Following on from this question about Apple ][ graphics:
What makes slot 7 of the Apple ][ unique from 1-6 and how was this used?

The original Apple ][ microcomputer was released for the US market and provided analogue video output in the NTSC format.

ITT manufactured under licence, a variant of the Apple ][, called the ITT2020. This was marketed in Europe with a modified video output using the European PAL standard.

What impact did this have on the ability to run ‘standard’ Apple software?

up vote 5 down vote accepted

The ITT2020 was sold in Europe from 1979 with the PAL format video display. It was withdrawn from sale once Apple launched the Apple ][ Europlus a couple of years later.

The higher scan rate of a PAL display meant that the Hi-Res graphics resolution was increased from the Apple’s 280 to the 2020’s 360 pixels per line. Vertical resolution was kept the same. This change to the aspect ratio meant that drawn circles were no longer circular.

In order to meet the increased video frequency, the number of bits provided to the video shift register was increased to 9 per memory location. This in turn required an extra RAM chip on the motherboard. The Hi-Res memory pages $2000 to $5FFF were thus 9-bit bytes.

Access to this altered memory layout required changes to the Applesoft Basic Interpreter. The ITT2020 had PALSOFT.

  • The graphics routines were larger, as code was included to manipulate the 9th bit which meant that something had to be removed due to the fixed size of the ROM.

  • The Apple Basic HPLOT command was reduced in scope. The PALSOFT HPLOT would only accept a single parameter.

  • Attempts to read and write directly to video memory, for example to manipulate sprites, failed as they would move 8 bytes per memory location, not 9.

  • Another consequence of the ROM changes was that entry points of some ROM routines changed. Software that made direct ROM calls, a common trick in microcomputers of that era, would fail as the call went to the wrong location.

The upshot of this is that several software packages written for the Apple ][ would not run on the ITT2020. Some that did run had unintended graphical anomalies.

  • How can you read/write 9 bits per location on an 8-bit system? – Wilson Apr 5 at 8:34
  • The 9th bit was stored in the additional memory chip - at a different memory address. – Chenmunka Apr 5 at 9:14

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