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Re: Crosspost: Did the cpu influence the display?

Linards Ticmanis wrote:

Alex Freed wrote:
I don't agree. No matter what the screen memory layout is, the video circuit will refresh all the DRAM as long as 128 consecutive addresses are accessed (or all 128 combinations of low 7 address bits are used). 


But this has to happen within a certain time frame, right?


As Alex points out, a linear mapping would do this as well.

His point is well taken--the Apple II display mapping is motivated
by the desire to display 40 characters/280 pixels per line while
leaving as few/small holes as possible and using as little logic
as possible.  DRAM refresh is not a primary consideration, since
its requirements are easily met by many mappings.

Nevertheless, the "alternate cycle free" 6502 memory usage,
coupled with a simple "DMA" video generator, negates any perceived
advantage of the Z-80 in a DRAM-based personal computer.

-michael

Music synthesis for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

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