The Spikeputor will be a fully functional computer with a CPU made exclusively from NMOS logic using about 5,000 MOSFETs (2N7000), resistors, and LEDs to visualize the logic. HP-5082-7340 Hex Display chips will be strategically added to display the numeric contents of the outputs of the major CPU components, although if you read binary, the LEDs will provide the same information. Electroluminescent wire will be added to visualize the logic pathways between the major components. The clock will be adjustable from maximum speed (predicted to be in the tens of thousands of hertz range) down to single step (design TBD). Since the primary driver of the Spikeputor is to visualize computation, speed was not a concern, although some steps will be taken (a clock tree, e.g.) to enhance the overall performance.
The initial CPU design was inspired by the Beta CPU, introduced in the MITx course, "Computation Structures" (highly recommended!) The Beta has a load-store, 32-bit RISC architecture. To save space, transistors, and the creator's sanity, the Spikeputor CPU was reduced to 16 bits. This necessitated several other major design changes, mainly centering on the fact that 16-bit opcodes could no longer support the inclusion of constants within the opcode.
Spikeputor CPU Design Features and Major Components:
- 16-Bit Address space
- Register Memory: Seven 16-bit registers, plus one register hard-coded to zero.
- Multi-function ALU
- Addition and Subtraction, supporting 2s Complement negative numbers
- Comparison (Equal, Less Than, Less Than or Equal)
- Boolean Logic (AND, OR, XOR, Identity)
- Variable Bitwise Shift Right or Left with or without Sign Bit Propagation
- Control Logic implemented with simple logic gates (no microcode or ROMs)
- Additional registers for Program Counter, Instruction, Constant, CPU Phase, and a few required status flags
- One-word opcodes for operations between registers
- Two-Word opcodes for operations between registers and 16-bit constants
- ALU Functions (see above)
- Memory functions: Load, Load Relative to PC, and Store
- Conditional (BEQ, BNE) and non-conditional branching with branch point storage
- IRQ and RESET handling (including automatically using register R6 as the Exception Pointer)
- Direct Memory Access (DMA) for all I/O
To be able to execute more than just trivial programs, the Spikeputor CPU will interface with high speed static memory chips (Fujitsu MB81C67 or similar). In addition, a mirrored write-only "screen memory," also made from (roughly 5,000) transistors, will be created, providing a 48 by 18 array of addressable LEDs. ROM may be added in the future to provide for a low-level operating system. Until then, the I/O controller (see below) will do the heavy lifting.
All I/O functions will be facilitated via custom software and hardware running on an Apple II Plus computer, vintage 1986. A custom peripheral card called BIAS (Board to Interface Apple to Spikeputor) will be designed as part of the project, as well as the software on the Apple to provide keyboard input, screen output, and long-term data storage and retrieval. The I/O controller will have direct access to the Spikeputor memory and will halt the Spikeputor CPU during all read and write operations.
The Spikeputor will be assembled on a series of mounted solderless breadboards. Each major component (ALU, Register Memory, Control Logic/Program Counter/Status Registers, and Screen Memory) will be laid out on a 4 foot by 6 foot pegboard. Each pegboard can contain a nine by six array (54) of breadbaords with some space left over for some smaller circuits as required. Each of the four pegboards will then be wall-mounted, with space below the installation for a desk with the Apple II Plus.
The power requirements, which will be discussed in a bit more detail in the project logs, will be somewhere on the order of 25 watts. The Spikeputor will run on +5V power, conveniently...Read more »