68000 Motherboard User’s Manual
Rev. A
Page 5 of 54
contemporary incarnations, including the ColdFire and CPU32 families, maintain the
bloodline with a sustaining market share into the future.
Through its life, the 68000 has enjoyed many notable design wins. Among computers, it
was the processor of the original Macintosh systems, the Atari ST and the Commodore
Amiga, among others. Its presence was also known among UNIX workstations,
including the original Sun Microsystems and SGI systems. And it was also the heart of
renowned game consoles, such as the Sega Genesis and Neo Geo systems, as well as
many stand-alone arcade games. It found a home in an array of computer peripherals,
networking equipment and other high-end gadgetry. My own childhood laser printer
succumbed to the screwdriver, yielding a traditional 64-pin PDIP Motorola 68000 at its
core. This processor defined a generation of computing in each market it touched.
From its original design philosophy born
within Motorola of the mid to late 1970’s,
this processor was crafted with an eye
toward the future. The processor design
was conceived under no burden of
confining software backward compatibility.
This forward-looking mindset enabled the
design engineers to “break away from the
past,” as the processor’s User’s Manual
cover art asserts. The first generation
sports a 32-bit architecture, masquerading
in 16-bit hardware. Although later
descendants expanded to true 32-bit form,
the 68000’s 32-bit data and address
registers are siphoned to the outside world
through a 16-bit data bus and 24-bit address
bus. These copious data and address
register banks, each with eight 32-bit
registers, provide a spacious backdrop to
efficient and elegant software development.
To further this elegance, the instruction set
of the 68000 was designed to be orthogonal, with a highly regular structure. The
instructions had nearly identical access to all addressing modes, moving beyond the
dedicated use of specific registers for specific functions. The 68000 internally features
two parallel 16-bit Arithmetic Logic Units for the fast calculation of addresses. From this
emerges the brilliant selection of available addressing modes, including the pre-
decrement and post-increment capabilities. The dazzling number of address modes takes
getting used to, when approaching with a background either in the primitive 8-bit devices
preceding the 68000 or the frugal RISC designs of today.
The generous selection of 14 addressing modes and impressive count of 56 instruction
types owes thanks to the 68000’s 16-bit data bus. This bus width enables a base op code
of 16-bits, rendering wide flexibility in defining the instruction’s operation. Processor
Summary of Contents for MB68k-100
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