7b3480b3   
   From: joe@user.com   
      
   "Jim Leonard"  wrote in message   
   news:ed82e6ae-270f-49c5-96b6-98d1a33e65ca@s19g2000vbp.googlegroups.com...   
      
   >On a PCjr with more than 128K, you run the jrmem driver (either IBM's   
   >or the excellent free replacment) and that starts DOS/program usage at   
   >the 128K mark, so while you can never get that memory back, you don't   
   >have to worry about clobbering it.  For a Tandy 1000 with more than   
   >128K, though, a quick search shows that I need to worry about whether   
   >or not my loaded program will stomp on allocated video memory and vice   
   >versa?   
      
   I'm not sure how the PCjr handles it but I can tell you how the 1000 does   
   it.   
      
   The 1000 starts memory at 00000.  The video is ALWAYS up at the video   
   window.  (B8000?  I'm having a brain freeze right now.  In any case, I'll   
   use   
   B8000 as my example)   
      
   Ok, you only have 128K of memory, from 00000 to 1FFFF.  What happens is   
   the machine tells you that you have 96K or whatever, depending on video   
   mode, and you only USE from 00000 to 17FFF for OS and program.  The   
   video is above that.  HOWEVER, you should NEVER access the video at   
   18000, always use the B8000 video window.   
      
   I should also point out that a 128K T1000 has no DMA.   
      
   When you add the first memory card (you also add DMA), it will be at 00000.   
   What happens is the BIOS will detect your added memory and configure the   
   video memory to sit 'on top' of it.  So, you add 128K from 00000 to 1FFFF.   
   The internal 'shared' memory now sits from 20000 to 3FFFF.  The 'video   
   access window' STILL sits at B8000, and your program has use of memory   
   from 00000 to 37FFF.   
      
   Add 256K of memory and the internal memory sits at 40000-5FFFF with the   
   top 32k from 58000 - 5FFFF for video, STILL accessed at B8000.   
   Add 512K of memory and the internal memory sits at 80000-9FFFF with the   
   top 32k from 98000 - 9FFFF for video, STILL accessed at B8000.   
      
   That's why T1000's always had a bit less memory available than what a   
   standard   
   PC would have.   
      
   On the very last T1000's, you could add a full 640K and if you did that, the   
   internal memory 'went away' except as video memory, and then you had a   
   full 640K for OS and programs. (Some people even hacked it to give 798K,   
   less the video used... )   
      
   Mike Yetsko   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)