Am386

The Am386 CPU is a 100%-compatible clone of the Intel 80386 design released by AMD in March 1991. It sold millions of units, positioning AMD as a legitimate competitor to Intel, rather than being merely a second source for x86 CPUs (then termed 8086-family).[1]

Am386
An AMD 80386DX-40 in a 132-pin PQFP, soldered onboard
General information
Launched1991
Marketed byAMD
Designed byAMD
Common manufacturer(s)
  • AMD
Product code23936
Performance
Max. CPU clock rate20 MHz to 40 MHz
FSB speeds20 MHz to 40 MHz
Cache
L1 cacheMotherboard dependent
L2 cachenone
Architecture and classification
ApplicationDesktop, Embedded (DE-Models)
Min. feature size1.5 μm to 0.8 μm
Microarchitecture80386
Instruction setx86 (IA-32)
Physical specifications
Cores
  • 1
Package(s)
  • DX variant:
    132-pin PGA
  • 132-pin PQFP
  • SX variant:
    88-pin PGA
  • 100-pin PQFP
  • DE variant:
    132-pin PGA
    132-pin PQFP
History
PredecessorAm286
SuccessorAm486

Features

CPU features table

History and design
Wafer of an Am386 processor with Intel logo.
Die of AMD Am386DX.

While the AM386 CPU was essentially ready to be released prior to 1991, Intel kept it tied up in court.[2] AMD had previously been a second-source manufacturer of Intel's Intel 8086, Intel 80186 and Intel 80286 designs, and AMD's interpretation of the contract, made up in 1982, was that it covered all derivatives of them. Intel, however, claimed that the contract only covered the 80286 and prior processors and forbade AMD the right to manufacture 80386 CPUs in 1987. After a few years in the courtrooms, AMD finally won the case and the right to sell their Am386 in March 1991.[3] This also paved the way for competition in the 80386-compatible 32-bit CPU market and so lowered the cost of owning a PC.[1]

While Intel's 386 CPUs had topped out at 33 MHz in 1989, AMD introduced 40 MHz versions of both its 386DX and 386SX out of the gate, extending the lifespan of the architecture. In the following two years the AMD 386DX-40 saw popularity with small manufacturers of PC clones and with budget-minded computer enthusiasts because it offered near-80486 performance at a much lower price than an actual 486.[4] Generally the 386DX-40 performs nearly on par with a 25 MHz 486 due to the 486 needing fewer clock cycles per instruction, thanks to its tighter pipelining (more overlapping of internal processing) in combination with an on-chip CPU cache. However, its 32-bit 40 Mhz data bus gave the 386DX-40 comparatively good memory and I/O performance.[5]

  • An Am386DX-25
  • The Am386DE-33 is an embedded version of the Am386DX-33
  • A PGA Am386DX-40
  • A PQFP Am386DX-40 on a 132-pin PGA adapter
  • AMD Am386DE Block Diagram There is not a Paging Unit like a DX CPU

Am386DX data
  • 32-bit data bus, can select between either a 32-bit bus or a 16-bit bus by use of the BS16 input
  • 32-bit physical address space, 4 Gbyte physical memory address space
  • fetches code in four-byte units
  • released in March 1991
The various models of the Am386DX, data from[1][2]
Model numberFrequencyFSBVoltagePowerSocket
AMD Am386DX/DXL-2020 MHz20 MHz5 V1.05 Watt132-pin CPGA
AMD Am386DX/DXL-2525 MHz25 MHz5 V1.31 Watt132-pin CPGA
AMD Am386DX/DXL-3333 MHz33 MHz5 V1.73 Watt132-pin CPGA
AMD Am386DX/DXL-4040 MHz40 MHz5 V2.10 Watt132-pin CPGA
AMD Am386DX-4040 MHz40 MHz5 V3.03 Watt132-pin PQFP

Am386DE data
  • 32-bit data bus, can select between either a 32-bit bus or a 16-bit bus by use of the BS16 input
  • 32-bit physical address space, 4 Gbyte physical memory address space
  • fetches code in four-byte units
  • no paging unit
The various models of the Am386DE, data from[1][2]
Model numberFrequencyFSBVoltagePowerSocketRelease date
AMD Am386DE-25KC25 MHz25 MHz3-5 V0.32-1.05 Watt132-pin PQFP?
AMD Am386DE-33KC33 MHz33 MHz5 V1.05-1.35 Watt132-pin PQFP?
AMD Am386DE-33GC33 MHz33 MHz5 V1.05-1.35 Watt132-pin CPGA?

AM386 SX

In 1991 AMD also introduced advanced versions of the 386SX processor -again not as a second source production of the Intel chip, but as a reverse engineered pin compatible version. In fact, it was AMD's first entry in the x86 market other than as a second source for Intel.[6] AMD 386SX processors were available at faster clock speeds at the time they were introduced and still cheaper than the Intel 386SX. Produced in 0.8 μm technology and using a static core, their clock speed could be dropped down to 0 MHz, consuming just some mWatts. Power consumption was up to 35% lower than with Intel's design and even lower than the 386SL's, making the AMD 386SX the ideal chip for both desktop and mobile computers. The SXL versions featured advanced power management functions and used even less power.[6]

  • An Am386SX-25
  • An Am386SX-33
  • An Am386SX-40

Am386SX data
  • 16-bit data bus, no bus sizing option
  • 24-bit physical address space, 16 Mbyte physical memory address space
  • prefetch unit reads two bytes as one unit (like the 80286).
The various models of the Am386SX, data from[2][6][7]
Model numberFrequencyFSBVoltagePowerSocketRelease date
AMD Am386SX/SXL-2020 MHz20 MHz5 V1.68/0.85 Watt100-pin PQFP1991
AMD Am386SX/SXL-2525 MHz25 MHz5 V1.84/1.05 Watt100-pin PQFP29 April 1991
AMD Am386SX/SXL-3333 MHz33 MHz5 V1.35 Watt100-pin PQFP1992
AMD Am386SX-4040 MHz40 MHz5 V1.55 Watt100-pin PQFP1991

80387 coprocessor

Floating point performance of the Am386 could be boosted with the addition of a 80387DX or 80387SX coprocessor, although performance would still not approach that of the on-chip FPU of the 486DX. This made the Am386DX a suboptimal choice for scientific applications and CAD using floating point intensive calculations. However, both were niche markets in the early 1990s and the chip sold well, first as a mid-range contender, and then as a budget chip. Although motherboards using the older 386 CPUs often had limited memory expansion possibilities and therefore struggled under Windows 95's memory requirements, boards using the Am386 were sold well into the mid-1990s; at the end as budget motherboards for those who were only interested in running MS-DOS or Windows 3.1x applications. The Am386 and its low-power successors were also popular choices for embedded systems, for a much longer period than their life span as PC processors.

  • An IIT 387SX-25 Coprocessor
  • A Cyrix FasMath 387DX-33 Coprocessor
  • An ULSI 387SX-40 Coprocessor

References
  1. "The AMD Am386 DX Processor". cpu-collection.de. 2011. Retrieved 12 November 2011.
  2. Shvets, Gennadiy (5 November 2011). "AMD 80386 microprocessor". CPU-World. Retrieved 29 November 2011.
  3. https://www.nytimes.com/1991/03/02/business/intel-loses-trademark-decision.html
  4. "386DX-40 and competitors". the red hill cpu guide. 2011. Retrieved 12 November 2011.
  5. Linderholm, Owen; Miller, Dan (1 December 1992). "486SX-25s vs. 386DX-40s: the upstart fights back. (evaluations of 30 microcomputers based on Intel Corp.'s 80486SX-25, 80386DX-40 microprocessors) (Hardware Review) (Systems: 486SX-25 vs. 386-40) (Evaluation)". Highbeam Business. Archived from the original on 1 January 2014. Retrieved 12 November 2011.
  6. "The AMD Am386 SX Processor". cpu-collection.de. 2011. Retrieved 30 November 2011.
  7. Archived 2016-03-04 at the Wayback Machine, AMD Datasheet no 15022.
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