Motorola 68040

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This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

The Motorola 68040 is a microprocessor from Motorola, released in 1990. It is the successor to the 68030 and is followed by the 68060. There was no 68050. In keeping with general Motorola naming, the 68040 is often referred to as simply the '040 (pronounced oh-four-oh or oh-forty).

In Apple Macintosh computers, the 68040 was found mainly in the high-end Macintosh Quadra. The fastest 68040 processor was clocked at 40 MHz and it was only used in the Quadra 840AV. The more expensive models in the (short-lived) mid-high Macintosh Centris also used the 68040, while the cheaper Centris and Macintosh Performa used the 68LC040. The 68040 was also used in other personal computers, such as the Amiga 4000 and Amiga 4000T, as well as a number of Workstations and later versions of the NeXT computers.

The 68040 was the first 680x0 family member with an on-chip Floating-Point Unit (FPU). It thus included all of the functionality that previously required external chips, namely the FPU and Memory Management Unit (MMU), which was added in the 68030. It also had split instruction and data caches of 4 kilobytes each. It was fully pipelined, with six stages.

Unfortunately, the 68040 ran into the transistor budget limit early in design. While the MMU did not take many transistors — indeed, having it on the same die as the CPU actually saved on transistors — the FPU certainly did. Motorola's 68882 external FPU was known as a very high performance unit and Motorola did not wish to risk integrators using the "LC" version with a 68882 instead of the more profitable full "RC" unit. (For information on Motorola's multiprocessing model with the 680x0 series, see Motorola 68020.) The FPU in the 68040 was thus made incapable of IEEE transcendental functions, which had been supported by both the 68881 and 68882 and were used by the popular fractal generating software of the time and little else. The Motorola floating point support package (FPSP) emulated these instructions in software under interrupt. As this was an exception handler, heavy use of the transcendental functions caused severe performance penalties.

Heat was always a problem throughout the 68040's life. While it delivered over four times the per-clock performance of the old-when-released 68020 and 68030, the chip's complexity and power requirements came from a large die and large caches. This affected the scaling of the processor and it was never able to run with a clock rate exceeding 40 MHz. A 50 MHz variant was planned, but canceled. Overclocking enthusiasts reported success reaching 50 MHz using a 100MHz oscillator instead of an 80MHz part and the then novel technique of adding oversized heat sinks with fans.

The 68040 offered the same features as the Intel 80486, but on a clock-for-clock basis could significantly outperform the Intel chip[citation needed]. However, the 80486 had the ability to be clocked significantly faster and it did not suffer from overheating problems. In 1993-1994, as the Macintosh desktop lineup transitioned to the '040, Apple was unable to offer the newer processor in their PowerBooks, which continued to be offered with 68030s. Even Macworld reviewers conceded that the best choice for power users was the PC compatible Texas Instruments 80486 notebook, rather than the top-of-the-line PowerBook 180[citation needed].

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