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The ITT 2020 was an Apple II computer clone manufactured by ITT under license from Apple Computer (the first licensed clone), specifically for the European market. In the Benelux, it was distributed by International Bell Telephone Company. It was distributed in the United Kingdom by Microsense Computer Limited.
The major difference, and the reason ITT believed this personal computer would be a success, was that the color video signal conformed to the European PAL standard, rather than the American NTSC standard. This meant color graphics could be viewed using a standard European monitor or TV set, rather than having to import an NTSC monitor from America or Japan as was the case for the Apple II. ITT sold this computer for a few years, starting in 1979. When Apple Computer started shipping the Apple II Europlus, ITT withdrew from the market, although the Europlus did not support color.
ITT also supplied a 113.75 kB single-sided floppy disk drive that was identical to the Apple II disk drive, supporting 13 sectors under DOS 3.2 on a 5+1⁄4-inch (133 mm) floppy disk. They never supplied the upgraded 140 kB single-sided, double-density drive supporting 16 sectors under DOS 3.3.
Source: [1]
ONERR GOTO
routine in the Apple II that required users to BLOAD a patch, was resolved in the ITT 2020.XDRAW
routine, causing strange shapes to appear under certain circumstances.CATALOG
listing showed Basic programs as type 'P' (for PALSoft) rather than type 'A' (for Applesoft)Any Apple II program using graphics had problems on the ITT 2020. Only programs written specifically for the ITT 2020 could make proper use of the Hi-res graphics, with horizontal co-ordinates of up to 359, rather than the 279 maximum of the Apple II
HPLOT
command with multiple parameters did not work properly.ONERR GOTO
bug would crash if an error occurred, as this bug was already fixed.BSAVE
and BLOAD
of graphics images did not work, as the 9th bit was not included in the file.ITT eventually released a technical note with an assembly language program that allowed users to load and save hi-res images.
The most significant difference between the ITT 2020 and the Apple II is the Hi-res graphics resolution. ITT increased the horizontal resolution from the 280 pixels the Apple II used, to 360 pixels. The vertical resolution however, remained the same. This made many Apple II programs incompatible with this computer.
The higher resolution was a necessary consequence of the higher frequency of the PAL color subcarrier. In order to provide enough bits to the video shift register to generate the higher PAL subcarrier frequency, 9 bits per memory location were needed, rather than the 7 bits the Apple II used. To achieve this, an extra 16Kx1 memory chip was added to the motherboard, which added a 9th bit to the Hires memory pages ($2000 to $5FFF). This made it necessary to modify the graphics routines in Apple's Applesoft BASIC interpreter in ROM (product 341-0021 to 341-0025). To emphasize this difference, ITT called this "PALSOFT".
To make room for the longer graphics routines (manipulating the 9th bit required extra code), the HPLOT
instruction was limited to a single parameter, rather than a string of parameters.
The extra RAM chip was basically a write-only memory: data was written to the chip by software, but read out by the video generation hardware. The data input of the chip was wired to the data input of the 8th bit (the MSB) of the main video memory and was therefore equal to this 8th bit during a write operation. By enabling the extra chip, the same data would be saved in the 9th bit as in the 8th bit.
For example, in order to set the word of memory location HiresLoc to 0b110100101, the following routine is necessary:
LDA$C05E; enable the extra RAM chipLDA#$80; set MSB to 1STAHiresLoc; save data to both bit 8 and bit 9LDA$C05F; disable extra RAM chipLDA#11010010; load the remainder of the dataSTAHiresLoc; save to main memory, but 9th bit remains set.
To change only the 9th bit requires even more programming:
LDAHiresLoc; read bits 1-8PHA; store on stackLDA$C05E; enable the extra RAM chipLDA#0; bit 9 must become 0STAHiresLoc; but bits 1-8 are also changedLDA$C05F; so disable extra RAM chipPLA; retrieve data from stackSTAHiresLoc; and put the old value back in 1-8
It is possible to read the 9th bit when this is necessary, for instance, to save a Hires image to disk.
This is achieved as follows:
LDAHiresLoc; read addressLDA$C063; MSB of this address is value of bit 9 of last address read inBPLnotset; branch if bit is zero
The bitmap graphics compatibility with the Apple II could be improved by installing a third-party Hires conversion board.
When enabled, this board removes the video artifacts caused by programs ignoring the 9th bit. This significantly improves compatibility with software intended for the original Apple II manipulating graphics memory directly. However, even with this board enabled the colors still differ from the original Apple II.
When using PALSOFT BASIC commands for drawing Hi-res graphics, the Hires conversion board should be disabled to see the graphics as intended.
Applesoft BASIC is a dialect of Microsoft BASIC, developed by Marc McDonald and Ric Weiland, supplied with Apple II computers. It supersedes Integer BASIC and is the BASIC in ROM in all Apple II series computers after the original Apple II model. It is also referred to as FP BASIC because of the Apple DOS command FP
used to invoke it, instead of INT
for Integer BASIC.
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