Tuesday, August 13, 2019

The Motorola 68000 microprocessor

The Motorola 68000 (MC68000) is a 16/32 -bit CISC microprocessor designed and marketed by Motorola (currently produced by Freescale). Introduced in 1979 deploying HMOS technology it was the first member of the successful family of 32-bit m68k microprocessors. The software created for this processor is usually fully compatible with future versions of the rest of the line, despite the fact that this first version is limited to an external 16-bit bus width. After three decades in production, the 68000 architecture is still in use today.

Notable computers using the Motorola 68XXX processor

The microprocessor owes its name to the number of transistors of which it’s composed. It has been used, among others, in the Commodore Amiga family, the Atari ST line of micro computers, the very first Macintosh, the Sharp X68000 and the first gaming video games used by Capcom. The MC68000 was launched in 1980 and is the first of a family of microprocessors that is made up of the Motorola 68010, Motorola 68020, Motorola 68030, Motorola 68040 and the Motorola 68060. This family of processors is often designated by the generic terms 680x0, m68k, 68k or 68000 family . Motorola also developed a 680x0 successor, called the Coldfire.


The 68000 emerged out of the the MACSS project (Motorola Advanced Computer System on Silicon, Motorola Advanced Silicon Computing Systems), which started in 1976 to develop a completely new architecture without maintaining compatibility with any previous processors. It would be the most powerful microprocessor to complement the 6800 8-bit production line instead of designing a compatible line. After all the 68000 provided a bus protocol with a compatibility mode for peripheral devices of the 6800 line. Thus a version with an 8-bit data bus was produced.

However, the designers focused mainly on the future or forward compatibility, which gave the m68k platform an advantage over subsequent 32-bit architectures in its instruction set. For example, the CPU registers were 32 bits wide, although only few autonomous structures in the processor worked in a 32 bits mode at that time. The MACSS team was very strongly influenced by the design of processors for minicomputers, such as the PDP-11 and VAX systems, which have a similar microcode.


The 68000 architecture is based on two banks of 8 32-bit registers. One bank represents data (Dn) and the other the pointers (An). It also has a 32-bit program counter and a 16-bit status register, the so called "System Byte" being high and the "User Byte" being low. Data registers (D0 to D7) can be used as 32-bit (.l), 16-bit (.w) and 8-bit (.b) registers. Any of them can be used for the accumulator, index- or pointer registers. This processor is made in HMOS technology and has 64 pins without signal multiplexing.

Address registers

Address registers (pointers) are very similar to those of data registers, but cannot be used as bytes and operations, because they do not affect carry registers when performing calculations. The A7 register represents the stack pointer and is duplicated, with one stack for the user mode and another for the supervisor mode.

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