Copyright © Philip M. Parker, INSEAD. Terms of Use.
(From Wikipedia, the free Encyclopedia)
Originally, the MC68000 was designed for use in household products (at least, that is what an internal memo from Motorola claimed). Luckily it was used for the design of homecomputers like the Amiga and Atari. It was also used in the Sega Genesis/MegaDrive, NeoGeo and several Arcade Machines as their main CPU. In the Sega Saturn, the 68000 was used as the sound processor, and in the Atari Jaguar they were used as a main controller for all the other dedicated hardware IC's.
Notable design wins include the Amiga, the Apple Macintosh, and the original Sun Microsystems and SGI UNIX machines. 68000 derivatives persisted in the UNIX market for many years, because the architecture so strongly resembles the Digital PDP-11 and VAX, and is an excellent computer for running C code. The 68000 eventually saw its greatest success as a controller. Thousands of HP, Printronix and Adobe printers used it. Its derivative microcontrollers, the CPU32 and Coldfire processors have been manufactured in the millions as automotive engine controllers. It also sees use by medical manufacturers and many printer manufacturers because of its low cost, convenience, and good stability. As of 2001, the Dragonball versions of the processor are used in the popular Palm series of PDAs from Palm Computing and Handspring's Visor, though the architecture is being phased out in favor of the ARM processor core.
The Voyage 200 PLT
The Motorola 68000 is also used in Texas Instruments' latest line of calculators. These four calculators (the TI-89, TI-92, TI-92 Plus, and Voyage 200 PLT) are the highest-end calculators. The TI-92 had a 10 MHz processor and 68KB RAM, and was released in 1995. It was upgraded to the TI-92 Plus (in 1998) with 188KB RAM, 384KB Flash ROM, and the same 10 MHz processor. Also released in that year was the TI-89, identical except it was in a more traditionally shaped case. Around 2000, the TI-92 Plus and the TI-89 were upgraded to Hardware Version 2, and gaines a 12 MHz processor and a few other hidden upgrades. In 2002, Texas Instruments relaeased its latest über-calculator: the Voyage 200 PLT. This had the Hardware Version 2 12MHz processor, 188 KB RAM, and a whopping 3.7 MB Flash ROM. It also had a clock and an Apps Desktop, both of which were summarily released as Flash ROM updates for the TI-92 Plus and TI-89.
Initial samples of the 68000 were released in 1982, and competed against the Intel 8086 and Intel 80286 with some success. Initial clock rates were a then-blisteringly fast 8MHz, with a rather slow eight to ten clocks per instruction. However, the instructions did more than Intel processors. Motorola ceased production of the 68000 in 2000, although derivatives, notably the CPU32 family, continue in production. As of 2001, Hitachi continued to manufacture the 68000 under license.
The 68000 was a clever compromise. When the 68000 was introduced, 16-bit busses were really the most practical size. However, the 68000 was designed with 32-bit registers and address spaces, on the assumption that hardware prices would fall. To address the perceived markets, the actual 68000 was designed in three forms. The base-form had a 24-bit address, and a 16-bit data bus. The short form, the 68008, had an 18-bit address (possibly 19 or 20 bits, at least one firm addressed 512KBytes with 68008s), and an 8-bit data bus. A planned future form (later the 68020) had a 32-bit data and address bus.
The CPU had 8 general-purpose data registers (D0-D7), and 8 address registers (A0-A7). The last address register was also the standard stack pointer, and could be called either A7 or SP. This was a good number of registers in many ways. It was small enough to make the 68000 respond quickly to interrupts (because only 15 or 16 had to be saved), and yet large enough to make most calculations fast.
Having two types of registers was mildly annoying at times, but really not hard to use in practice. Reportedly, it allowed the CPU designers to achieve a higher degree of parallelism, by using an auxiliary execution unit for the address registers.
The 68000 comparison, arithmetic and logic operations set bits in a status register to record their results for use by later conditional jumps. The bits were "Z"ero, "C"arry, e"X"tend, and "N"egative. The eXtend bit deserves special mention, because it was separated from the Carry. This permitted the extra bit from arithmetic, logic and shift operations to be separated from the carry for flow-of-control and linkage.
The designers attempted to make the assembly language orthogonal. That is, instructions were divided into operations and address modes. Almost all address modes were available for almost all instructions. Many programmers disliked the "near" orthogonality, while others were grateful for the attempt.
At the bit level, the person writing the assembler would clearly see that these "instructions" could become any of several different op-codes. It was quite a good compromise because it gave almost the same convenience as a truly orthogonal machine, and yet also gave the CPU designers freedom to fill in the op-code table.
The minimal instruction size was huge for its day at 16 bits. Furthermore, many instructions and addressing modes added extra words on the back for addresses, more address-mode bits, etc.
Many designers believed that the M68000 architecture had compact code for its cost, especially when produced by compilers. This belief in more compact code led to many of its design wins, and much of its longevity as an architecture.
Most embedded system designers are acutely aware of the costs of memory.
This belief (or feature, depending on the designer) continued to make design wins for the instruction set (with updated CPUs) up until the ARM architecture introduced compressed Thumb op-codes that were more compact.
The CPU, and later the whole family, implemented exactly two levels of privilege. User mode gave access to everything except the interrupt level control. Supervisor privilege gave access to everything. An interrupt always became supervisory. The supervisor bit was stored in the status register, and visible to user programs.
A real advantage of this system was that the supervisor level had a separate stack pointer. This permitted a multitasking system to use very small stacks for tasks, because the designers did not have to allocate the memory required to hold the stack frames of a maximum stack-up of interrupts.The 68000 family
People who are familiar with the PDP-11 or VAX usually feel comfortable with the 68000. With the exception of the split of general purpose registers into specialized data and address registers, the 68000 architechture is in many ways a 32-bit PDP-11.
History
Architecture
Address bus
Internal registers
Status register
The instruction set
Privilege levels
Source: the above text is adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Motorola 68000."
Copyright © Philip M. Parker, INSEAD. Terms of Use.
