Atari 2600 Emulator Basics

Emulating Atari 2600

• An emulator for the Atari 2600 is created by emulating its three main chips:
  • CPU: A 6502-variant handling program execution.
  • RIOT: ("RAM Input Output and Timers") chip for input/output functions.
  • TIA: (Television Interface Adapter) for graphics and sound output.
• Atari 2600 game code is stored in cartridges with a ROM chip.
• The Atari 2600 has 128 bytes of RAM.

2600 Memory Map

• Memory is organized into three sections:
  • ROM: Highest addresses (FFFF to 0000), initial execution address fetched here.
  • TIA: First 128 bytes (0000 to 007F).
  • RAM: Next 128 bytes (0080 to 00FF).
• RIOT chip occupies addresses from 2000 onwards.
• The 6502 CPU, while only addressing up to FFFF, attempts to access higher memory addresses.

Emulating the 6502 CPU

• The 6502 CPU is used in classic systems like Atari 2600, Commodore 64, NES.
• It has three general-purpose registers:
  • A (Accumulator): 8-bit for calculations and data storage.
  • X: 8-bit for addressing and indexing.
  • Y: 8-bit for addressing and indexing.
• Memory management components:
  • Stack Pointer (S): 8-bit, stores temporary data and return addresses, addresses from 100 to 1FF, overlapping with RAM.
  • Program Counter (PC): 16-bit, holds the address of the next instruction.
  • Processor Flags: 8-bit, status flags like negative, zero, overflow, interrupt enable, decimal mode, break.

6502 CPU Emulation

• CPU's registers and flags emulated with program variables (A, X, Y, S, PC, Flags).
• Initial address stored in memory locations FFFC and FFFD.
• ROM code specifies the program's starting address; the emulator fetches this to set the program counter.
• Emulator understands each CPU instruction (opcode).
• 6502 instructions are typically one byte long.
• Each instruction emulated for register & flag changes.
• Emulator handles different CPU addressing modes (immediate, zero page, relative, etc.).

Addressing Modes - Breaking Down Instructions

• 6502 instructions grouped by addressing modes and instruction types (LDA, ADD, SUB).
• CPU uses instruction byte bits to differentiate modes & types.
• Reducing emulator routines through separating instruction & mode bits.

Emulator Complexity

• Initial emulators are straightforward, replicating CPU behavior.
• Complex systems (like game consoles) require accurate timing and synchronization between hardware components (TIA, joysticks).
• Precise timing for gameplay is crucial in emulating game consoles.
• Simpler emulators (word processors) can process data as it becomes available.
• Emulation complexity depends on the target system architecture.
• 6502 is simpler than 68000 (Amiga, Atari ST).
• 68000 has variable instruction lengths & execution times, demanding precise synchronization.
• Emulator timing must match real-system behavior; this often requires detailed testing.
• Documentation may not fully describe all hardware interactions, leading to unique gameplay experiences.
• Accurate emulation requires precise replication of CPU and hardware interactions, including timing.
• Complex synchronization between hardware components is necessary.
• Precise timing replication for emulator actions (rendering a line of graphics).
• Tracking CPU cycles to advance other hardware emulation.
• Emulators for different architectures (x86 on ARM) require proper memory models.
• x86's strict memory ordering must be emulated on ARM.
• Achieving complete accuracy in emulation is a complex task.
• While generally accurate, "edge cases" (specific interactions) might not be fully replicated.