pcjs/blog/2015/02/23

Early 80386 CPUs

Assembling a detailed and accurate history of the 80386, including a complete listing of all the "steppings" (revisions), when they were released, what "errata" (problems) each stepping suffered from, and which of those problems were fixed by a later stepping, seems virtually impossible at this late date.

I won't make the attempt here, either. I'm just going to throw together everything I know into one pile, as I begin adding 80386 support to PCjs.

Let's start with steppings (revision levels), then move on to errata, and finally undocumented and/or deprecated instructions.

Steppings

As the "INTEL 80386 PROGRAMMER'S REFERENCE MANUAL 1986", section 10.1, explains:

The contents of EAX depend upon the results of the power-up self test. The self-test may be requested externally by assertion of BUSY# at the end of RESET. The EAX register holds zero if the 80386 passed the test. A nonzero value in EAX after self-test indicates that the particular 80386 unit is faulty. If the self-test is not requested, the contents of EAX after RESET is undefined.

DX holds a component identifier and revision number after RESET as Figure 10-1 illustrates. DH contains 3, which indicates an 80386 component. DL contains a unique identifier of the revision level.

But what steppings did Intel produce, and what "revision level" was associated with each stepping?

I've read reports that the first stepping (A0) had serious problems, like an inability to return to real mode (reminiscent of the 80286). Since it was never released commercially, and we know so little about it, let's move on to the A1 stepping.

From "Advanced 80386 Programming Techniques" by James L. Turley (C) 1988:

Evolution of the 80386

A1 Stepping

This is the first revision after the initial (A0) introduction of the 80386 [in late 1985]. The A1 stepping became available around the first of the year in 1986.

• Hardware Peculiarity: Unless pin F13 of the 80386 is connected to the +5V power supply, the 80386 never terminates a memory cycle, hanging the processor.

• Loading LDTR with a Null Selector: If you load the local descriptor table register with a null selector (0000 through 0003) without performing a memory read immediately afterward, the 80386 behaves erratically. LDTR can be loaded either as the result of an LLDT instruction or as part of a task-switch operation if the incoming TSS has a null selector in its LDT field.
• Loading Bad Stack Selectors: The 80386 does not check the privilege level of the desired stack segment selector during an LSS instruction. Normally, the RPL of the selector and the DPL of the segment must exactly equal the DPL of the currently executing code segment.
• Misaligned Selectors: If a 16-bit memory operand is loaded into a segment register, the 80386 hangs if the selector is not word-aligned. This can happen with a MOV instruction and with the LDS, LES, LFS, LGS, and LSS instructions.
• Testing Null Selectors: The 80386 hangs if an LAR, LSL, VERR, or VERW instruction is used to test a null selector.
• Popping Selectors: When popping a segment selector from the stack, the 80386 performs all of the privilege-related checks backward. That is, you are allowed to pop a selector to a segment with more privilege, but not to one with less privilege.
• Indirect FAR JMP to Same Privilege Level: An indirect JMP instruction is one in which the new segment selector and offset are stored in memory. If the new code segment is at the same privilege level as the current code segment, the 80386 does not read the selector portion of the JMP operand. The result is a NEAR JMP to a new offset within the current code segment.
• Spurious Breakpoint Exceptions: A MOV instruction into or out of the debug registers while hardware breakpoints are enabled may cause a breakpoint fault to be reported. You should temporarily disable the hardware breakpoints while reading or writing the debug registers and then execute a JMP instruction to flush the prefetch queue before reenabling them.
• Successive Floating-Point Instructions: If two floating-point instructions are executed close together, the 80386 may force the coprocessor to start the second one too soon if the first one did not require any memory operands.
• Bad Floating-Point Instructions: The execution of certain undefined floating-point instructions causes the 80386 to hang rather than reporting an invalid instruction fault.
• Misaligned Floating-Point Instructions: If 80287 and/or 80387 instructions are not word-aligned, the 80386 passes the wrong instruction to the coprocessor, causing unpredictable behavior.
• Misaligned Descriptor Tables: All segment descriptors should be dword-aligned, or the 80386 may not read them correctly.
• Incorrect Interrupt Vector: If a maskable interrupt occurs immediately after the 80386 has executed an instruction with an 8-bit operand, the interrupt is always assigned a vector number of 0.
• Handling Exceptions with 80286 Tasks: If an exception that pushes an error code is handled through a task gate to an 80286 task, the 80386 generates a double fault (exception 8) as soon as the 80286 task switch is completed.
• Page Fault During Task Switch: If the new task uses different page tables than the old task and a page fault occurs during the task-switch operation, the 80386 hangs instead of reporting the page fault.
• Returning from a Nested Task: If you execute an IRET instruction when the NT (Nested Task) flag in EFLAGS is set, the 80386 returns to your "parent" task. However, it will neglect to clear NT in the outgoing task's TSS.
• Task-Switch Trap Bit: When a task switch occurs and the incoming TSS has its task-switch bit set, the 80386 generates a debug fault (exception 1). However, it will neglect to set the BT flag in register DR6 before invoking the exception 1 handler.
• Invalid TSS Faults: When an invalid TSS fault (exception 10) occurs as the result of a task-switch operation, the 80386 pushes the wrong values for EIP and ESP on the error handler's stack.
• Wrong Paging Information During Task Switch: When a task switch occurs and the new task uses a different set of page tables than the old task (that is, CR3 in the TSS is different), the 80386 reads the new TSS descriptor and updates the old TSS using the page tables of the new task.
• Masking NMI: If an NMI is serviced through a task gate, the 80386 neglects to reenable NMI inputs after the NMI-handling task terminates. The 80386 does not recognize NMI inputs while it is servicing an NMI.
• Incorrect Error Code: If an exception that pushes an error code is handled through a task gate, the 80386 may push an error code of 0 instead of the correct error code.
• Not-Present LDT: If a task switch occurs and the new task's TSS selector selects an LDT descriptor that is marked no present, the 80386 reports a not-present fault (exception 11), instead of an invalid TSS (exception 10).
• Self-test: The self-test feature does not work on the A1 stepping of the 80386.
• Interlevel Transfers: If you perform a control transfer to a higher privilege level and the procedure at that level uses an expand-down stack segment for its stack, as opposed to an expand-up data segment, the 80386 generates a spurious stack fault (exception 12).
• Debug Access Fault: If an attempt is made to read or write the debug registers while GD is set in DR7, the 80386 generates a debug fault (exception 1). However, the wrong value for EIP will be pushed onto the exception handler's stack.
• Multiply and Divide Errors: If the 80386 executes a MUL, IMUL, DIV, or IDIV instruction with a memory operand and general protection fault (exception 13) occurs, the processor hangs rather than reporting the fault.
• Intersegment Transfers: If the last 2 bytes of a direct FAR JMP or FAR CALL instruction are beyond the end of the code segment, the 80386 hangs rather than reporting a general protection fault.
• Intersegment Transfers II: If the last 2 bytes of a direct FAR JMP or FAR CALL instruction lie on the other side of a page boundary, the 80386 hangs, regardless of whether the other page is present or not. Remember, a page boundary occurs every 4kb.
• Bit-Test Instructions: The bit-test instructions that perform read-modify-write cycles to memory (BTC, BTR, and BTS) do not work properly unless your memory runs without wait states.
• Bit Scan Forward Instruction: If a BSF instruction is executed with a memory operand, and a general protection fault (exception 13) or a page fault (exception 14) occurs, the 80386 hangs rather than reporting the fault.
• Accessing CR3, TR6, and TR7: Reading or writing registers CR3, TR6, or TR7 produces incorrect results if your memory requires wait states. This is particularly obscure, since these instructions do not reference memory.

B0 Stepping

The B0 stepping became available around the middle of 1986. As is the case with earlier steppings, the "B0" marking can sometimes be found on the outside of the device. If not, the code "S40336" or "S40337" may appear. Starting with the B0 stepping, the 80386 would leave the component identifier, 3, in DH and the revision identifier, also 3, in DL [These B0 values need to be independently confirmed, because they are the same values reported for the B1 stepping. -JP]

• Interrupts and Privilege Violations: If a hardware interrupt occurs immediately before an IOPL-sensitive instruction when CPL is greater than IOPL, the 80386 behaves erratically. Generally, it will produce spurious stack faults (exception 12). Note that this occurs only if the IOPL-sensitive instruction was going to fail.

• Infinite Page Faults: If a page fault (exception 14) occurs while the 80386 is attempting to invoke a page fault handler, it generates another page fault rather than reporting a double fault (exception 8).
• Invalid TSS Fault: If an invalid TSS fault (exception 10) is caused by an attempt to switch to a TSS that is too small, and the exception is handled through a task gate, the 80386 shuts down.
• Invalid TSS Fault II: If you execute an IRET instruction while the NT (Nested Task) flag is set in EFLAGS, and the "parent" TSS is too small, the 80386 generates a double fault (exception 8).
• Invalid TSS Fault III: If the previous error occurs and the double fault is handled through a trap gate, the 80386 shuts down.
• Invalid TSS Fault IV: If the gate descriptor for an invalid TSS fault (exception 10) is bad and an invalid TSS fault occurs for any reason, the 80386 shuts down.
• Tracing a REP MOVS Instruction: The 80386 does not correctly single-step (trace) repeated string move instructions. Instead, it generates a debug trap after every other iteration of the repeated MOVS instruction.
• Breakpointing a REP MOVS Instruction: If a data breakpoint is enabled and it is hit during a repeated string move instruction, the 80386 reports it only after an even number of iterations. This gives you a fifty-fifty chance of learning about your breakpoint one instruction late.
• 16-Bit and 32-Bit Control Transfers: Whenever control is transferred from 16-bit code to 32-bit code through a task gate or through a gate that causes a change in privilege level, the 80386 discards the upper half of the offset address in EIP. If the destination address was greater than 64Kb, this will cause a program error.
• 16-Bit and 32-Bit Control Transfers II: When an IRET instruction that transfers control to a Virtual 8086 mode task is executed, the 80386 neglects to truncate the offset address on the stack to 16 bits.
• 16-Bit and 32-Bit Control Transfers III: If a 32-bit call, trap, or interrupt gate is used to transfer control from a 16-bit code segment to a 32-bit code segment without changing privilege levels, the 80386 treats the gate as a 16-bit gate.
• Mixing Address Sizes: If a LOOP, MOVS, LODS, STOS, CMPS, INS, or OUTS instruction is followed by an instruction that uses a different address size, the 80386 may use the incorrect address size when updating either the loop counter or the string index.

B1 Stepping

The B1 stepping became available in late 1986. It corrected many of the bugs in earlier versions, but new ones were either introduced or discovered. The best known of these was the widely publicized multiply failure discovered in mid-1987. The B1 stepping is identifiable either by the "B1" mark or by the code "S40343," "S40344," or "S40362." As is the case with the B0 stepping, the B1 revision leaves a binary three in DL after reset. [Again, I would like to see the B0 values independently confirmed. -JP]

• IBTS and XBTS Instructions Removed: The Insert Bit String (IBTS) and Extract Bit String (XBTS) instructions were removed from the 80386's instruction set. It was determined that they took up too much space on the microprocessor and that their functionality could be duplicated with the SHLD and SHRD instructions. The opcodes 0F A6 and 0F A7 now produce invalid opcode faults.

• Multiplication Errors: Certain 80386 microprocessors produce erroneous results when performing multiplication. Not all B1 stepping 80386s suffer from this bug. It is aggravated by increases in the processor's operating frequency, elevations in the ambient temperature, or decreases in the power supply voltage. This failure is extremely pattern sensitive; certain patterns will produce errors readily, while others never will. [See sample program below]
• Double Page Faults: The bug that appeared in the B0 stepping regarding page faults that occurred during page faults has been made a permanent feature of the 80386, with one minor change. If a third page fault occurs while the processor is servicing the first two, the 80386 shuts down. [See Intel Errata #18 below]
• Disabling Page Translation: The 80386 does not stop translating linear addresses to physical addresses when paging is disabled. Any page-translation entries that are still in the cache will be used, regardless of the setting of PG in CR0. To completely disable paging, flush the TLB by clearing CR3.
• Page Translation Affects I/O Addresses: When paging is enabled, the MMU sometimes erroneously translates I/O addresses above 0FFF as well as memory addresses. Coprocessor references (which appear in the I/O space) are also affected. The I/O addresses are translated as though they were linear memory addresses, using the memory translation tables cached in the TLB. If the "linear" I/O address is not in the cache, no translation will take place; only cached entries produce this effect. [See Intel Errata #10 below]
• Page Fault Error Codes: Under certain circumstances, the 80386 pushes an incorrect error code onto the page fault handler's stack. [See Intel Errata #9 below]
• Four-Gigabyte Code Segments: If you define a 4Gb code segment (limit = FFFFF, G = 1), the base address of that segment must be dword-aligned, or the 80386 generates a general protection fault (exception 13) when it fetches an instruction from the beginning of the segment. This feature is expected to become permanent.
• Wrong Loop Counter: After a REP INS instruction finishes its last iteration, register ECX holds the value FFFFFFFF instead of 0, if the next instruction after the REP INS references memory. [See Intel Errata #11 below]
• LSL Instruction and Stack Pointer: If the LSL instruction is followed by an instruction that references the stack, register ESP may become corrupted. [See Intel Errata #14 below]
• Not-Present LDT: If a task switch occurs to a Virtual 8086 mode task and the incoming task's TSS holds a selector to an LDT descriptor that is marked not present, the 80386 generates a not-present fault (exception 11) rather than reporting an invalid TSS fault (exception 10). [See Intel Errata #16 below]
• Reading from CR3, TR6, or TR7: If hardware breakpoints are enabled, reading from CR3, TR6, or TR7 may cause spurious debug faults to be reported. It is recommended that you disable breakpoints and then execute the MOV instruction followed by a JMP instruction before reenabling breakpoints.
• Privilege Checking a Null Selector: If you perform an LAR, LSL, VERR, or VERw instruction using a null selector (0000 through 0003), the 80386 actually checks the descriptor in slot 0 of the GDT instead of always failing. [See Intel Errata #15 below]
• Privilege Checking Bad Selectors: An LAR, LSL, VERR, or VERW instruction that checks an unreachable selector causes the 80386 to hang unless there is a JMP, CALL, or memory-related instruction already in the prefetch queue. An unreachable selector is one that either is beyond the limit of its descriptor table or references a non-existent LDT. The processor will remain hung until it receives an interrupt.
• Faulting Floating-Point Instructions: If the second byte of a floating-point instruction is located in the first byte of a page that will cause a page fault (either because it is not present or because of an impending privilege violation), the 80386 hangs. The processor will remain hung until it receives an interrupt. [See Intel Errata #17 below]

The following sample program has been calculated to produce the [aforementioned multiplication] error. An 80386 that fails one or more of these multiply instructions is obviously faulty. However, passing does not guarantee a perfect part. To their credit, Intel agreed to test all 80386s for a limited time and report on their success or failure. Since then, all 80386s have been tested before shipping. Those that fail have been marked "For Sixteen-Bit Software Only." [To be exact: "16 BIT S/W ONLY" -JP] Those that passed have been marked with a double sigma sign. All 80386s produced after the B1 stepping should be free of this defect.

; Perform various 16-bit and 32-bit multiply operations...

K1      DD      41h                 ;memory-based constant 1
K2      DD      81h                 ;memory-based constant 2

		MOV     EAX,0042E8h         ;load EAX with operand
		MUL     K1                  ;EAX = EAX * 41H
		CMP     EAX,10FCE8h         ;check answer
		JNE     fail                ;failure if not equal

		MOV     EAX,085D00h
		MUL     K1
		CMP     EAX,021F9D00h
		JNE     fail

		MOV     EAX,042E80000h
		MUL     K1
		CMP     EAX,0FCE80000h
		JNE     fail
		CMP     EDX,010h
		JNE     fail

		MOV     EAX,0417A000h
		MUL     K2
		CMP     EAX,0FE7A000h
		JNE     fail
		CMP     EDX,0002h
		JNE     fail

		MOV     DX,0AB66h
		MOV     AX,09AE8h
		MUL     DX
		CMP     AX,0B070h
		JNE     fail

		MOV     DX,0FDF3h
		MOV     AX,09AE8h
		MUL     DX
		CMP     AX,05238h
		JNE     fail

		MOV     DX,0B554h
		MOV     AX,0E8EAh
		MUL     DX
		CMP     DX,0A4F9h
		JNE     fail

		MOV     DX,0B4C6h
		MOV     AX,0E8EAh
		MUL     DX
		CMP     AX,0ACFCh
		JNE     fail
		CMP     DX,0A478h
		JNE     fail

		MOV     DX,0B318h
		MOV     AX,0E8EAh
		MUL     DX
		CMP     DX,0A2F1h
		JNE     fail

		MOV     DX,0B43Bh
		MOV     AX,0E8EAh
		MUL     DX
		CMP     DX,0A3FAh
		JNE     fail

---

As for the B0 stepping, the October 15, 1991 issue of PC Magazine confirms what Turley noted above:

You can tell if you have a B0 or B1 Step level 386 by looking at the markings on the chip.
If it has the ID number S40336 or S40337 stamped on it, then it's a Step B0; if it's marked
with S40343, S40344, or S40362, it's a Step B1. Some B0 and B1 chips were marked B0 or B1
rather than with an ID number.

Most of the information I have obtained about the 80386 begins with the B1 stepping. From a December 17, 1986 Intel document titled "80386-B1 STEPPING INFORMATION":

80386-B1 component identifier readable in DH after reset: 03H
80386-B1 revision  identifier readable in DL after reset: 03H

At this time, B1 stepping parts are identified with one of the marks shown
below:

                                       |
            ii                         |    ii
                                       |
            ii  A80386-16              |    ii  A80386-20
            ii  S40344                 |    ii  S40362
            ii  (FPO number)           |    ii  (FPO number)
            ii   m  c  i '85 '86       |    ii   m  c  i  '85 '86
                                       |
    ----------------------------------- ----------------------------------
                                       |
            ii                         |
                                       |
            ii  A80386 ES B1           |
            ii                         |
            ii                         |
            ii   m  c  i  '85 '86      |
                                       |

So, the B1 stepping set DL to 0x03 on reset. This would lead one to believe that the revision number for a B0 stepping was 0x02, but according to Turley (see above), the B0 and B1 steppings report the same revision; as I noted above, it would be nice to see some independent confirmation.

The 80386 CPU on my Compaq DeskPro 386 "Version 2" System Board is labeled as:

            A80386-16
            S40344
            L8260347
            (m)(c)i '85 '86

The "S40344" indicates that it's a B1 stepping, although I'm not currently able to power it and run any tests.

According to OS/2 Museum proprietor Michal Necasek, the "L8260347" means:

[I]t was actually manufactured in mid-1988. It took me many years to realize how Intel marked the chips.
The initial 'L' (or whatever number/letter it is) can be ignored and the following 3 digits (826) are the
date code. The first is the last digit of the year.... The last two digits are the week of manufacture,
so week 26 of 1988. Many Intel CPUs actually have 2 or 3 date codes on them, probably related to when the
die was made, when the package was assembled, and when it was tested/binned and stamped. On your 386,
there might be another date code on the bottom side, but if there is, it's not going to be too far apart
from the one on the top and it's going to be slightly older.

Fortunately, my 80386-B1 CPU is also marked with a "ΣΣ" (double sigma), which is how Intel marked parts that tested safe for 32-bit multiplication. Some 80386 CPUs suffered from a manufacturing defect that could occasionally result in multiplication errors; defective parts that Intel caught were marked with "16 BIT S/W ONLY" instead of the double sigma.

The only other information I have about revision levels comes from a March 30, 1987 document titled "80386-C0 STEPPING INFORMATION":

80386-C0 component identifier readable in DH after reset: 03H
80386-C0 revision  identifier readable in DL after reset: 04H

However, that document does not indicate how a 80386-C0 part was marked, and I've found very little other information on the C0 stepping; it may have been quickly superseded by the D0 stepping, which used revision identifier 0x05.

Errata

It's fair to say that the 80386 B1 stepping had a lot of problems. Although, for a CPU of significantly greater complexity than its predecessors, that doesn't seem terribly surprising.

From the aforementioned December 17, 1986 document, here's what the world knew about 80386-B1 problems at that time:

1. Opcode Field Incorrect for FSAVE and FSTENV
2. FSAVE, FRESTOR, FSTENV and FLDENV Anomolies [sic] with Paging
3. Wraparound Coprocessor Operands
4. IRET to TSS with Limit too Small
5. Single-Stepping First Iteration of REP MOVS
6. Task Switch to Virtual 8086 Mode Doesn't Update Prefetch Limit
7. Wrong Register Size for String Instructions in Mixed 16/32-bit Addressing Systems
8. FAR Jump Located Near Page Boundary in Virtual 8086 Mode Paged Systems
9. Page Fault Error Code on Stack Not Reliable
10. Certain I/O Addresses Incorrect when Paging is Enabled
11. Wrong ECX Update by REP INS
12. NMI Doesn't Always Bring Chip Out of Shutdown in Obscure Condition with Paging Enabled
13. HOLD Input During Protected Mode Interlevel IRET when Paging is Enabled
14. Protected Mode LSL Instruction Should not be Followed by PUSH/POP
15. LSL/LAR/VERR/VERW Instructions Malfunction with Null Selector
16. "Not Present" LDT in VM86 Task Raises Wrong Exception
17. Coprocessor Instructions Crossing Page/Segment Boundaries
18. Double Page Faults Do Not Raise Double Fault Exception

An errata update dated March 26, 1987, produced internally by IBM rather than Intel, noted two additional issues:

• Maximum Sized Segments Need Alignment
• CR3/TRx Move Corrupts LIP (Linear Instruction Pointer)

As an aside, an 80386-C0 document dated March 30, 1987 lists none of the above errata, suggesting that the C0 stepping fixed all the above problems -- with the exception of #18, which was apparently reclassified from errata to "feature":

Double Page Faults Do Not Raise Double Fault Exception

Problem: If a second page fault occurs, while the processor is attempting to enter the service routine for the first, then the processor will invoke the page fault (exception 14) handler a second time, rather than the double fault (exception 8) handler. A subsequent fault, though, will lead to shutdown.

Workaround: No workaround is necessary in a working system.

On April 30, 1987, another errata update omitted #18, as well as the two new issues documented by IBM (which, like #18, Intel must have decided were not significant problems "in a working system"), and added the following:

• Breakpoints Malfunction after Reading CR3, TR6, or TR7
• Return Address Incorrect for Segment Limit Fault during FNINIT

On September 1, 1987, the final 80386-B1 errata update that I'm aware of continued to list previous errata (#1-#17), plus the two errata from April 30 (#18 and #19), along with the following new errata:

• VERR/VERW/LAR/LSL Instructions Malfunction with Bad Selector
• Coprocessor Malfunctions with Paging Enabled

This brought the total number of 80386-B1 errata to 21.

There was also a manufacturing problem that caused 32-bit multiplications to fail on some parts, which Intel publicly acknowledged in April 1987.

From the San Jose Mercury News, April 11, 1987:

INTEL DISCOVERS A BUG IN ITS NEW 80386 CHIP
Author(s): CHRISTOPHER H. SCHMITT AND JIM BARTIMO, Mercury News
Business Writers Date: April 11, 1987 Section: Business

Intel acknowledged Friday that a bug has cropped up in its new flagship microprocessor chip.

Microprocessors serve as the electronic brains of devices ranging from personal computers to
home appliances.

Intel said the bug in its 80386 microprocessor produces incorrect answers when the chip
performs certain mathematical operations. "When you multiply one 32-bit number by another
32-bit number, you get the error," said Intel spokesman Jim Jarrett. A 32-bit number is one
that utilizes the full data path of the chip.

However, operating system software that would let the chip fully use its 32-bit characteristics
is still being developed, so the problem shouldn't immediately affect personal computer users.
Microsoft Corp. of Redmond, Wash., has announced its intention to deliver in early 1988 a 32-bit
operating system known as OS/2.

Even when using the 32-bit capabilities of the chip, the possibility of an error is "rare,"
Jarrett said.

Spokesman Bruce LeBoss said not all of the 100,000 80386 chips Intel has produced so far are
affected, but he declined to elaborate. Intel is sticking with a previously announced estimate
that it will sell from 500,000 to 1 million 80386s this year.

To address the problem, Intel said it is notifying computer companies and other customers and
extending the chip's warranty from one to two years. Anticipating that it will replace some
80386s already in the field, the company charged off an undisclosed amount to cover the
expense. One analyst estimated the charge at about $7 million.

Intel will test the chips it has already sold to determine which ones have the bug. But it will
not replace the chips for free. Instead, Intel will work with customers on an upgrade program.

Intel said it will be up to individual computer makers to work with consumers who have
purchased machines using the 80386 chip.

The problem may be most acutely felt by Houston-based Compaq Computer, which so far has
been the biggest producer of 80386-based computers. Compaq said it will disclose by early
next week plans for coping with the possibility that versions of its Deskpro 386 computer model
contain flawed chips.

The bug may cause some shortages of the 80386. The problem won't be fixed until July, so the
only guaranteed chips are those that have already been produced and have passed Intel's test.
"Everybody wants perfect chips and we'll have fewer of those to go around for a while," Jarrett
said. Analysts indicated that in the long run, the bug is more likely to be embarrassing than
anything else. "You have a complex chip here," said Daniel Klesken, an analyst for Montgomery
Securities in San Francisco. "One should not be surprised that (problems) happen occasionally."

The problem affected the B1 stepping; it's unknown whether other (earlier) steppings were affected as well.

What follows is some additional information about early 80386 processors from various online sources.

---

Excerpt from "CPU Identification by the Windows Kernel", by Geoff Chappell:

80386

Finer identification of 80386 processors is largely academic. Whatever the model or stepping, the 80386 processor is unsupported since [Windows NT] version 4.0, and soon causes the bug check UNSUPPORTED_PROCESSOR (0x5D), though not without the kernel having worked its way through more tests for defects to identify models and steppings. For any 80386 processor that passes all tests, the model and stepping leap ahead to 3 and 1. Version 3.51, which was the last to support the 80386 (and only then in a single-processor configuration), rejects any 80386 that does not pass all these tests.

Family  Model   Stepping    Test
------  -----   --------    ----
  3       0       0         32-bit MUL not reliably correct
  3       1       0         supports XBTS instruction
  3       1       1         set TF bit (0x0100) in EFLAGS causes Debug exception (interrupt 0x01) only at completion of REP MOVSB
  3       3       1

The particular multiplication that distinguishes model 0 is of 0x81 by 0x0417A000. This same test was used by Microsoft at least as far back as Windows 3.10 Enhanced Mode, to advise:

The Intel 80386 processor in this computer does not reliably execute 32-bit
multiply operations. Windows usually works correctly on computers with this
problem but may occasionally fail. You may want to replace your 80386 processor.
Press any key to continue...

The instruction whose support is tested for model 1 stepping 0 has opcode bytes 0x0F 0xA6 followed by a Mod R/M byte and by whatever more this byte indicates is needed for the operand. This opcode is disassembled as XBTS by Microsoft’s DUMPBIN utility from Visual C++, and has been since at least the mid-90s. However, the same opcode was apparently reused for the CMPXCHG instruction on some 80486 processors. The confusion seems to have left a lasting mark: Intel’s opcode charts leave 0x0F 0xA6 unassigned even now. The specific test performed by the Windows kernel is to load EAX and EDX with zero and ECX with 0xFF00. If executing XBTS ECX,EDX does not cause an Invalid Opcode exception and clears ecx to zero (which CMPXCHG ECX,EDX would not), then XBTS is deemed to be supported and the processor is model 1 stepping 0. This case of 80386 processor also was known to Windows 3.10 Enhanced Mode, and was rejected as fatal:

Windows may not run correctly with the 80386 processor in this computer.

Upgrade your 80386 processor or start Windows in standard mode by typing
WIN /s at the MS-DOS prompt.

When string instructions such as MOVSB are repeated because of a REP prefix, each operation is ordinarily interruptible. As Intel says (for REP in the Intel 64 and IA-32 Architectures Software Developer’s Manual Volume 2B: Instruction Set Reference N-Z), this “allows long string operations to proceed without affecting the interrupt response time of the system.” It ordinarily applies also to the Debug exception, such as raised by the processor at the end of executing an instruction for which the TF bit is set in the EFLAGS when the instruction started. Programmers may have noticed this in the real world of assembly-language debugging. If the debugger actually does implement its trace command as a trace, as opposed to setting an INT 3 breakpoint where the instruction is calculated to end, then a two-byte REP MOVSB may take many keystrokes to trace through! That model 1 stepping 1 traces through a REP MOVSB without interruption may be helpful when debugging, but it is surely a defect.

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More examples of problems with early 80386 CPUs are posted in "The Old New Thing" blog. Here are some highlights from "My, what strange NOPs you have!", by Raymond Chen:

[I]f the instruction following a string operation (such as movs) uses opposite-sized addresses from that in the string instruction (for example, if you performed a movs es:[edi], ds:[esi] followed by a mov ax, [bx]) or if the following instruction accessed an opposite-sized stack (for example, if you performed a movs es:[edi], ds:[esi] on a 16-bit stack, and the next instruction was a push), then the movs instruction would not operate correctly.

...

[T]here was one bug that manifested itself in incorrect instruction decoding if a conditional branch instruction had just the right sequence of taken/not-taken history, and the branch instruction was followed immediately by a selector load, and one of the first two instructions at the destination of the branch was itself a jump, call, or return. The easy workaround: Insert a NOP between the branch and the selector load.

...

[T]he B1 stepping did not support virtual memory in the first 64KB of memory. Fine, don't use virtual memory there.

...

If virtual memory was enabled, if a certain race condition was encountered inside the hardware prefetch, and if you executed a floating point coprocessor instruction that accessed memory at an address in the range 0x800000F8 through 0x800000FF, then the CPU would end up reading from addresses 0x000000F8 through 0x0000000FF instead. This one was easy to work around: Never allocate valid memory at 0x80000xxx.

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Instructions

Here's more information on the opcodes (IBTS and XBTS) that were removed from the B1 stepping.

IBTS (0x0F 0xA7)

Opcode IBTS

CPU: 80386 step A0-B0 only 
Type of Instruction: User 

Instruction: IBTS base,bitoffset,len,sorc 

Description: 
Write bit string length bits from 
[bits .. 0 ]	(lowest bits) to bitfield, 
defined by and bitsoffset from this base 
to start of the field to write. String write from this start 
field bit to higher memory addresses or register bits. 

Flags Affected: None 

CPU mode: RM,PM,VM 

+++++++++++++++++++++++ 
Physical Form:	IBTS	r/m16,AX,CL,r16 
IBTS	r/m32,EAX,CL,r32 
COP (Code of Operation)	: 0FH A7H 

Clocks:	IBTS 
80386:	12/19

XBTS (0x0F 0xA6):

Opcode XBTS

CPU: 80386 step A0-B0 only
Type of Instruction: User

Instruction: XBTS dest,base,bitoffset,len

Description:
Write bit string length bits from bitfield, defined by
and bitsoffset from this base to start of
the field to read. String read from this start field bit to
higher memory addresses or register bits.
And after it string placed to operand, lowest bit of
register or memory to bit 0 of .

Note:   Use SHLD/SHRD instructions for extract bits strings.
On 80386 steps B1+ this opcode generation INT 6,
and on some of 486 other instruction replace this
instruction opcode.

Flags Affected: None

CPU mode: RM,PM,VM

+++++++++++++++++++++++
Physical Form:  XBTS    r16,r/m16,AX,CL
XBTS    r32,r/m32,EAX,CL
COP (Code of Operation) : 0FH A6H

Clocks: XBTS
80386:  6/13

@jeffpar February 23, 2015 (Updated March 9, 2015 with information from "Advanced 80386 Programming Techniques")