Cyrix 5x86
| Company type | Fabless semiconductor company |
|---|---|
| Industry | Microprocessors |
| Founded | 1988 |
| Fate | Ceased production of 5x86 in 1996 |
| Headquarters | United States |
| Products | x86 microprocessors (including 5x86 series) |
| Parent | National Semiconductor (from 1997) |
The Cyrix 5x86 series consists of x86 microprocessors developed by Cyrix and introduced on June 5, 1995. As a fabless entity, Cyrix contracted IBM for chip fabrication. Launching roughly five months prior to the acclaimed Cyrix 6x86, the 5x86 ranked among the highest-performing CPUs compatible with Socket 3 platforms. It outperformed many applications running on an Intel Pentium at 75 MHz, aiming at a segment seeking mid-range processors that fit into 486 Socket 3 motherboards—systems which typically could not accommodate Pentium CPUs except via the Pentium Overdrive upgrade.
The IBM 5x86C represents IBM’s branded variant of the Cyrix Cx5x86 processor. IBM’s earlier x86 offerings, including the IBM 386SLC and IBM 486SLC, were derived from altered Intel architectures.
Design
Known internally as "M1sc," the Cyrix 5x86 CPU was a reduced version of the "M1" core found in the Cyrix 6x86, delivering approximately 80% of its performance while utilizing half the number of transistors. It maintained a 32-bit memory interface similar to conventional 486 processors but incorporated internal features akin to fifth-generation CPUs such as the Cyrix 6x86, AMD K5, and Intel Pentium, with partial support for Pentium instructions. Certain performance-enhancing functions were initially disabled due to stability concerns but could be activated through specialized software tools.
The STMicroelectronics ST5x86 and IBM 5x86C were licensed rebranded versions of the Cyrix design, manufactured by Cyrix on their behalf. These variants were nearly identical, although IBM offered a unique 75 MHz model not available from Cyrix and required slightly different operating voltages. It is important not to confuse the Cyrix 5x86 with the AMD Am5x86, which was a clock-quadrupled 486 delivering comparable performance and Socket 3 compatibility but featured a distinct architecture.
Cyrix’s 5x86 had a relatively short market presence of about six months, likely discontinued to prevent internal competition with its newer 6x86 product line.
Controversies and anomalies
The official Cyrix 5x86 documentation acknowledged that certain features were disabled by default in production chips, notably the branch prediction mechanism used in performance comparisons against Intel’s Pentium processors. Enabling these functions via software often resulted in instability, especially on early revisions running 32-bit applications.
There have been persistent rumors regarding an uncommon 133 MHz clock-quadrupled variant, primarily accessible to upgrade kit manufacturers such as Gainbery. Some units rated at 100 and 120 MHz support a 4X multiplier and can potentially operate at 133 MHz; however, these chips generally exhibit limited overclocking headroom, with 120 MHz approaching their manufacturing limits. An additional variant running at 80 MHz (2×40 MHz) is known but may never have been officially released.
IBM’s version, the 5x86C, was conservatively specified and functioned at a lower voltage (3.3V). For instance, IBM marketed parts that Cyrix designated as 100 MHz at only 75 MHz. IBM offered both 75 and 100 MHz models, with some early-produced 120 MHz units likely discontinued before speed reductions were implemented. While IBM continued manufacturing the 5x86C until late 1998 (QFP package), Cyrix ceased production in 1996. No existing processors are known to incorporate the 4X multiplier or Stepping 1 Rev 3 cores.
Specifications
- iDX4WB pinout featuring 168 pins
- Compatible with Socket 3
- Fabricated with 2.0 million transistors utilizing a 0.65 micrometre manufacturing process
- Die dimensions measure 144 mm2
- Operates at a voltage of 3.45 volts
- Includes a unified level-one cache of 16 kilobyte
Offered variants consist of:
- 100 MHz versions supporting 33 MHz (33×3) and 50 MHz (50×2) front side bus speeds
- 100 MHz models compatible with 33 MHz (33×3) and 25 MHz (25×4) front side bus frequencies
- 120/133 MHz options for 40 MHz (40×3) and 33 MHz (33×4) front side bus configurations
External links
- Performance comparison benchmarks
- Overview of Cyrix 5x86
- Brief on Cyrix 5x86 Processor
- Listing in the chart of 486 processors
- Tool to activate 5x86 "register bits"
- Cache write-back performance utility provided by Evergreen Technologies (refer to the "Cyrix5x86" section and the "et9603.exe" link)