68XX

Overview

The 68xx architecture, also known as the Motorola 6800 series, is an 8-bit microprocessor architecture developed by Motorola in the mid-1970s. It was widely used in early home computers, automotive applications, and embedded systems due to its robust design and ease of use.

History of 68xx Architecture

Motorola 6800 (1974): The original 8-bit processor that started the 68xx architecture.
Motorola 6801/6803 (1978): Enhanced versions of the 6800 with additional features like on-chip RAM and I/O ports.
Motorola 6809 (1979): An advanced 8-bit processor with more addressing modes and improved performance.

Key Concepts

8-bit Architecture: The 68xx architecture is based on an 8-bit data bus, meaning it can process 8 bits of data at a time.
Registers: The 68xx has a small set of registers for operations.
Addressing Modes: The 68xx supports various addressing modes for accessing memory.
Simple Instruction Set: The 68xx has a relatively simple and small instruction set, making it easy to learn and use.

68xx Architecture

Registers

Accumulator (A, B): Used for arithmetic and logic operations.
Index Register (X): Used for indexing and loop control.
Stack Pointer (SP): Points to the current position in the stack.
Program Counter (PC): Points to the next instruction to be executed.
Condition Code Register (CCR): Holds the processor status flags.

Example: Simple Assembly Program

    ORG $8000
    LDAA #$00        ; Load A with 0
loop:
    ADDA #$01        ; Add 1 to A
    STAA $0200       ; Store A at address $0200
    INX              ; Increment X
    CPX #$10         ; Compare X with 16
    BNE loop         ; Branch to loop if X is not equal to 16
    SWI              ; Software interrupt (end of program)

Differences Between 68xx and Other Architectures

8-bit vs. 16/32/64-bit: The 68xx is an 8-bit architecture, while modern architectures like x86 and ARM are 16, 32, or 64-bit.
Simple Instruction Set: The 68xx has a simpler and smaller instruction set compared to more complex architectures like x86.
Limited Registers: The 68xx has fewer registers compared to modern architectures, which have more general-purpose and special-purpose registers.

Understanding the 68xx architecture is crucial for low-level programming, performance optimization, and system development in environments where 68xx processors are used.

Last modified: 14 December 2024