QL Issue 5 A18 & A19

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Whopper
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QL Issue 5 A18 & A19

Postby Whopper » Fri Jan 05, 2018 3:20 pm

Sorry about this, but it's stupid question time.

Address lines A18 and A19. Are they used at all? Are they even connected?

A confused Whopper.


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tofro
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Re: QL Issue 5 A18 & A19

Postby tofro » Fri Jan 05, 2018 3:42 pm

Connected? Yes. To the CPU and the expansion port.
Used? No, not in an unexpanded QL. They are only needed for expansion cards like memory or I/O.

In case you wonder how the QL internal address decoding circuitry makes sure it's not shadowing the lower 256k of the memory map in upper address space:

It doesn't. At least not by technical means, but rather with two paragraphs in the manual (That's how you build cheap computers!) The QL relies on expansion cards being faster than its own internal address decoding and "grabbing" an address in the upper address range as soon as they see relevant addresses on the bus, then disabling the internal address decoding by asserting DSMC. That (mandatory) mechanism is described on pg. 64 of the Technical Guide

Tobias
Last edited by tofro on Fri Jan 05, 2018 3:48 pm, edited 2 times in total.


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Dave
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Re: QL Issue 5 A18 & A19

Postby Dave » Fri Jan 05, 2018 3:44 pm

A18 and A19 are pins 8a and 7a on the expansion connector. They are used if you have a memory expansion. Nothing internal to the QL uses them.


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Whopper
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Re: QL Issue 5 A18 & A19

Postby Whopper » Fri Jan 05, 2018 4:38 pm

Thank you both for this information.

Whopper


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