The Orthogonal Masterpiece

When computer historians look back at the 16-bit era, one chip stands above the rest as the engine of the “Creative Revolution”: the Motorola 68000. While the Intel 8086 dominated the business world through the IBM PC, the 68000 (or “68k”) powered the machines that artists and dreamers used: the Apple Macintosh, the Commodore Amiga, the Atari ST, and the Sega Genesis.

Released in 1979, the 68000 was a hybrid 16/32-bit processor. Internally, it was a 32-bit beast, capable of crunching large numbers and addressing vast amounts of memory. Externally, it used a 16-bit data bus to keep motherboard costs reasonable.

A Clean-Sheet Architecture

The most striking feature of the 68000 was its elegance. The Intel x86 architecture was a messy evolution of the 8-bit 8080, burdened by “Segments” and “Offsets” to access more than 64KB of RAM. The 68000, by contrast, had a Flat Linear Address Space.

24-bit Address Bus: It could directly address 16 Megabytes of RAM. To a programmer, memory was just a single, continuous array from $000000 to $FFFFFF.
32-bit Registers: It featured 16 general-purpose 32-bit registers, split into Data (D0-D7) and Address (A0-A7).

The Joy of Orthogonality

Programmers loved the 68k because it was Orthogonal. This means that almost any instruction could use almost any addressing mode with any register. There were very few “special cases” to memorize.

Compare the x86 and 68k approaches:

Intel 8086 (Restrictive):

MOV AX, [SI]    ; OK: Can read from Source Index
MOV AX, [DX]    ; ERROR: Cannot use Data Register as pointer
ADD AX, BX      ; OK: Add BX to AX
STOSB           ; Special instruction just to write a byte and increment DI

Motorola 68000 (Flexible):

MOVE.L (A0), D0     ; Move Long (32-bit) from addr in A0 to D0
ADD.L  D1, D0       ; Add D1 to D0
MOVE.B D0, (A1)+    ; Move Byte from D0 to addr in A1, then increment A1
SUB.W  (A2)+, D3    ; Subtract Word from addr in A2 (and inc A2) from D3

The addressing modes like Post-Increment (An)+ and Pre-Decrement -(An) made working with arrays, stacks, and buffers incredibly concise. A memory copy loop (memcpy) in 68k assembly is a thing of beauty:

; Copy a block of memory
; A0 = Source, A1 = Dest, D0 = Count (Num words - 1)
CopyLoop:
    MOVE.W  (A0)+, (A1)+    ; Copy word, increment both pointers
    DBRA    D0, CopyLoop    ; Decrement D0, Branch if not -1

The DBRA (Decrement and Branch) instruction was a hardware loop counter that made tight loops extremely fast.

Supervisor and User Modes

The 68000 introduced a robust privilege system, separating User Mode from Supervisor Mode.

User Mode: Standard applications ran here. They could not execute privileged instructions (like STOP or RESET) or modify the Status Register.
Supervisor Mode: The OS kernel ran here. Triggered by hardware interrupts or software “Traps” (Exceptions).

This architecture laid the groundwork for true preemptive multitasking operating systems like AmigaOS and Unix, years before Windows moved away from the cooperative multitasking of the DOS era.

The Instruction Set Highlights

The 68k instruction set was rich and expressive:

Bcc (Branch on Condition): BEQ (Equal), BNE (Not Equal), BGT (Greater Than), etc.
MULU / MULS: Hardware multiply (Unsigned/Signed). A luxury in 1979!
DIVU / DIVS: Hardware divide.
SWAP: Swap the upper and lower 16-bit halves of a 32-bit register.
LINK / UNLK: Instructions specifically designed to create and destroy stack frames for high-level languages like C and Pascal.

The Amiga Connection

In the Amiga, the 68000 typically ran at 7.09 MHz (NTSC) or 7.16 MHz (PAL). This precise frequency was chosen to sync with the video signal. Because the Amiga had custom chips (DMA) to handle the heavy lifting of graphics and sound, the 68000 was free to run game logic.

Interleaved Access: The Amiga’s memory bus was designed so that the CPU and the custom chips took turns accessing RAM. In many display modes, the custom chips would steal cycles only during the “dead time” of the CPU memory access, effectively allowing the CPU to run at full speed even while the screen was being refreshed.

Legacy and Evolution

The 68000 line evolved into the 68020 (true 32-bit data bus), 68030 (MMU), 040 (FPU), and 060 (Superscalar). It powered the workstation market (Sun Microsystems, Silicon Graphics) and the arcade market (Capcom CPS-1/2/3, SNK Neo Geo) for almost two decades.

While the desktop world eventually standardized on x86, the 68000 lives on in embedded systems and in the hearts of coders who remember the elegance of MOVE.L (A0)+, (A1)+. It was a processor designed by engineers, for engineers.

Technical Modules

Technical Demo: Starfield - 3D projection math relies heavily on the 68000’s MULS instruction.
Technical Demo: Twister - Sine table lookups are optimized using the (An)+ addressing mode.

Source & Further Reading

“M68000 Programmer’s Reference Manual”: The bible for 68k coders.
“The Art of Assembly Language Programming” (Amiga Version): excellent tutorials on 68k specific optimizations.
“Easy68k”: A modern Windows-based 68000 simulator and assembler for learning the architecture. External Link