Commodore the Inside Story by David Pleasance

Commodore the Inside Story by David Pleasance

Author:David Pleasance
Language: eng
Format: epub
Publisher: Unicorn Publishing Group
Published: 2021-09-15T00:00:00+00:00


SYS 123, 45, 6

So I was asked to help out on a new project for a little while … another engineer, Jeff Porter, was working on a new 8-bit computer, but it was battery powered. Simply called the Commodore LCD, this was 1984’s answer to a laptop computer. It had a really nice display for the era as Commodore actually owned an LCD company in those days, and they were developing a very good suite of built-in applications – this was no Plus/4 fiasco. All seemed good with this project.

But before long, Bil called to say he needed help on another new system, which became the Commodore 128. This was going to be a first: an 8-bit computer with advanced features that would run C64 software and hardware.

I worked on timing analysis (in spreadsheets in the end) and on getting the system compatible with existing C64 hardware, trying out all kinds of add-ons, looking at what they did wrong, figuring out either how to fix their thing or, more likely, how to get the C128 to work with it. We actually had to make a few things as broken and bad-idea as they were on the C64, just to ensure compatibility.

And some of this was pure nutty, this compatibility thing. There was this idea that the C64 character set was a little ugly, so the software folks had put in a new font. It certainly looked better, but it also had a few unintended compatibility issues. We knew, for example, that the C64 as it existed was pretty unchangeable. When you put the C128 into C64 mode, it ran a real C64 ROM, for example, so programs that just randomly jumped into the C64 ROM would still work.

We loaded up this graphics painting software from Island Graphics, a pretty popular program on the C64. When it started up it presented a splash screen where it drew a nice island scene or something, then proceeded to write its name. The problem was that it was doing this by reading the character ROM and blowing up those characters to big graphics. Then it used its own painting routines to fill in colour, things like that, designed to show off a few features. But when it went to paint the dot in the ‘i’ from the new ROM it missed … and wound up painting the background. This erased a bunch of stuff, so each new drawing operation messed up, and it would take 20–25 minutes to get this splash screen to go.

Because of that, we had to keep the C64 character set. But, it turned out, a double-sized character ROM wasn’t much more expensive, so in every C128, there’s a double-sized character ROM with the top address line driven by the C128/C64 signal, a hardware line that identifies which mode the system is running. Additional compatibility issues had the Z80 processor actually boot up first, look for the C= (Commodore) key to be held or a few other issues, and decide if it should boot the system into C128 or C64 mode.



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