Content by David L. Beem (original HERE). Edited by Tomáš Slavotínek.
Additional data sourced from CPU-World and cpu-museo.it.
Last update: 07 Jul 2022
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
and is ignored:
# | Description |
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):
# |
Description |
0 |
486DX-25/-33 and some 487SX (single clock w/ FPU), 8 KB WT L1 cache |
1 |
486DX-50 and later DX-33 (single clock w/ FPU), 8 KB WT L1 cache |
2 |
486SX (single clock w/o FPU) and some 487SX, 8 KB WT L1 cache |
3 |
486DX2 (clock doubled w/ FPU), 8 KB WT L1 cache |
4 |
486SL (low-power 486DX, PQFP packaging), 8 KB WT L1 cache |
5 |
486SX2 (clock doubled w/o FPU), 8 KB WT L1 cache |
7 |
486DX2, 8 KB WB L1 cache |
8 |
486DX4 (clock tripled w/ FPU), 16 KB WT L1 cache |
9 |
486DX4, 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 its 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 its 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 A-step 386 was falsely identifying
the 486 chip A-step version, the CMPXCHG instruction was moved elsewhere for
the B-step 486.
CPUID |
OTF |
Processor |
S-spec |
© Date |
Stepping |
Notes |
0400h |
No |
486DX |
? |
|
A0, A1 |
|
0401h |
No |
486DX-25 |
SX308 |
'89 |
B2 - B6 |
|
? |
No |
486DX-25 |
SX249 |
'89 |
? |
|
? |
No |
486DX-25 |
SX250 |
'89 |
? |
|
? |
No |
486DX-25 |
SX493 |
'89 |
? |
"LOWPOWER" stamp |
? |
No |
486DX-33 |
None |
'89 |
? |
IBM brand, FRU P/N 63G9145 |
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 |
0414h |
? |
486DX-33 |
SX778 |
'89, '92 |
|
PQFP |
0415h |
Yes |
486DX |
? |
|
aB0 |
SL Enhanced |
0415h |
Yes |
486DX-33 |
SX810 |
'89, '92 |
? |
|
|
0420h |
No |
486SX-20 |
SX406 |
'89 |
? |
PGA |
0420h |
No |
486SX-16 |
SX431 |
'89 |
? |
PGA |
0420h |
? |
486SX-16 |
SX540 |
|
? |
PGA |
? |
No |
486SX-16 |
SX545 |
'89 |
? |
"LOWPOWER" stamp |
? |
No |
486SX-16 |
SX548 |
'89 |
? |
PGA |
0420h |
No |
486SX-25 |
SX411 |
'89 |
A0 |
PGA |
0420h |
No |
486SX-20 |
SX467 |
'89 |
A0 |
PGA |
0420h |
No |
486SX-25 |
SX468 |
'89 |
A0 |
PGA |
0420h |
No |
487SX |
? |
'89 |
A0 |
|
? |
No |
ODP486SX-20 |
SZ675 |
'89 |
A2? |
PGA |
0421h |
No |
487SX |
SZ494 |
'89 |
B0 |
|
0422h |
No |
486SX-25 |
SX673 |
'89 |
B0 |
PQFP |
0422h |
No |
486SX-25 |
SX679 |
'89 |
B0 |
PGA |
0422h |
No |
486SX-33 |
SX680 |
'89 |
B0 |
PGA |
0422h |
No |
486SX-25 |
SX826 |
'89 |
B0 |
PGA |
? |
No |
486SX-25 |
SX765 |
'89 |
? |
PQFP, 3.3 volt |
? |
No |
486SX-25 |
SX893 |
'89 |
? |
PGA |
0423h |
Yes |
486SX-33 |
SX797 |
'89, '92 |
D |
PGA, SL Enhanced |
0423h |
Yes |
486SX-25 |
SX798 |
'89, '92 |
D |
PGA, SL Enhanced |
0423h |
Yes |
486SX-33 |
SK798 |
'89, '92 |
D |
PGA, SL Enhanced |
0423h |
Yes |
486SX-33 |
SX902 |
'89, '92 |
D |
PGA, SL Enhanced |
0423h |
Yes |
486SX-25 |
SX903 |
'89, '92 |
D |
PGA, SL Enhanced |
0423h |
Yes |
486SX-33 |
SK903 |
'89, '92 |
D |
PGA, SL Enhanced |
0424h |
Yes |
486SX |
? |
|
gAx |
PGA, SL Enhanced, 3.3 volt |
? |
No |
486SX-33 |
SX724 |
'89 |
? |
PQFP |
? |
Yes |
486SX-25 |
SX763 |
'89, '92 |
? |
PQFP, PGA mount |
0427h |
No |
486SX-25 |
SX586 |
'89 |
cA0? |
PQFP |
0428h |
No |
486SX-25 |
SX683 |
'89 |
cB0 |
PQFP |
0428h |
No |
486SX-33 |
SX684 |
'89 |
cB0 |
PQFP |
042Ah |
Yes |
486SX-33 |
SX789 |
'89, '92 |
E, aA0, aA1 |
PGA & PQFP, SL |
042Ah |
Yes |
486SX-25 |
SX790 |
'89, '92 |
E, aA0, aA1 |
PGA |
042Bh |
Yes |
486SX-25 |
SX854 |
'89, '92 |
aB0, aC0 |
PQFP |
042Bh |
Yes |
486SX-33 |
SX855 |
'89, '92 |
aB0, aC0 |
PQFP |
042Bh |
Yes |
486SX-25 |
SX930 |
'89, '92 |
aB0, aC0 |
PGA |
|
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 |
? |
|
0433h |
No |
486DX2-66 |
SX759 |
'89 |
? |
|
0433h |
? |
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 |
IBM brand, FRU P/N 92F2582 |
0434h |
Yes |
486DX2 |
? |
|
aA0, aA1 |
SL Enhanced |
0435h |
Yes |
486DX2-50 |
None |
'89, '92 |
aB0, aC0 |
IBM brand, FRU P/N 03H4938 |
0435h |
Yes |
486DX2-66 |
None |
'89, '92 |
aB0, aC0 |
IBM brand, FRU P/N 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 v3.0, SL, 5 volt |
0435h |
Yes |
ODP486SX-33 |
SZ875 |
'89, '92 |
aB0, aC0 |
ODP v3.0, SL, 5 volt |
0435h |
Yes |
ODP486DX-33 |
SZ878 |
'89, '92 |
aB0, aC0 |
ODP v3.0, SL, 5 volt |
0435h |
Yes |
DX2ODP50 |
SZ902 |
'89, '92 |
aB0, aC0 |
ODP v3.0, SL, 5 volt |
0435h |
Yes |
DX2ODP66 |
SZ903 |
'89, '92 |
aB0, aC0 |
ODP v3.0, SL, 5 volt |
0435h |
Yes |
DX2ODPR66 |
SZ904 |
'89, '92 |
aB0, aC0 |
ODP v3.0, SL, 5 volt |
0435h |
Yes |
DX2ODP50 |
SZ932 |
'89, '92 |
aB0, aC0 |
ODP v4.0, SL, 5 volt |
0435h |
Yes |
DX20DP66 |
SZ933 |
'89, '92 |
aB0, aC0 |
ODP v4.0, SL, 5 volt |
0435h |
Yes |
DX2ODPR50 |
SZ934 |
'89, '92 |
aB0, aC0 |
ODP v4.0, SL, 5 volt |
0435h |
Yes |
DX2ODPR66 |
SZ935 |
'89, '92 |
aB0, aC0 |
ODP v4.0, 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 |
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 |
Various |
'89, '94 |
A |
Any 0490h CPU in WT mode |
|
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 |
OTF - retrieving the CPUID at any point - "On-The-Fly"
Further Reading
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