65C816

Overview

The 65C816, also known as the W65C816S, is a 16-bit microprocessor developed by the Western Design Center (WDC). It is an enhanced version of the 8-bit 6502 microprocessor, which was widely used in the late 1970s and early 1980s. The 65C816 was designed to provide a smooth transition from 8-bit to 16-bit processing, offering backward compatibility with the 6502 while introducing several advanced features and enhancements.

Architecture

The 65C816 microprocessor features a 16-bit data bus and a 24-bit address bus, allowing it to address up to 16 megabytes of memory. This is a significant improvement over the 6502's 64-kilobyte address space. The processor operates in two modes: Emulation Mode and Native Mode. Emulation Mode ensures compatibility with 6502 code, while Native Mode unlocks the full 16-bit capabilities of the processor.

Registers

The 65C816 includes several registers, some of which are extensions of the 6502's registers:

**Accumulator (A)**: 16-bit register used for arithmetic and logic operations.
**Index Registers (X and Y)**: 16-bit registers used for indexing and loop control.
**Stack Pointer (SP)**: 16-bit register pointing to the top of the stack.
**Program Counter (PC)**: 16-bit register holding the address of the next instruction to be executed.
**Direct Page Register (D)**: 16-bit register used to define a base address for zero-page addressing.
**Processor Status Register (P)**: 8-bit register holding the processor's status flags.
**Data Bank Register (DB)**: 8-bit register used to select the current data bank in memory.
**Program Bank Register (PB)**: 8-bit register used to select the current program bank in memory.

Instruction Set

The 65C816 retains the 6502's instruction set and adds several new instructions to support 16-bit operations and enhanced functionality. Some of the new instructions include:

**PEA (Push Effective Address)**: Pushes a 16-bit address onto the stack.
**PER (Push Effective Relative Address)**: Pushes a 16-bit relative address onto the stack.
**PLX/PLY (Pull X/Y Register)**: Pulls a 16-bit value from the stack into the X or Y register.
**STP (Stop the Processor)**: Halts the processor until a reset occurs.
**WAI (Wait for Interrupt)**: Puts the processor into a low-power state until an interrupt occurs.

Memory Management

The 65C816's 24-bit address bus allows it to access a much larger memory space than the 6502. The processor uses a combination of bank switching and direct page addressing to manage memory efficiently.

Bank Switching

The 65C816 divides its 16 MB address space into 256 banks, each 64 KB in size. The Data Bank Register (DB) and Program Bank Register (PB) are used to select the current data and program banks, respectively. This allows the processor to access different regions of memory without changing the base address.

Direct Page Addressing

The Direct Page Register (D) allows the 65C816 to define a base address for zero-page addressing. This feature enhances the processor's ability to access frequently used variables and data structures quickly.

Interrupts and Exceptions

The 65C816 supports several types of interrupts and exceptions, including:

**IRQ (Interrupt Request)**: A maskable interrupt used for general-purpose interrupt handling.
**NMI (Non-Maskable Interrupt)**: A non-maskable interrupt used for critical events.
**RESET**: A signal that resets the processor and initializes its state.
**ABORT**: An interrupt used to handle memory access violations and other critical errors.
**COP (Co-Processor)**: A software interrupt used to invoke co-processor functions.

Applications

The 65C816 has been used in a variety of applications, including:

**Home Computers**: The 65C816 was used in the Apple IIGS, an advanced model of the Apple II series.
**Game Consoles**: The 65C816 was the core processor in the Super Nintendo Entertainment System (SNES), providing enhanced graphics and sound capabilities.
**Embedded Systems**: The 65C816 has been used in various embedded systems, including industrial controllers and automotive applications.

Development Tools

Several development tools are available for the 65C816, including assemblers, simulators, and debuggers. These tools facilitate the development of software for the processor and help developers optimize their code for performance and efficiency.

Assemblers

Assemblers for the 65C816 translate assembly language code into machine code. Some popular assemblers include:

**CA65**: Part of the cc65 cross-compiler suite, CA65 supports the 65C816 and provides a rich set of features for assembly language programming.
**XA**: A cross-assembler for the 65C816 that supports various output formats and advanced macro capabilities.

Simulators and Debuggers

Simulators and debuggers allow developers to test and debug their code in a controlled environment. Some popular tools include:

**BSNES**: An emulator for the SNES that accurately simulates the 65C816 processor and its peripherals.
**No$sns**: A debugger and emulator for the SNES that provides advanced debugging features for 65C816 code.

Performance and Limitations

The 65C816 offers several performance improvements over the 6502, including faster instruction execution and enhanced memory management. However, it also has some limitations:

**Backward Compatibility**: While the 65C816 is backward compatible with the 6502, some 6502-specific code may not run correctly in Native Mode.
**Complexity**: The 65C816's advanced features and extended instruction set can make it more challenging to program compared to the simpler 6502.
**Market Adoption**: Despite its advantages, the 65C816 did not achieve the same level of market adoption as the 6502, limiting its use in commercial products.

Legacy

The 65C816 has left a lasting legacy in the world of microprocessors. Its design influenced the development of later processors and contributed to the evolution of computer architecture. The 65C816's backward compatibility with the 6502 ensured that it could be used in a wide range of applications, from home computers to game consoles and embedded systems.