Virtual addresses were then normally mapped onto physical addresses by the paging system except when it was disabled. ("Real" mode had no "virtual" addresses.) Just as with the 80386, circumventing memory segmentation could substantially improve performance in some operating systems and applications. On a typical PC motherboard, either four matched 30-pin (8-bit) SIMMs or one 72-pin (32-bit) SIMM per bank were required to fit the 80486's 32-bit data bus. The address bus used 30-bits (A31..A2) complemented by four byte-select pins (instead of A0,A1) to allow for any 8/16/32-bit selection. This meant that the limit of directly addressable physical memory was 4 gigabytes as well (230 "32-bit" words = 232 "8-bit" words). There are several suffixes and variants. (see Table). Other variants include:  The specified maximal internal clock frequency (on Intel's versions) ranged from 16 to 100 MHz. The 16 MHz i486SX model was used by Dell Computers. One of the few 80486 models specified for a 50 MHz bus (486DX-50) initially had overheating problems and was moved to the 0.8-micrometre fabrication process. However, problems continued when the 486DX-50 was installed in local-bus systems due to the high bus speed, making it rather unpopular with mainstream consumers, as local-bus video was considered a requirement at the time, though it remained popular with users of EISA systems. The 486DX-50 was soon eclipsed by the clock-doubled i486DX2, which although running the internal CPU logic at twice the external bus speed (50 MHz), was nevertheless slower due to the external bus running at only 25 MHz. The i486DX2 at 66 MHz (with 33 MHz external bus) was faster than the 486DX-50, overall. More powerful 80486 iterations such as the OverDrive and DX4 were less popular (the latter available as an OEM part only), as they came out after Intel had released the next-generation P5 Pentium processor family. Certain steppings of the DX4 also officially supported 50 MHz bus operation, but it was a seldom used feature. *"WT" = write-through cache strategy, "WB" = write-back cache strategy  80486 compatible processors have been produced by other companies such as IBM, Texas Instruments, AMD, Cyrix, UMC, and SGS Thomson.