Re: 400/800 OS in a XL

KMK <draco@xxxxxxxxxxxxxx> writes:

>> No. By writing all zeros, they remain as input.
>>
>> You are right about the coldstart issue, you can't get a warmstart by
>> pressing the reset button. And you obviously, cannot enable/disable basic
>> with the Option key. But they are minor problems. The OS unpatched should
>> work.
>
> PIA ports are programmed as output by default, i.e. after power up.

No, they are input in the power-up or reset state. This is described in
the datasheet, and it's the normal behavior for every I/O port chip that I
know. If the default state of any data port would be output, then it might
provoke all kind of damage including short circuit if the design is to be
an input.

> The
>OS zeroes I/O registers starting at minor addresses and proceeding
>towards higher addresses. So it first zeroes $D300, then $D301, and
>after that there's no next instruction to execute, because the XL
>hardware switched these addresses to RAM (bit 0 is cleared = no ROM).

This is completely harmless because (as explained above) the ports are
programmed as input. Furthermore, it is actually a NOP operation because
all PIA registers are zeroed automatically on reset.

>The XL OS does pretty the same thing, but first it zeroes the PBCTL
>register.

Again, PBCTL, as all other PIA registers are already zero after reset.

> The unpatched 800 OS does not have any chance to startup on XL hardware.

If you would have read the whole thread you would have see that the
original poster did run successfully the 800 os in XL hardware. He got the
memo pad. The reason that it wasnt working for him before is because he
put the OS in the wrong EPROM location.

> I don't see a reason why anyone could want to use that anyways.

To have a translator in hardware, that's the reason.

.

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