Intel 486 & 486 POD (Pentium Overdrive)
CPUID, S-spec & Steppings

Created by David Beem (original HERE). Edited by Tomáš Slavotínek.

The CPUID (or "Reset Signature") is an encoded value reported from the chip to denote the particular CPU Family, Series, and Stepping of the CPU. Most Intel 486 CPUs with copyright dates of '92 or later (with the exception of the 486SL CPUs) support retrieving the CPUID at any point ("OTF", or "On-The-Fly") when the processor is operating. Otherwise the CPUID has to be gained from the system BIOS saving the Reset Signature away early on in the boot process. This isn't done by all 486 BIOS versions, but as a general rule IBM PS/2s (and later IBM products) do.

The first digit of the 486 CPUID denotes the processor type on Intel CPUs. Other manufacturers like IBM (Where it is the letter "A") use it as a vendor indication. The only Pentium-class CPUs listed here are the "POD (Pentium Overdrive) for 486" (CPUID 153xh) that fit in an extension of the 486 socket. The "Second CPU" (CPUID 2xxxh) value does not come into definition here & is ignored:

 0 "Normal" CPU
 1 Overdrive (ODP) CPU
 2 Second CPU on a dual CPU system

The second digit denotes the CPU Family. This will be "4" for all the 486s on this page, with the sole exception of the PODs that have a Pentium-class core & able to run a Pentium instruction set. In those cases it will be "5".

The third digit denotes the particular 486 processor series, with the exception of the POD CPUs (The PODs multiply the base processor clock by 2.5 and have an onboard Floating Point Unit, or "FPU" math coprocessor):

 0  486DX-25, DX-33, or some 487SX (single clock speed with onboard FPU), with 8 KB WT L1 cache
 1  486DX-50 or later DX-33 (single clock speed with onboard FPU), with 8 KB WT L1 cache
 2  486SX (single clock speed without onboard FPU) or some 487SX, with 8 KB WT L1 cache
 3  486DX2 (clock doubled internal speed with FPU) with 8 KB WT L1 cache
 4  486SL (low-power 486DX in PQFP packaging) with 8 KB WT L1 cache
 5  486SX2 (clock doubled internal speed without FPU) with 8 KB WT L1 cache
 7  486DX2 with 8 K8 WB L1 cache
 8  486DX4 with 16 KB WT L1 cache (clock tripled internally with onboard FPU)
 9  486DX4 with 16 KB WB L1 cache

The fourth digit is an encoded value of the stepping depending on the processor series. I have listed the steppings for the different CPUIDs. In many cases I have only been able to narrow the CPUID and S-spec down to a range of stepping values. On a few Intel 486 processors with L1 cache that can be set to either Write-Through (WT) or Write-Back (WB) modes, the CPU will shift CPUIDs to show which mode the L1 cache is in at the time.

There can be confusion between the CPUID used on some Intel 486 processors, and those of it's competitor's 486 (AMD, Cyrix/TI, & UMC) models. In most cases the emulation of Intel CPUIDs was deliberate: They wanted the software unaware of whether it was running on a true Intel processor or their version. This is undoubtedly why Intel made the OTF CPUID function return a 'Vendor ID' string (Intel's value is "GenuineIntel"). For earlier 486 CPUs not supporting the Vendor ID there is a couple ways to tell the common CPUIDs apart. One is that Intel removed the 'UMOV' instruction that was on the 286 and 386 from their 486 CPUs. It is still present on the AMD models, although it has been made hard to detect in some cases. Cyrix/TI models also don't have the 'UMOV', but there are documented ways of telling them apart from other vendor's CPUs.

The Intel "S-spec" notation (only left off two 486 CPUs rebranded to IBM) is a 5-character marking in the form "S", uppercase letter (usually "X" on the 486 CPUs), digit, digit, digit. It is simply a batch version encoding and doesn't otherwise represent any processor capabilities on it's own. The numbers may not be necessarily ordinal, but for the early 486 CPUs they are somewhat in order of production. Each unit of a particular S-spec always has the same capabilities and CPU stepping, but there may be more than one S-spec for a particular CPUID or stepping. Intel also paired certain S-specs together, the lower value representing one speed and the next representing the other speed.

The 486 steppings cover a few different CPU core types. The original core is usually encoded by an uppercase letter followed by a digit (as in "A2"), although there was single uppercase letters (i.e. "D") at the end of the cycle. Later "SL Enhanced" cores (lower power requirements) are encoded lowercase letter, uppercase letter, digit (as in "aB0"). The last Intel 486 core had only one stepping, "A", that denoted the CPU had either a 8 or 16 KB L1 cache that is settable to Write-Back mode (earlier 486 have an 8 KB L1 cache in Write-Through mode). The POD cores (with two separate 8 KB data & code L1 caches, settable to Write-Back or Write-Through modes) are in a completely different stepping notation much like the early 486s: Uppercase letter-digit.

I know the purpose of only one 486 stepping change, although a little background is needed: When Intel initially produced the 32-bit 80386 (before even the "DX" marking came into place), they had added an instruction that was later removed (The 'XBTS', or 'Extract Bit String'). Since the instruction was only present in the 386 'A' step, some programmers began to check for it in software to know if the CPU was the 'A' step or a later 386 that had some initial bugs fixed. Intel inadvertently selected the same location of the removed 'XBTS' opcode for a new 'CMPXCHG' ('Compare, Exchange') 486 instruction. When the code to detect the 386 'A' step was falsely identifying the 486 chip 'A' step version, the CMPXCHG instruction was moved elsewhere for the 'B' step 486. I am currently working on code to detect the stepping based on the CMPXCHG opcode position when the BIOS CPUID & OTF CPUID (signifying an older 486) are not available.

CPUID OTF Processor S-spec © Date Stepping Notes
0400h No 486DX ?   A0, A1  
0401h No 486DX-25 SX308  '89 B2 - B6  
? No 486DX-25 SX250 '89 ?  
0402h No 486DX-25 SX328 '89 C0  
0402h No 486DX-33 SX329 '89 C0  
0403h No 486DX ?   C1  
0404h No 486DX-25 SX418 '89 D0  
0404h No 486DX-33 SX419 '89 D0  
0404h No 486DX-33 SX668 '89 D0  
0404h No 486DX-33 SX729 '89 D0  
0410h No 486DX ?   cA2, cA3  
0411h No 486DX-50 SX408 '90 cB0, cB1  
0411h No 486DX-50 SX546 '89 cB0, cB1  
0413h No 486DX-50 SX710 '89 cC0  
0413h No 486DX-33 SX829 '89 cC0  
0414h Yes 486DX-33 SX767 '89, '92 aA0, aA1 SL Enhanced
0415h Yes 486DX ?   aB0 SL Enhanced
? Yes 486DX-33 SX810 '89, '92 ?  
? No 486SX-20 SX406 '89 ? PGA packaging
? No 486SX-16 SX431 '89 ? PGA packaging
? No 486SX-16 SX545 '89 ? "LOWPOWER" stamp
? No 486SX-16 SX548 '89 ? PGA packaging
0420h No 486SX-25 SX411 '89 A0 PGA packaging
0420h No 486SX-20 SX467 '89 A0 PGA packaging
0420h No 486SX-25 SX468 '89 A0 PGA packaging
0420h No 487SX ? '89 A0  
? No ODP486SX-20 SZ675 '89 A2? PGA packaging
0421h No 487SX SZ494 '89 B0  
0422h No 486SX-25 SX673 '89 B0 PQFP packaging
0422h No 486SX-25 SX679 '89 B0 PGA packaging
0422h No 486SX-33 SX680 '89 B0 PGA packaging
0422h No 486SX-25 SX826 '89 B0 PGA packaging
? No 486SX-25 SX765 '89 ?  
? No 486SX-25 SX893 '89 ? PGA packaging
0423h Yes 486SX-33 SX797 '89, '92 D PGA packaging, SL Enhanced
0423h Yes 486SX-25 SX798 '89, '92 D PGA packaging, SL Enhanced
0423h Yes 486SX-33 SK798 '89, '92 D PGA packaging, SL Enhanced
0423h Yes 486SX-33 SX902 '89, '92 D PGA packaging, SL Enhanced
0423h Yes 486SX-25 SX903 '89, '92 D PGA packaging, SL Enhanced
0423h Yes 486SX-33 SK903 '89, '92 D PGA packaging, SL Enhanced
0424h Yes 486SX ?   gAx PGA, SL Enhanced, 3.3 volt
? No 486SX-33 SX724 '89 ? PQFP packaging
? Yes 486SX-25 SX763 '89, '92 ? PQFP packaging, PGA mount
0427h No 486SX ?   cA0 PQFP packaging
0428h No 486SX-25 SX683 '89 cB0 PQFP packaging
0428h No 486SX-33 SX684 '89 cB0 PQFP packaging
042Ah Yes 486SX-33 SX789 '89, '92 E, aA0, aA1 PGA & PQFP packaging, SL
042Ah Yes 486SX-25 SX790 '89, '92 E, aA0, aA1 PGA packaging
042Bh Yes 486SX-25 SX854 '89, '92 aB0, aC0 PQFP packaging
042Bh Yes 486SX-33 SX855 '89, '92 aB0, aC0 PQFP packaging
042Bh Yes 486SX-25 SX930 '89, '92 aB0, aC0 PGA packaging
0432h No 486DX2-50 SX626 '89 A0 - A2 Can be ODP486SX A2 step
0432h No 486DX2-50 SX627 '89 A0 - A2 Can be ODP486SX A2 step
0433h No 486DX2-50 SX641 '89 B1  
0433h No 486DX2-66 SX645 '89 B1  
0433h No 486DX2 SX731 '89 B1  
0433h No 486DX2-66 SX744 '89 B1  
0433h No 486DX2-50 SX749 '89 B1  
0433h No 486DX2-66 SX750 '89 B1  
? No 486DX2-40 SX722 '89 ?  
? No 486DX2-66 SX752 '89 ?  
? No 486DX2-66 SX759 '89 ?  
? ? 486DX2-66 SX762 '89 ?  
0433h No ODP486DX-25 SZ697 '89 B1 ODP, 5 volt
0433h No ODP486DX-33 SZ698 '89 B1 ODP, 5 volt
0433h No ODP486SX-25 SZ800 '89 B1  
0433h No ODP486SX-33 SZ801 '89 B1  
0433h No ODP486DX-33 SZ802 '89 B1 ODP, 5 volt
0433h No 486DX2 None '89 B1 Rebranded IBM, FRU 92F2582
0434h Yes 486DX2 ?   aA0, aA1 SL Enhanced
0435h Yes 486DX2-66 None '89, '92 aB0, aC0 Rebranded IBM, FRU 03H4939
0435h Yes 486DX2-66 SX807 '89, '92 aB0, aC0 SL Enhanced
0435h Yes 486DX2-50 SX808 '89, '92 aB0, aC0 SL Enhanced
0435h Yes ODP486SX-25 SZ874 '89, '92 AB0, aC0 ODP Version 3.0, SL, 5 volt
0435h Yes ODP486SX-33 SZ875 '89, '92 aB0, aC0 ODP Version 3.0, SL, 5 volt
0435h Yes ODP486DX-33 SZ878 '89, '92 aB0, aC0 ODP Version 3.0, SL, 5 volt
0435h Yes DX2ODP50 SZ902 '89, '92 aB0, aC0 ODP Version 3.0, SL, 5 volt
0435h Yes DX2ODP66 SZ903 '89, '92 aB0, aC0 ODP Version 3.0, SL, 5 volt
0435h Yes DX2ODPR66 SZ904 '89, '92 aB0, aC0 ODP Version 3.0, SL, 5 volt
0435h Yes DX2ODP50 SZ932 '89, '92 aB0, aC0 ODP Version 4.0, SL, 5 volt
0435h Yes DX20DP66 SZ933 '89, '92 aB0, aC0 ODP Version 4.0, SL, 5 volt
0435h Yes DX2ODPR50 SZ934 '89, '92 aB0, aC0 ODP Version 4.0, SL, 5 volt
0435h Yes DX2ODPR66 SZ935 '89, '92 aB0, aC0 ODP Version 4.0, SL, 5 volt
0435h Yes 486DX2-66 SX911 '89, '92 aB0, aC0 "i66" marking on underside, SL
0435h Yes 486DX2-50 SX912 '89, '92 aB0, aC0 "i50" marking on underside, SL
? Yes 486DX2-50 SX950 '89, '92 ? PQFP
0436h Yes 486DX2-50 SX954 '89, '92 AB0, aC0 "i50" marking on underside
0436h Yes 486DX2-66 SX955 '89, '93 A "i66" marking on underside
0440h No 486SL-25 SX741 '89, '92 A PQFP, Low-power 486DX
0441h No 486SL-33 SX805 '89, '92 A PQFP, Low-power 486DX
0441h No 486SL-25 SX806 '89, '92 A PQFP, Low-power 486DX
045Bh Yes 486SX2-50 SX845 '89, '92 aC0  
0470h Yes 486DX2-50 SX954 '89, '93 A 0436h CPUID in L1 WB mode
0470h Yes 486DX2-66 SX955 '89, '93 A 0436h CPUID in L1 WB mode
0480h Yes DX4ODP75 SU001 '89, '93 A ODP V1.1, 5 volt
0480h Yes DX4ODP100 SU002 '89, '93 A ODP V1.1, 5 volt
0480h Yes DX4ODPR75 SU003 '89, '93 A ODP V1.1, 5 volt
0480h Yes DX4ODPR100 SU004 '89, '93 A ODP V1.1, 5 volt
0480h Yes DX4ODP75 SZ956 '89, '93 A ODP V1.1, 5 volt
0480h Yes DX4ODP100 SZ957 '89, '93 A ODP V1.1, 5 volt
0480h Yes DX4ODPR75 SZ958 '89, '93 A ODP V1.1, 5 volt
0480h Yes DX4ODPR100 SZ959 '89, '93 A ODP V1.1, 5 volt
0480h Yes 486DX4-100 SK047 '89, '93 A 3 volt
0480h Yes 486DX4-100 SK050 '89, '93 A 3 volt
0480h Yes 486DX4-100 SK051 '89, '93 A 3 volt
0480h Yes 486DX4 SK053 '89, '93 A 3 volt
0480h Yes 486DX4 SK099 '89, '93 A 3 volt
0480h Yes 486DX4-75 SX870 '89, '93 A 3 volt, "Mobile" CPU
0480h Yes 486DX4-75 SX876 '89, '93 A 3 volt
0480h Yes 486DX4-100 SX877 '89, '93 A 3 volt
0480h Yes 486DX4-75 SX884 '89, '93 A 3 volt
0480h Yes 486DX4-100 SX900 '89, '93 A 3 volt
0480h Yes 486DX4-100 SX906 '89, '93 A 3 volt, "Mobile" CPU
0480h Yes 486DX4-100 SX896 '89, '94 A 3 volt
0483h Yes 486DX4-100 * '89, '94 A
0490h Yes 486DX4-100 SX158 '89, '94 A 3 volt, 16 KB WB L1
0490h Yes 486DX4-100 SX209 '89, '94 A 3 volt, 16 KB WB L1
0490h Yes 486DX4-100 SK896 '89, '94 A 3 volt, 16 KB WB L1
0490h Yes 486DX4-100 SK096 '89, '94 A 3 volt, 16 KB WB L1
1480h Yes DX4ODPR75 SZ925 '89, '93 A ODP V1.0, 5 volt
1480h Yes DX4ODPR100 SZ926 '89, '93 A ODP V1.0, 5 volt
1531h Yes POD63 SZ953 '89, '94 B1 Pentium core, 5 volt, ODP
1531h Yes POD63 SZ990 '89, '94 B2 Pentium core, 5 volt, ODP
1532h Yes POD63 SU013 '92, '94 C0 Pentium core, 5 volt, ODP
1532h Yes POD83 SU014 '92, '94 C0 Pentium core, 5 volt, ODP
* - Any of the CPUID 0490h in L1 WT mode
? - Unknown
OTF - retrieving the CPUID at any point - "On-The-Fly"


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