CL:
> >           s->timers[1].latch = (s->timers[1].latch & 0xff00) | val;
> 
> Hmmm. The reference document I used for QEMU's 6522 device is at
> http://archive.6502.org/datasheets/mos_6522_preliminary_nov_1977.pdf and
> according to page 6 and the section "Writing the Timer 1 Registers" writing to
> the high byte of the latch does indeed clear the T1 interrupt flag.
> 
> Side note: there is reference in Gary Davidian's excellent CHM video that
> 6522s obtained from different manufacturers had different behaviours, and
> there are also web pages mentioning that 6522s integrated as part of other
> silicon e.g. IOSB/CUDA also had their own bugs... :/
> 

The MOS document you've cited is much older than the Synertek and Rockwell 
devices. The datasheets for the Synertek and Rockwell parts disagree with 
MOS about T1LH behaviour. Apple certainly used SY6522 devices in my Mac II 
and I'd assumed Apple would have used compatible logic cores in the custom 
ICs found in later models. But I don't really trust assumptions and 
datasheets so I wrote the Linux patch below and ran it on my Quadra 630.

diff --git a/arch/m68k/mac/via.c b/arch/m68k/mac/via.c
index 3d11d6219cdd..ed41f6ae2bf2 100644
--- a/arch/m68k/mac/via.c
+++ b/arch/m68k/mac/via.c
@@ -634,3 +634,27 @@ static u64 mac_read_clk(struct clocksource *cs)
 
        return ticks;
 }
+
+static int baz(void)
+{
+       u8 a, b, c;
+
+       local_irq_disable();
+
+       while (!(via1[vIFR] & VIA_TIMER_1_INT))
+               continue;
+       a = via1[vIFR] & VIA_TIMER_1_INT;
+       via1[vT1LH] = via1[vT1LH];
+       b = via1[vIFR] & VIA_TIMER_1_INT;
+       via1[vT1LL] = via1[vT1LL];
+       c = via1[vIFR] & VIA_TIMER_1_INT;
+
+       printk("a == %2x\n", a);
+       printk("b == %2x\n", b);
+       printk("c == %2x\n", c);
+
+       local_irq_enable();
+
+       return 0;
+}
+late_initcall(baz);

Based on the Synertek datasheet* one would expect to see b equal to a but 
I got this result instead:

[   10.450000] a == 40
[   10.450000] b ==  0
[   10.450000] c ==  0

This amounts to a MOS design flaw and I doubt that this result from my 
Quadra 630 would apply to other Mac models. So it would be great to see 
the output from a Quadra 800. But until then, let's disregard this patch.

* http://archive.6502.org/datasheets/synertek_sy6522.pdf