Cyrix 5x86 ("M1sc")

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Cyrix 5x86 ("M1sc")

Despite having the same name as AMD's 5x86 processor, the Cyrix 5x86 is a totally different animal. While AMD designed its 5x86 by further increasing the clock on the 486DX4, Cyrix took the opposite approach by modifying its M1 processor core (used for the 6x86 processor) to make a "lite" version to work on 486 motherboards. As such, the Cyrix 5x86 in some ways resembles a Pentium OverDrive (which is a Pentium core modified to work in a 486 motherboard) internally more than it resembles the AMD 5x86. This chip is probably the hardest to classify as either fourth or fifth generation.

The 5x86 employs several architectural features that are normally found only in fifth-generation designs. The pipeline is extended to six stages, and the internal architecture is 64 bits wide. It has a larger (16 KB) primary cache than the 486DX4 chip. It uses branch prediction to improve performance.

The 5x86 was available in two speeds, 100 and 120 MHz. The 5x86-120 is the most powerful chip that will run in a 486 motherboard--it offers performance comparable to a Pentium 90 or 100. The 5x86 is still a clock-tripled design, so it runs in 33 and 40 MHz motherboards. (The 100 MHz version will actually run at 50x2 as well, but normally was run at 33 MHz.) It is a 3 volt design and is intended for a Socket 3 motherboard. It will run in an earlier 486 socket if a voltage regulator is used. I have heard that some motherboards will not run this chip properly so you may need to check with Cyrix if trying to use this chip in an older board. These chips have been discontinued by Cyrix but are still good performers, and for those with a compatible motherboard, as good as you can get. Unfortunately, they are extremely difficult to find now.

Look here for an explanation of the categories in the processor summary table below, including links to more detailed explanations.

General Information	Manufacturer	Cyrix
	Family Name	5x86
	Code name	"M1sc"
	Processor Generation	Fourth
	Motherboard Generation	Fourth
	Version	5x86-100	5x86-120
	Introduced	1996?
	Variants and Licensed Equivalents	--
Speed Specifications	Memory Bus Speed (MHz)	33 / 50	40
	Processor Clock Multiplier	3.0 / 2.0	3.0
	Processor Speed (MHz)	100	120
	"P" Rating	P75	P90
Benchmarks	iCOMP Rating	~610	~735
	iCOMP 2.0 Rating	~67	~81
	Norton SI	264	316
	Norton SI32	~16	19
	CPUmark32	~150	~180
Physical Characteristics	Process Technology	CMOS
	Circuit Size (microns)	0.65
	Die Size (mm^2)	144
	Transistors (millions)	2.0
Voltage, Power and Cooling	External or I/O Voltage (V)	3.45
	Internal or Core Voltage (V)	3.45
	Power Management	SMM
	Cooling Requirements	Active heat sink
Packaging	Packaging Style	168-Pin PGA
	Motherboard Interface	Socket 3; or 168-Pin Socket, Socket 1, Socket 2 (with voltage regulator)
External Architecture	Data Bus Width (bits)	32
	Maximum Data Bus Bandwidth (Mbytes/sec)	127.2	152.6
	Address Bus Width (bits)	32
	Maximum Addressable Memory	4 GB
	Level 2 Cache Type	Motherboard
	Level 2 Cache Size	Usually 256 KB
	Level 2 Cache Bus Speed	Same as Memory Bus
	Multiprocessing	No
Internal Architecture	Instruction Set	x86
	MMX Support	No
	Processor Modes	Real, Protected, Virtual Real
	x86 Execution Method	Native
Internal Components	Register Size (bits)	32
	Pipeline Depth (stages)	6
	Level 1 Cache Size	16 KB Unified
	Level 1 Cache Mapping	4-Way Set Associative
	Level 1 Cache Write Policy	Write-Through, Write-Back
	Integer Units	1
	Floating Point Unit / Math Coprocessor	Integrated
	Instruction Decoders	1
	Branch Prediction Buffer Size / Accuracy	!? entries / !? %
	Write Buffers	!?
	Performance Enhancing Features	--

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