eec20afe   
   From: joe@user.com   
      
   "Jim Leonard"  wrote in message   
   news:cd42bebe-349a-497f-b02c-9b1f3ba85390@a70g2000hsh.googlegroups.com...   
      
   > Are you saying that some Tandy 1000 models did not have DMA?  If so,   
   > that is news to me; I was under the impression all Tandy 1000s had   
   > DMA.   
      
   The original 1000 had no DMA until you added the first memory card.  As   
   did the HX and I believe the EX, but I'm not certain about the last one.   
      
   There were two different 256K memory cards.  Different in that one had   
   a DMA chip on it, the other didn't.  When you added memory, the first   
   card had to include the DMA chip.  The PCBs for the cards were the same,   
   just didn't stuff some parts.   
      
   The BIOS on the 1000 assumed DMA if it found more than 128K of   
   memory, meaning that if you blew the DMA, the system wouldn't boot   
   with the memory card.  Everything looked fine, just the floppy wouldn't   
   work.  This was a common problem in that there was no 'load' on the   
   -12v line on the PC bus.  As a result if a user turned off the computer and   
   then plugged in or removed the memory card, if it 'cocked slightly' it would   
   blow one of the TTL chips on the main board that routed the DMA signal.   
   The -12v line on the connectors would hold -12v in a dangerous state for   
   MANY minutes after power off if there was nothing attached to load it down.   
      
   A common fix was to just 'piggy back' another chip on the blown chip, since   
   the blown chip had an open output and the others were in parallel.  I seem   
   to   
   recall a 74LS16, but I may be mistaken on the chip number.  Oh for the days   
   of discrete logic.   
      
   Anyway, with a 128K the RAM was mapped at 00000h or the bottom of   
   memory.  The video RAM was in the first 128K, along with all the   
   other use of RAM.  When you added the first memory card, either 128K or   
   256K, the card RAM would start at address 00000h and the onboard RAM   
   would configure to 'bump up' to sit on top of it.  The video use was   
   'hardcoded' and that didn't change.  This 'offset' that you programmed   
   into the onboard memory made the memory design pretty simple actually.   
      
   When you added 512K of RAM in cards (the most Tandy supported at the   
   time), then your onboard memory sat at 80000H or the top memory just   
   below 640K.  Until I think it was the TX or maybe it was the TL machines   
   came along.  Then you could add 640K and the 'video' memory onboard   
   essentially was video only.   
      
   Mike   
      
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