Handcrafted Replica of Commodore 64, Instrumented in Turing's Style
A hobbyist named DrMattRegan has embarked on an ambitious project to build a Commodore 64 without relying on its original chipset, offering a challenge for retrocomputing enthusiasts seeking more than simple assembly. In lieu of the core 6502 microprocessor, DrMattRegan is emulating the CPU's functionality with EPROMs (Erasable Programmable Read-Only Memory) and SRAM (Synchronous Random Access Memory) chips, along with conventional logic gates.
As DrMattRegan explains, he stores the 6502 CPU instructions and their micro-operations as microcode in EPROMs. These EPROMs serve as a "rulebook," sequencing the internal steps of each instruction to effectively control the datapath and execution flow, allowing the emulated CPU to behave identically to its real-world counterpart.
The working memory elements, such as CPU registers, program counter, stack pointer, and other internal state bits, are held in SRAM chips acting as a "notebook." These SRAM chips store intermediate values manipulated during instruction execution.
The control logic is implemented as a Finite State Machine (FSM) constructed from TTL (Transistor-Transistor Logic) chips. This FSM orchestrates the microcode steps and data flow signals to and from the SRAM and other logic, recreating the fetch-decode-execute cycle of the CPU at a logic gate level.
Thus far, the emulated 6502 CPU has been built on a printed circuit board (PCB) and is capable of running the Apple II version of the iconic game Pac-Man as a benchmark. Enthusiasts eagerly anticipate more updates in this intriguing series.
For those interested in DrMattRegan's previous work, his VIC-less VIC-20 project may provide valuable insights. The development of this project was helped by a tip from [Clint].
[1]: Microcode implementation for emulating the 6502 CPU:- EPROMs store microcode for sequencing internal steps of each instruction.- Control flow and datapath are controlled by reading from EPROMs.- Emulates the behavior of a real 6502 in a fully discrete and transparent manner.
[2]: SRAM for data and address storage:- Holds CPU registers, program counter, stack pointer, and other internal state bits.- Stores intermediate values manipulated during instruction execution.
[3]: TTL Finite State Machine (FSM) for control logic:- Controls microcode steps and data flow signals.- Orchestrates fetch-decode-execute cycle at a logic gate level.
[4]: Authentic, scratch-built replica of the 6502's behavior:- Rebuilds the CPU at a lower abstraction level than the original chip.- Transparent and open for study or modification.
- Assorted: One
- Assorted: Two
- Turing 6502 Project
- DrMattRegan's emulated 6502 CPU for his Commodore 64 project uses EPROMs to store microcode that sequences the internal steps of each instruction, effectively mimicking the behavior of a real 6502 CPU.
- The working memory elements in DrMattRegan's project, including the CPU registers, program counter, stack pointer, and intermediate values, are held in SRAM chips for data and address storage, replicating the functions of these elements within the original 6502 CPU.
