Issue 7.5: Exploring low-level computing concepts with Ruby

The article shows how to build a small 6502 simulator in JRuby that is just complete enough to run Snake6502. It uses the game as a guide to explain machine code, disassembly, memory as a shared store for code and data, and the role of the program counter and stack in control flow.

It then adds memory-mapped I/O for random numbers, keyboard input, and pixel output, and introduces a CPU model with registers and flags. From there, it explains addressing modes, config-driven opcode lookup, and a simulator loop that reads bytes from a ROM and executes the matching instruction definitions.

Reading notes

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• The issue is directly inspired by Nick Morgan’s Easy 6502 tutorial, and the Snake6502 bytecode is used as the concrete example throughout.
• Machine code is presented as a stream of bytes that can be disassembled into assembly instructions such as JSR, LDA, STA, BEQ, and RTS.
• Addressing modes are treated as different ways of turning an operand into an effective address, so instruction behavior can be defined after the address is computed.
• The storage model uses a 16-bit address space with 8-bit values, a program counter starting at $0600, and stack storage between $0100 and $01ff.
• Jump, branch, jsr, and rts behavior are built on top of the program counter and stack.
• Memory-mapped I/O handles reads from $fe as random bytes, reads from $ff as the last key pressed, and writes to $0200-$05ff as screen pixels.
• The CPU model tracks A, X, Y, and P, with carry, negative, and zero flags implemented for the Snake6502 game.
• CMP and BEQ are explained as a combination of flag setting and conditional branching.
• The article lists the addressing modes used by Snake6502, including implicit, relative, immediate, zero page, zero page indexed by X, absolute, indexed indirect, and indirect indexed.
• A configuration object loads opcode mappings from CSV and instruction definitions from a DSL file, and the simulator executes operations by looking up the opcode, computing the effective address, and evaluating the matching definition.