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The Vectrex OS and its functions

There is no 'official' ROM listing, and therefor there are no official function names. I learned the inner workings of the vectrex ROM from a documented dissassembled ROM. As far as I know there are two such dissassembled listings, one done by Fred Taft and one by Bruce Tomlin (based on Freds work). Throughout this document I use the function names given in Bruce's version.

Many thanks for the nice job you both have done.

The OS is located at memory address $F000 to $FFFF (well nearly, the last few bytes are reserved vectors of the 6809 CPU). Most functions require parameters, some functions return some values. This information is most of the time passed via registers of the 6809 microproccessor. There is no 'standard' in what registers are used. Look at the functions for the information which registers are used. As you will know the 6809 CPU has a register called DP (direct page register), this register is used to point to 256byte pages within the memory. The 6809 can address 256*256byte pages, dp therefor can have values from 0-255. The dp register if used wisely can shorten and speedup programs, because direct page accesses are faster and shorter than extended addressing. There are two values that you will encounter frequently while programming vectrex, as $c8 and $d0. You can see, eyeing the memory map above, these are direct page pointers to 1. ($c8) RAM used by the BIOS (or the programmer) and 2. ($d0) the base address of the VIA 6522 (VIA Vercatile Interface Adapter, sometimes called PIA Periphal Interface Adapter). All output and timer functionality are regulated/accessed by that chip, it is pretty important :-). For a detailed describtion of that chip see Appendix C. Actually input is also done via VIA, but indirectly, by accessing the AY-3-8912 soundchip, see Appendix D for a detailed describtion of that chip.

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