IBM 5100 tinkering

From: J.C. Wren <jcwren_at_jcwren.com>
Date: Sun Jul 13 19:58:00 2003

        I'm not sure what video controllers the PETs used, but Motorola 6845 type
controllers had a register that set the starting scan address of the video
memory. So scrolling was as easy as changing this pointer.

        IBMs have always scrolled by doing screen copies. I don't know if you
remember Xenix, but Xenix used to scream on a Hercules graphics card. That
was because they used a similiar technique, and weren't limited to the BIOS
implementation.

        I wonder if the difference in speed you are seeing is a result of this type
of comparison?

        --John

On Saturday 12 July 2003 10:49 am, Hans Pufal wrote:
> We did some tinkering with our IBM 5100 the other day with results which
> might be of interest.
>
> Our 5100 is a model A4; it has 64Kbytes of memory and APL (only).
>
> When we opened up the beastie for a service a year or so ago and
> speculated on the fact that there were no empty slots in the card cage
> (we do seem to have every built-in option made) which led us to propose
> that perhaps the BASIC interpreter was also included but made
> unavailable by the absence of the language selector switch on the front
> panel.
>
> Examining the cable harness behind the panel showed a particularly thick
> section encased in insulation tape just behind where the language select
> switch would be installed and tempting evidence of one, possibly two
> unconnected wires.
>
> At that time we went no further.
>
> This week we were prompted to revisit the 5100 with a view of seeing if
> we could in fact run BASIC. Carfully cutting open the insulating tape
> around the cable harness behind the language select switch position
> revealed two unconnected wires complete with tiny push on connectors.
>
> We jumpered the two wires, powered on the machine and waited with bated
> breath. Following the familiar self-test display we were presented with
> an unfamiliar prompt, not the APL one at all. A little fiddling with the
> keyboard quickly confirmed that we had BASIC operational!
>
> So at least the APL only versions of the 5100 are configured with the
> BASIC interpreter which is disabled simply by the absence of the
> language select switch. Our IBM engineer speculates that the reverse is
> not the case : the BASIC only configuration does not include the APL ROM
> code. Anyone care to experiment and confirm that?
>
> We are now installing a language select switch (though alas not of the
> same type as the others) and look forward to playing with BASIC on the
> machine.
>
> Does anyone know the price difference between the A4 and C4 models? I
> would hazard a guess that that switch cost several hundered dollars at
> least ;-)
>
> Oh, what prompted us to revist the 5100? We recently acquired a
> Commodore PET 2001 and on compiling some techinal notes for it found a
> web site stating that the PET was 2 to 3 times faster than the 5100.
> That statemnet surprised me and I wanted to confirm it. Early
> indication show that the two machines are remarkably alike in
> performance (another suprise) the following program runs in the same
> time (to the second) on both machines :
>
> 10 a = 0
> 20 print a, a*a, sqr(a)
> 30 a = a + 1
> 40 if a < 100 goto 20
> 50 stop
>
> The 5100 seems about 30% faster on though with the following program:
>
> 10 a = 0
> 20 b = a*a
> 25 c = sqr(a)
> 30 a = a + 1
> 40 if a < 100 goto 20
> 50 stop
>
> So it seems it is the display which slows it down.
>
> Regards,
>
> -- hbp
Received on Sun Jul 13 2003 - 19:58:00 BST