Path: utzoo!attcan!utgpu!watmath!clyde!att!osu-cis!killer!igloo!andyross
From: andyross@igloo.UUCP (Andrew Rossmann)
Newsgroups: comp.sys.tandy
Subject: Re: Tandy 1000 graphics/sound in Turbo C/Pascal?
Message-ID: <1384@igloo.UUCP>
Date: 10 Jan 89 22:43:35 GMT
References: <1945@ogccse.ogc.edu> <1946@ogccse.ogc.edu> <1370@igloo.UUCP> <75@raider.MFEE.TN.US>
Reply-To: andyross@igloo.UUCP (Andrew Rossmann)
Organization: igloo, Northbrook, IL
Lines: 34

In article <75@raider.MFEE.TN.US> gdh@raider.MFEE.TN.US (Gordon Hull) writes:
>In article <1370@igloo.UUCP>, andyross@igloo.UUCP (Andrew Rossmann) writes:
>>   The easiest way to access the Tandy/PCjr graphic modes is to use the BIOS
>> (ugh!). First, be sure to release unnecessary memory, as the computer will
>> have to expand the RAM allocated to video from 16K to 32K. In Pascal, make
>> sure to specify a small Heap size, 0 if you don't need it at all, otherwise
>> TP will grab all of available memory.
>
>I didn't think that video memory had to be allocated; i.e. the extra 16k is
>already there somewhere.  Am I wrong on this?  It is still at $b800, isn't
>it (unless you specicfically set otherwise)?
>
>Gordon Hull

  On the 1000 series, references to the $B800 segment are actually
re-routed to the top 16K of memory. (About $9C00 in a 640K machine.) When
you set up the 320x200x16 or 640x200x4 graphics modes, that 16K must be
expanded to 32K. Also, references to $B800 will now access the top 32K of
RAM. (About $9800.)
  In reality (at least on my SX) there are 4 64Kx4 RAMs (128K total). Any
or all of this is available for video or normal use. These RAM chips are
ALWAYS the top 128K, whether you have 384K or 640K.  Normally, the top 16K
is set for video, and the other 112K is available for normal RAM.
Unfortunately, any access to this memory is subject to additional wait
states. This is because the chip can only be accessed 1 part at a time, and
waits are added to prevent video 'snow.'
  My Technical reference manual shows that there is the possibility of up
to 256K of RAM for video, but I think only 128K is possible on the SX. That
is why you can only allocate up to 128K of video RAM using Tandy BASIC's
CLEAR command. The newer L series may support larger video RAM (they have a
640x200x16 color mode, in addition to Hercules [if you have a true mono
monitor.])

andyross@igloo.UUCP, andyross@ddsw1.MCS.COM