Z-80 code question about a loop that depends on the contents of the refresh register

[Ethan Dicks]

On Wed, Dec 14, 2016 at 2:08 AM, Alexis Kotlowy
<thrashbarg at kaput.homeunix.org> wrote:
> On 14/12/2016 09:19, Ethan Dicks wrote:
>> So far, this loop hangs on all three emulators I've tried - simh's
>> altairz80, simcpm010 for AmigaDOS, and EMUZ80 for Raspberry Pi...
>
> Have you tried running it on ZEMU? (Windows only unfortunately, but
> should run under WINE).
>
> http://www.z80.info/z80emu.htm#EMU_CPU_W32

Thanks for the tip, Alexis.  It did run under ZEMU under Wine on my
Mint laptop, but what an awful interface.  My next goal is finding
something less mousy like fixing altairz80.

> I tried single stepping through it just now and it looks like it's doing
> its job, at least as a possible random number generator.

Yep.  No tweaks.  It just ran as expected under ZEMU once I got the
files in there.

> I can't find any information on what the MSB is set to when the
> accumulator is loaded with R, and what the Sign flag is set to. The
> datasheet says the Sign and Zero flags are changed by the instruction.
> If either of these flags are set, the routine enters an infinite loop.

The upper bit of R can be set or cleared with LD R,A but won't change
state due to counter activity.  I checked the program - it only does
LD A,R to read the register.  It never sets it.

Thanks again for the tip.  Once I get a few more things checked out,
I'll be revealing what this is all about.  I think a few people may be
interested, or at least amused.

-ethan