IBM PCjr

From: Captain Napalm <spc_at_armigeron.com>
Date: Tue Jun 24 17:32:46 1997

It was thus said that the Great Jeff Hellige once stated:
>
> On 24-Jun-97, A.R. Duell wrote:
>
> >The video circuitry in the PC-jr can output a few more modes that a CGA
> >card can't (320*200 in 16 colours, etc), but the monitor is a normal CGA
> >one.
>
> The PCjr is what Tandy used for the basis of thier video modes in the 1000
> series machines. I believe they were even seen by software as the same thing,
> though I could be wrong about this one.

  For the normal CGA modes, yes. But for the 320x200x16 and 640x200x4 no.
For some reason, when in those modes, they did not appear at the standard
CGA address in the IBM PCjr, which made programming for those modes on the
PCjr real fun (gotta love the way MS-DOS can't deal with non-contiguous
memory).

  In the PCjr, the video memory could reside anywhere in the first 128K of
RAM, but there was circuitry to echo that to the standard CGA addresses.
And due to the video memory sharing main memory, the PCjr wasn't as fast as
the PC (that, and due to lack of DMA).

  When programming those modes under MS-DOS, I had to boot a particular
setup where I knew the address of the video, which left me less memory to
work with MS-DOS, since MS-DOS was loaded PAST the video memory (which if I
recall, meant I lost something like 64 or 96K of RAM - oh, the video memory
couldn't reside just anywhere, there were alignment restrictions. Sigh).

  -spc (Funny that the Tandy's were more compatible than the PCjr ... )
Received on Tue Jun 24 1997 - 17:32:46 BST