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Okay I think Dandelion BASIC is starting to come together.
The ROM memory locations had all been mapped out some time ago, but I had been confused about how the GAS assembly directives mapped RAM.
This has finally fallen into place, and the RAM is now completely mapped out by hand from the top of SRAM @ 0x0200 to 0x05FF.
So, even including the registers, we are not using very much RAM at all.
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There is something very very powerful about assembly language. You can do quite a bit of computation with teeny tiny amounts of memory.
If you plan with care, you can memorise where every part of the program is in the machine's memory.
This Arduino mega I am targeting has 8KB of SRAM, so with all the RAM segment carefully defined, we are only really using 1KB of memory.
Get ready to see quite a convoluted project some to life.
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This particular implementation of Tiny BASIC is Tiny BASIC Extended, otherwise known as TBX.
It was originally implemented for the Intel 8080, and has, rather excitingly, FOR NEXT loops, and 2 dimensional DIM arrays.
It does so with a virtual machine, which is what is actually running BASIC. All the assembly code does is make up the instructions that comprise the virtual machine.
Rather crazily, you can assign registers to the Arduino in a way that simulates an 8080 CPU.
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So what has actually happened with this project is that the Arduino simulates an Intel 8080, which in turn runs a virtual machine.
Rather convoluted, but in the end we will end up with a powerful Tiny BASIC.
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The benefit of all this is that it may be possible to cram the interpreter into a small enough memory footprint to squeeze in the famous TVOut arduino library alongside it.
This will allow a true computer in a chip, and will keep costs down as the project gets released.
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