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Large Software Characters
W D Shaw
10 April 1975
FR80 Technical Paper 3
FR80 hardware characters have a maximum height of 277 raster units, inadequately small for some graphics users. To draw larger characters they must be simulated using vectors, produced either by each host software package or oy the FR80 itself. It has been decided to modify the FR80 Displayer to simulate characters larger than 277 raster units. The difficulty with this method is that source of the FR80 Displayer is not available. All is not lost however; FRAH has spent most of his evenings recently decoding the FR80 binary. The character routines are now thought to be understood.
2. LARGE CHARACTER OUTPUT BY THE FR80 DISPLAYER
Large software characters will be simulated in the FR80 Displayer by a patch at the point at which it issues the character IOT, HSCP (Plot Character). Proper character and line spacing will be output without patching. Control returns to the FRSO character routines as if an HSCP had been executed.
3. ADDITIONS TO STANDARD DATA FORMAT TO IMPLEMENT LARGE CHARACTERS
The format or Standard Data commands Set Character Spacing, Set Text Line Spacing and Select Character Height will be modified to accept spacing and height values in the range 0 to 16383. As only 9 bits are currently available in these command words, anan optional second word will be added. If a value of 511 is entered in the first command word, the second command word will contain the 16-bit spacing or height.
What follows is a description of how each command is changed so that existing usage will continue but now with the option to output much larger characters by software.
All commands currently included in the Standard Data Format will continue to work after the proposed modifications to the FR80 Displayer, including Set Character Rotation and Set Character Size. The latter command should not be used by the host packages; Set Character Height should be used instead. The single word version of the command should be used where possible. Note that hardware characters will not be output when character height is greater than 277 raster units.