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 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 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 100 MHz oscillator instead of an 80 MHz part and the then novel technique of adding oversized heat sinks with fans. The 68040 offered the same features as the 80486, but on a clock-for-clock basis could significantly outperform the Intel chip in integer and floating-point instructions according to Motorola's own benchmark results. Independent measurements of the processor's floating-point performance in workstation systems suggested rather more modest gains over the 80486 and SPARC in Motorola's chosen
LINPACK benchmark, largely confined to double-precision operations, and vendor literature such as that describing the
NeXTstation Color and Turbo Color models advertised somewhat lower ratings than Motorola's own literature. However, the 80486 had the ability to be clocked significantly faster without suffering from overheating problems, and also quickly spawned a mobile derivative for laptops. Newer-generation 80486-based PC laptops were introduced in 1992, resulting in the 100-series PowerBook being stuck with the aging
68030 which were no longer competitive, and Apple was unable to ship a 68040-equipped PowerBook until the
PowerBook Duo 280 and
PowerBook 500 series in 1994. ==Variants==