Parallel Computer

Almost all computers today use only one processor limiting their operation to one instruction at a time. Parallel computers combine many processors in a single computer to greatly increase the throughput of instructions.

Crossroads is a roleplaying game set about 300 years in the future and computers for an important part of society. It is impossible to know how computers will work in the future but we can look at current technology and make a few plausible guesses.

1. Processors will have reach a fundamental speed limit dictated by physics.
2. Faster computers will only be achievable by using multiple processors.
3. Transistors and circuits will become ever smaller to the extent that minor impacts and background radiation has a noticeable affect on processors.
4. Processor failure will become an everyday occurrence and computers will have to compensate for such failures.

This assessment may be wrong but it introduces the idea of massively-parallel fault-tolerant computers.

Computer Architecture

Processors in the computer are arranged in a 2D grid which easier to implement than a 3D grid. Each processor is connected only to its four nearest neighbours by power and communications lines. As fault tolerance is a high priority individual processors are independent. To this end processor communication is limited to a post box system where messages are deposited in a small buffer between a pair of processors and will stay there until read. This means that each processor must poll its post boxes every so often to read its mail.

Instruction Set

To make for easy simulation and design of the computer a RISC architecture was chosen. An instruction would either load a byte from memory, store a byte to memory, or perform an operation on a register. A 16-bit instruction set was eventually chosen and all memory locations hold 16 bits rather than the more usual 8 bits found in other computers. Two main addressing modes are used - immediate mode and source conditional mode - along with a hybrid - immediate conditional mode. Immediate mode instructions operate on one register and one numerical value, source conditional mode instructions operate on two registers. Conditional mode instructions may or may not be executed depending on the state of the control register.