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Diff in ram handling in 800XL vs 130XE


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I'm working on trying to understand the difference in how the 800XL and 130XE handled ram. I know that the 130XE uses banking to access the extra ram on board. But there seems to be a difference in how the base ram itself is accessed.

 

For instance on the 800XL pbi you have the ~CAS and ~RAS signals. Now this is the 800XL with at that time a max of 64k already installed. Why would you need to access more ram?

 

Consequently on the 130XE ECI those signals are missing.

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Ras and Cas are further down the chain though. It's at the MMU and EMMU stage where it's decided whether a memory access should go to Ram, Rom, IO or otherwise. Providing Ras/Cas at the PBI would have allowed less circuitry at the expansion peripheral end assuming it provided dynamic Ram.

 

Why missing on 130XE? Maybe they thought other things were more important or helpful. XE provides the D1xx signal which means less addr decoding work needed at the peripheral side. Possibly by the mid 80s they assumed expansions would be using Static Ram?

 

You'd probably need input from CTIRAD, but I think that aside from the different physical interface the 320XL and 320XE type expansions are practically the same. Essentially they're taking over for any Ram access although at the least the 320XL has the jumper option to allow normal non PortB banked Ram accesses to be from the default source.

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