XPost: comp.sys.sinclair, comp.sys.atari.8bit, comp.sys.apple2   
   XPost: comp.sys.cbm   
   From: mjmahon@aol.com   
      
   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/   
      
   "The wastebasket is our most important design   
   tool--and it is seriously underused."   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)