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

[Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>]

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... :/


ATB,

Mark.