PROP-6502 Propeller laptop


Dennis Ferron developed this Parallax Propeller Laptop with a 6502 co-processor and 64K of static RAM as his entry to a Parallax Propeller design contest. He observed that hardware prototyping has evolved from building-block hardware (TTL) to programmable hardware (PLA’s and FPGA’s). He believes that the Propeller chip represents the next revolution: 100% software-based virtual hardware, and he built this laptop to prove it! While his design does not go so far as to prove that this computer can emulate every other 6502 computer (like the Commodore 64, Apple II, or even Nintendo NES), the possibility exists. This project earned David an Honorable Mention in the Parallax contest.

The hardware for this computer is minimal. The entire address space of the 6502 is devoted to a single 64K static RAM chip. The system includes no ROM in the 6502 memory map, no startup BIOS, nor any dedicated I/O at any address. How then does the program get loaded? How does it communicate with the outside world? That’s the key to why this system is so great: by not providing any dedicated addresses, the Propeller is free to define the memory map in any way imaginable!

But how does it actually work? The 6502 is connected to the static RAM chip on its address bus, and its data bus, but the control bus is not directly connected. All the control signals go through the Propeller. So, if the 6502 wants to read or write RAM, the Propeller reads that signal from the 6502 and controls the RAM appropriately. Thing is, the Propeller is under no obligation to do what the 6502 asks it to. Instead of turning on the RAM, the Propeller can say, “Hmmm… I think I’ll intercept this byte myself.” The software on the Propeller can then read or write the Propeller’s hub RAM, but set the data bus up so that the 6502 does not know anything odd happened. Since the Propeller can run objects simultaneously in other cogs, these bytes mapped to hub RAM could be used to map virtual hardware, such as emulating video memory, or the input buffer of a disk controller.

David asks, “What classic computer would you program this laptop to be?”

The PDF containing the full project description can be downloaded from Parallax.

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