On Wed, 26 Apr 2000, Daniel Jorge Caetano wrote:
> >For MSX this means that IM2 is not available. Neither is IM0, but that
> >was unused anyway.
>
> There is no problem, I think. MSX standard is about IM1, right?
I'm not sure what the standard says. But there are IM2 program on MSX. The
question is how many and whether they can be modified to use IM1 instead.
> >Another incompatiblity with Z80:
> >"The Z80 has some undocumented instructions, such as being able to load the
> >upper or lower 8 bits of the index register (IY and IX) independently.
> >This was used by a handful of programs, especially some "copy protected"
> >code. The AB180-20 does not support all of the undocumented Z80
> >instructions."
> >But Z180 doesn't have IXH/IXL instructions either, right?
>
> Yes, but Z180 has trap... the AB180 has trap feature?
It has. But a trap only works if the opcode is unused. So if they made a
new instruction with that same opcode, trap won't work. There is a complete
instruction map in the PDF file on their site, so someone could check
whether the opcodes are free or not.
> >This may be a serious one:
> >"The on-chip register addresses are located in the I/O address space from
> >0000H to 00FFH (16-bit I/O addresses). In order to access the on-chip I/O
> >registers (using I/O instruction), the high-order 8 bits of the 16-bit I/O
> >address must be 0."
> >This means the MSX I/O ports will be inaccessable if the high-order 8 bits
> >happen to be zero. Those high-order bits come from one of the registers,
> >right? I thought it was B, but Laurens said A. Anyway, the value of that
> >register is unknown in MSX programs.
>
> I think this can be solved by hardware (add something to "trash" the upper
> 8 bits always they are 0x00... (-;
That's not what I'm worried about.
The problem is, if you do an "OUT (#20),A" and the upper bits are zero, the
hardware outside of the processor will never see the OUT, because it is
sent to the internal I/O ports. Also, writing the wrong values to the
internal I/O ports may crash your system.
> >By the way, the DMA on this processor looks real cool. It can also perform
> >DMA from memory to I/O and vice versa. It could be used for example to
> >upload samples to the MoonSound or Music Module.
>
> I'm waiting Ademir's answer. I talk to him by phone and he liked a lot
> of the 20MIPS at 20Mhz idea... (-;
Note that 20MIPS is the best case: it will only be reached if you use only
1-byte instructions. But even with longer instructions this processor is
likely to be quite fast.
Bye,
Maarten
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