Boot without PIA?

[RodCastler]

Hi you architecture wizards,

 

I received a few 800XLs that had their PIAs cannibalized.

 

I've taken a quick look at the sequence diagrams in "de Re Atari" but I don't see a lot going on with the PIA.

 

While I read harder:

 

- Will an XL boot without the PIA chip installed? why/why not?

- How does the OS check the presence of supporting chips such as POKEY AND PIA at boot time?

- Can an atari be tricked to believe there's a PIA installed at boot sequence? (if I ever want to have it boot and then operate without SIO, Joystick or PIA related activities).

 

 

This is all out of curiosity as I will somehow procure the missing PIAs. I still want to learn.

 

 

Thanks

[Van]

No the PIA's port B is used for memory management and the CA and CB bits are used for the SIO. Mouser has a modern replacement, the W65C21, for ~$6.00. The N model is a backwards drop-in replacement.

Yogi

[Rybags]

PIA provides the /COMMAND line which means SIO operations would be impossible. Cassette IO would actually be "almost" possible but the PIA provides motor control which by default would probably be off.

 

The PORTB lines - the default state of those should be ones (they use external pullups) - I think that state should persist just fine without the actual PIA in place.

The architecture of the machine is such that the default state provides us with OS in, Basic out, SelfTest out, Ram expansion >64K disabled from POV of both CPU and Antic.

 

My theory is that a machine without PIA should startup just fine but with no controller ports usable aside from paddles but not buttons and necessary to use a cartridge to provide Basic if required.

How the machine operates though - the IRQ handler checks PIA bits so it's hard to say what e.g. a BRK would do. Normally it's the last priority of IRQs but absence of a PIA might mean the IRQ status addresses return false positives.

[HiassofT]

The XL OS switches in the selftest ROM during power-up so it needs a working PIA.

 

so long,

 

Hias

[Marius]

No the PIA's port B is used for memory management and the CA and CB bits are used for the SIO. Mouser has a modern replacement, the W65C21, for ~$6.00. The N model is a backwards drop-in replacement.

Yogi

 

I even have a few XE computers which had some kind of '6821P' installed from factory...

[Van]

 

I even have a few XE computers which had some kind of '6821P' installed from factory...

 

As I understand, the 6521 (even the NOS versions) is a improved version of the 6520/6820 and is backwards compatible. The new Western Digital W65C21N is described as a drop-in replacement but has lower power consumption and higher speed, ~16MHz ( but that really doesn't come into play with the Atari ).

Good to know that they were used in the XE

Yogi