The Clone machines underwent many improvements along the way, the clock speed increased to 8, 10 and then 12MHz, and some of the last generation of XT type clone machines used an 80286 processor running at 16MHz, instead of an 8088 chip. NEC produced a better clone of the 8088 chip, the V20, and this was used in many of the last generation of XT type computers. Some manufacturers used an 8086 chip or the NEC V30, and accessed the RAM 16 bits at a time, instead of 8 bits at a time.
The XT type computer with its eight bit external data bus, and 20 bit address bus, was the most common PC until 1987 when 80286 based computers (PC/AT), with their 16 bit external data bus and 24 bit address bus, started to become affordable.
By 1987 the entry level machine had progressed to an 80286 based computer. IBM used the name PC/AT for these computers, clone manufacturers usually called them 286 computers. 1987 saw the introduction of the 80386 chip and by 1991 the entry level machine had progressed to an 80386SX based machine. The rest of the progression to the present day and beyond roughly follows the following list.
| Year | Processor |
|---|---|
| 1992 | 386DX40 |
| 1993 | 486SX25 or 33 |
| 1994 | 486DX33 |
| 1995 (first half) | 486DX2/66 |
| 1995 (second half) | 486DX4/100 |
| 1996 (first half) | Pentium 75 |
| 1996 (second half) | Pentium 100 |
| 1997 (first half) | Pentium 133, Cyrix/IBM or AMD equivalent |
| 1997 (second half) | Pentium 166 to 200, Cyrix/IBM or AMD equivalent |
| 1998 (first half) | Pentium MMX 200, Cyrix/IBM or AMD equivalent |
| 1998 (second half) | Celeron or Celeron A 300MHz, AMD K6-2/300, or 6x86/300 |
The 486SX chip was first available for a 25MHz clock speed and then for 33MHz and eventually a 486SX2/66 was produced. The 486DX chip was first produced operating at 33MHz but a 50MHz version was also available from Intel and a 40MHz version from AMD.
The next step was to double the Clock Speed inside the Processor. The 486DX2/66 Processor operates with an External Clock Speed of 33 MHz and an Internal Clock Speed of 66 MHz. The DX4/100 processors operate with an External Clock Speed of 33MHz and an Internal Clock Speed of 100 MHz. AMD produced 486DX2/80 and 486DX4/120 chips that operated with an External Clock Speed of 40MHz.
OverDrive processors are mainly used by Corporate and Government users who often have budgets to upgrade or repair existing Equipment, rather than replace equipment. IBM and Cyrix also looked at this market for a while but it appears as though there is only a small market for this type of product.
Two other products, the Evergreen and Power Leap chips are re-packaged and re-badged 80586 chips mounted on a sub-board that plugs into a 80486 System Board. The board is required to provide the 3.3 volt supply to the processor because many older 80486 System Boards only have a 5 volt supply to the processor chip.
NexGen (now part of AMD) produced a processor called the Nx586, a 486 compatible device similar to the Pentium OverDrive from Intel. Another similar device was also marketed by IBM/Cyrix as the 5x86 chip. All of these chips had a 32-bit External Data bus and a 64-bit Internal Data bus.
| Back to the The Pentium and beyond chapter | Back to the opening index | Book one index |
| Whats inside the first generation Pentium Processor | Time line of Intel processors | Internal and external speeds of older Pentium type processors | The Sub US$1000 PC |