caeb1f42   
   From: tlake@twcny.rr.com   
      
   "Jim Leonard"  wrote in message   
   news:5ab8eb31-963e-41e0-ad39-556215d8daf8@y21g2000hsf.googlegroups.com...   
   > On Sep 9, 9:12 am, "Tom Lake"  wrote:   
   >> The jr had its video memory at a fixed location no matter how much RAM   
   >> was installed.   
   >> That's why you needed all those fixes when adding memory past the 128K   
   >> mark.   
   >> The 1000 moved video memory based on RAM size and so could use it all   
   >> with no tricks.   
   >   
   > So I could, if I wanted, use *more* than 128KB RAM for video pages on   
   > a 640K Tandy 1000? Holy crap.   
      
   The location was variable.  The size was, too, up to a point.  I don't   
   remember   
   being able to expand it beyond 128K.  (Then again, I don't remember what I   
   had for breakfast this morning!)   
   >   
   >> In the original 1000s IRQ5 was for disk support, same as jr.   
   >   
   > That's incorrect; on the PCjr, IRQ 5 is triggered when vertical   
   > retrace occurs.  I just confirmed this in the PCjr tech ref last   
   > night, but I haven't attempted to verify it through programming it   
   > yet.   
      
   You're right.  IRQ5 was used for the hard disk on the XT and later   
   models of the 1000 (optionally)   
   Vertical retrace could be tested for by a loop which reads bit 3 of I/O   
   port 03DAh or by using the vector at 34h.  I wish they had   
   included horizontal trace as well as the Color Graphics Adapter did   
   on the PC.   
      
   Tom Lake   
      
   --- SoupGate-Win32 v1.05   
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