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(C) Copyright 1993, 1994 By Harald Feldmann Revision 04, Nov 3rd 1994.
Hamarsoft's 86BUGS list, (C) 1993/94 By Hamarsoft (R)
??????????????????????????????????????????????????????????????????????????????
The 86BUGS list, distributed with Ralf Brown's Interrupt list, is maintained
and provided to you by Hamarsoft, the maker of the HAP & PAH datacompression
program. Latest version of HAP & PAH is 3.14e. If you like this list you are
encouraged to register the HAP 3.00 shareware program. You will receive
the latest, registered, version of HAP 3.14e by air-mail on 3.5" diskette.
FTP to garbo.uwasa.fi and get pc/arcers/hap300re.zip for more info.
????????????????????????????????????????????????????????????????????????????
To contact Hamarsoft, write to ? or send e-mail over Internet to:
? harald.feldmann@almac.co.uk
Hamarsoft, New Address! ????????????????????????????????????????????
Harald Feldmann, ? or send e-mail to HARALD FELDMANN over
P.o. Box 451, ? Ilink in the international COMPRESS echo
6400 AL Heerlen, ? The p.o. box will be maintained if e-mail
The Netherlands ? should no longer be possible.
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Various people have contributed to this list. They are mentioned in a
separate page, click on <acknowledgements> to see their names and e-mail
addresses. These people are not employed by or affiliated with Hamarsoft.
Hamarsoft and all people who contributed to the 86BUGS list do not accept
any liability whatsoever regarding the use, inability to use, correctness
or completeness of the information presented in the 86BUGS list.
Attention authors: if you mention this list in your article or book, please
send a courtesy copy to the P.o. box address by airmail. Thank you.
This is 86BUGS list revision level 04, issued November 3rd 1994.
(C) Copyright 1993, 1994 By Harald Feldmann.
Acknowledgements
??????????????????????????????????????????????????????????????????????????????
This file lists undocumented and buggy instructions of the Intel 80x86
family of processors as well as features of processors compatible with
Intel products. Note that Intel does not support the special features and
may decide to drop opcode variants and instructions in future products.
Wherever the notation 88,86,87,186,286,287,287xl,386,386sx,387,387sx,
486,486sx,487 and Pentium is used, Intel CPUs are referenced unless
noted otherwise.
All mentioned trademarks and/or tradenames are owned by the respective
owners and are acknowledged.
I would like to give credit to those who provided useful information or
who in another way contributed to the 86BUGS list.
9308 Chris Lueders (chris_lueders@zaphod.fido.de) iAPX program & mul bugs
9311 Anthony Naggs (amn@ubik.demon.co.uk) NEC differences and CPU tests
9407 Christian Ludloff (Ludwig-K?hn-Str. 15, 09123 Chemnitz, Germany)
Discovered CPUID instruction on 486.
9410 Robert Mashlan (rmashlan@r2m.com) BOUND difference on NEC V20
9410 Anthony Naggs (amn@ubik.demon.co.uk) POP CS & MOV CS on 86/88
SETALC on NEC & i186 BOUND difference, NEC specific
instructions.
9410 Christian Ludloff (see above for address) Pentium extensions (MSRs),
INFO and STAT programs.
If you contributed, but are not listed, please send a note.
AAA Adjust After BCD Addition
??????????????????????????????????????????????????????????????????????????????
Mnemonic: AAA
Opcode : 37 (88=8, 86=8, 286=3, 386=4, 486=3 clocks)
Bug in : Different implementation in 88 and 86 versus 286+
Function:
The 88 and 86 processors would not add a carry out of al into ah if an
invalid operand would be in al (FF), the newer processors _will_, yielding
different results for the same _invalid_ operand. Execution is effectively
the same when valid operands are loaded.
Highest 4 bits of AL are always cleared.
AAD Adjust After BCD Division
??????????????????????????????????????????????????????????????????????????????
Mnemonic: AAD
Opcode : D5 imm8 (88=60, 86=60, 286=14, 386=19, 486=14 clocks)
Bug in : Is an opcode variant on Intel's 88,86,286,386,486
Variant does not work on NEC's V-series, probably not on AMD CPUs
Function:
This instruction regularly performs the following action:
- unpacked BCD in AX example (AX = 0104h)
- AL = AH * 10d + AL (AL = 0eh )
- AH = 00 (AH = 00h )
The normal opcode decodes as follows: d5,0a
The instruction itself is an instruction plus operand. By replacing the
second byte with any number in the range 00 - ff you can build your own
instruction AAD for various number systems in those ranges. For example
by coding d5,10 you achieve an instruction that performs:
- AL = AH * 16d + AL.
- AH = 00
This feature of Intel's chips can be used to determine whether there is
a true Intel CPU installed in a system.
(NEC difference supplied by Anthony Naggs)
AAM Adjust After BCD Multiplication
??????????????????????????????????????????????????????????????????????????????
Mnemonic: AAM
Opcode : D4 imm8 (88=83, 86=83, 286=16, 386=17, 486=15 clocks)
Bug in : Is an opcode variant on Intel's 88,86,286,386,486
Function:
This instruction regularly performs the following action:
- binary number in AL
- AH = AL / 10d
- AL = AL MOD 10d
Thus creating an unpacked BCD in AX. The normal opcode decodes as follows:
d4,0a. The instruction itself is an instruction plus operand. By replacing
the second byte with any number in the range 00 - ff you can build your own
instruction AAM for various number systems in that range. For example by
coding d4,07 you achieve an instruction that performs:
- binary number in AL
- AH = AL / 07d
- AL = AL MOD 07d
AAS Adjust After BCD Subtraction
??????????????????????????????????????????????????????????????????????????????
Mnemonic: AAS
Opcode : 3F
Bug in : Intel's documentation
Function:
Adjusts result of two subtracted BCD numbers to form a valid new BCD number.
Highest 4 bits of AL are always cleared.
ADD4S Addition of packed BCD strings (NEC V20/30 only)
??????????????????????????????????????????????????????????????????????????????
Mnemonic: ADD4S
Opcode : 0F 20 (7+19n clocks, n is the number of bytes per operand)
Bug in : Rarely documented, except in NEC manuals
Function:
Adds the packed BCD string at DS:SI to the packed BCD string at ES:DI. The
length of the string, in BCD digits, is specified in CL. Unlike Intel string
operations CL, DI & SI are unchanged by the operation. The Zero Flag (ZF) is
set if both operands are zero. The Carry Flag (CF) and Overflow Flag (OF)
appear to be set by the addition of the most significant digits.
Note that 0F is treated as <POP CS> on the 88/86 and prefixes newer
instructions on 286+ CPUs.
(Supplied by Anthony Naggs)
See also SUB4S, CMP4S, ROL4, ROR4
BOUND Checks register against limits
??????????????????????????????????????????????????????????????????????????????
Mnemonic: BOUND reg,mem
Opcode : 62 [mod:reg:r/m]
Bug in : NEC V20 handles it differently from Intel 286+. But apparently,
according to Intel documentation, equal to 186.
Function:
Bound checks a register against limits and generates exception 5 if the
value falls outside the limit. On NEC CPUs the mnemonic is apparently also
referred to as 'CHKIND'.
Note that the mem component refers to two consecutive memory locations, of
size 'reg' which contain the lower and upper limit for the value in 'reg'
as [low limit][high limit].
'reg' size: 'mem' specifies address of:
word dword
dword qword
Normally, on Intel 286+ CPUs, the exception saves the CS:IP pointing TO the
BOUND instruction. On the NEC V20, the saved CS:IP point to the instruction
following the BOUND instruction.
According to Intel's documentation the 186 handles this exception the same
way the NEC does. It has been verified on a 486 that the CS:IP of BOUND on
that CPU indeed points TO the instruction itself and not the following one.
Also, contrary to what one might expect, BOUND only allows word or dword
registers to be tested. Byte registers are invalid.
(V20 supplied by Robert Mashlan)
(186 difference & 'CHKIND' supplied by Anthony Naggs)
Breakpoint errors while debugging
??????????????????????????????????????????????????????????????????????????????
Mnemonic: N/A
Opcode : N/A
Bug in : some 386, some 486
Function:
Breakpoints are used in the process of debugging programs.
On the 386+, debug registers may be used instead of a one byte opcode.
386 specific debugging bugs occurring on some 386s:
Breakpoints are missed under the following conditions:
- A data breakpoint set to a mem16 operand of a VERR, VERW, LSL or LAR while
the segment with selector at mem16 is not accessible.
- A data breakpoint is set to the write operand of a REP MOVS instruction
and the read cycle of the next iteration generates a fault.
- A code or data breakpoint is set on the instruction following a MOV or
POP to SS while the instruction needs more than two clocks.
(see <MOV> and <POP>)
Random breakpoints may occur under the following condition:
- Breakpoints set using debug registers DR0 to DR4 may produce spurious
breaks if breakpoints were enabled before a MOV from CR3, TR6 or TR7 took
place. These unreliable breaks may continue to occur until the next JMP
instruction is executed. A workaround would be to:
= disable breakpoints before any MOV from CR3, TR6 or TR7
= MOV the values
= perform a JMP
= enable breakpoints.
Single stepping is not disabled in the handler for a TSS fault if the code
that caused the fault was being single-stepped and a task gate was used to
handle the fault.
486 specific debugging bugs occurring on some 486s:
A code breakpoint set on control transfer instructions (like CALL, RET, JMP
etc.) will clear the lowest four bits of DR6 when the breakpoint is taken.
A code breakpoint set on an instruction immediately following a RETN, JCXZ,
intrasegment indirect CALL (CALL word ptr [bx] for example) or
intrasegment indirect JMP (JMP word ptr [bx] for example) will always be
satisfied, even when the control instruction is taken. A breakpoint set at
the target of these control transfer instructions will not be taken,
even if control is transferred to them, because the buggy breakpoint sets
the RF (Resume Flag). There is said to be no workaround other than to avoid
the situation, however, coding a nop after the control transfer instruction
and setting the breakpoint to the instruction following the nop may,
according to my view, very well solve the problem. (untested)
BRKEM Break for emulation (NEC V20/30 only)
??????????????????????????????????????????????????????????????????????????????
Mnemonic: BRKEM imm
Opcode : 0F FF imm (38 clocks)
Bug in : Rarely documented, except in NEC manuals
Function:
(8080 is written here as 8O8O to avoid visual confusion with the 8088).
This is the basic instruction used to switch to 8O8O emulation mode.
The BRKEM instruction is used in a similar way to an INT instruction,
(referred to as BRK by NEC). The mode flag (MD) is set to zero, the Flags,
CS & IP are pushed onto the stack then CS & IP are loaded from the
specified interrupt vector.
In 8O8O emulation mode the V30 registers and flags are mapped to 8O8O
registers and flags.
General purpose register names:
?????????????????????????????????????
8O8O name???????? A ? B ? C ? D ? E ? H ? L ? SP? PC?
Intel x86 name??? AL? CH? CL? DH? DL? BH? BL? BP? IP?
V30 name????????? AL? CH? CL? DH? DL? BH? BL? BP? PC?
?????????????????????????????????????
Individual flag names:
?????????????????????
8O8O name???????? C ? Z ? S ? P ? AC?
Intel x86 name??? CF? ZF? SF? PF? AF?
V30 name????????? C ? Z ? S ? P ? AC?
?????????????????????
In 8O8O emulation mode the segment used for instructions is determined
by the CS (PS) register. The DS (DS0) register determines the segment
used for data.
When an interrupt occurs during 8O8O emulation the CPU switches to native
V30 mode to process the interrupt. When the interrupt handler is complete
the IRET, (RETI in NEC nomenclature), will return to 8O8O emulation mode.
From 8O8O emulation mode RETEM (Return from Emulation, opcode ED FD) returns
to native mode, setting MD flag and restoring flags, CS & IP from the native
stack. Alternatively CALLN imm8 (Call Native, opcode ED ED imm) can be used
to call native V30 interrupts, (just like a regular INT).
Note that 0F is treated as <POP CS> on the 88/86 and prefixes newer
instructions on 286+ CPUs.
(Supplied by Anthony Naggs)
BSF, Bit Scan Forward
??????????????????????????????????????????????????????????????????????????????
Mnemonic: BSF op1,op2
Opcode : 0F BC
Bug in : Intel's documentation
Function:
Finds the first (lowest) bit set to 1 in op2, sets ZF=1 and returns the bit
position in op1. If op2 is 0, ZF=0 and the value of op1 is undetermined,
some 386's leave the old value in op1, some early 486's load garbage into
op1 and later 486's leave op1 unchanged.
BSWAP reg32 Byte Swap
??????????????????????????????????????????????????????????????????????????????
Mnemonic: BSWAP reg32
Opcode : 0F C8+reg# (00001111 11001rrr)
Bug in : 486
Function:
Swaps all bytes in 32 bit registers, changing the sequence from ABCD to
DCBA, handy for converting numbers to a CPU format where the byte order
is reversed. Bug appears when BSWAP is not preceded by prefix 66h to
indicate 32 bit registers in 16 bit mode or when it IS preceded by 66h
in 32 bit mode.
Do not use this instruction with 16 bit registers as operand.
Results are undefined in that case. Use XCHG reg8,reg8 instead if you need
to swap 2 bytes in a 16 bit register like AX.
BT op1,op2 Bit Test
??????????????????????????????????????????????????????????????????????????????
Mnemonic: BT
Opcode : 0F A3 op1,op2
Bug in : No bug, avoid use on ports in 386, 486
Function:
Basically copies bit(op2) from op1 into CY. Memory variant is more complex.
Do not use on memory mapped I/O ports or memory operands that span into or
lie completely within nonexistent memory.
In the case of memory mapped I/O ports, use MOV and TEST instead.
BTC op1,op2 Bit Test and Complement
??????????????????????????????????????????????????????????????????????????????
Mnemonic: BTC op1,op2
Opcode : 0F BB reg1,reg2
0F BA reg,mem
Bug in : No bug, avoid use on ports in 386, 486
Function:
Basically copies bit(op2) from op1 into CY and complements bit(op2) of op1.
Memory variant is more complex. Do not use on memory mapped I/O ports or
memory operands that span into or lie completely within nonexistent memory.
In the case of memory mapped I/O ports, use MOV and TEST instead.
BTR op1,op2 Bit Test and Reset
??????????????????????????????????????????????????????????????????????????????
Mnemonic: BTR op1,op2
Opcode : 0F B3 [mod:reg:r/m]
0F BA [mod:110:r/m] imm8
Bug in : No bug, avoid use on ports in 386, 486
Function:
Basically copies bit(op2) from op1 into CY and sets bit(op2) of op1 to 0.
Memory variant is more complex. Do not use on memory mapped I/O ports or
memory operands that span into or lie completely within nonexistent memory.
In the case of memory mapped I/O ports, use MOV and TEST instead.
BTS op1,op2 Bit Test and Set
??????????????????????????????????????????????????????????????????????????????
Mnemonic: BTS
Opcode : 0F BA [mod:101:r/m] imm8 / 0F AB [mod:reg:r/m]
Bug in : No bug, avoid use on ports in 386, 486
Function:
Basically copies bit(op2) from op1 into CY and sets bit(op2) of op1 to 1.
Memory variant is more complex. Do not use on memory mapped I/O ports or
memory operands that span into or lie completely within nonexistent memory.
In the case of memory mapped I/O ports, use MOV and TEST instead.
Chip Step information for Intel CPUs
??????????????????????????????????????????????????????????????????????????????
CPUs are manufactured in models (like the 80386). While these models are
manufactured, errors in the mask layout and mask design may become
apparent. These errors may be corrected before a new batch of chips is
made. To distinguish between these revisions an identification code is
placed within the mask design on 386+ CPUs. By testing the CPU with CPUID
or by performing a RESET, this information is copied to specific registers.
The register used to hold mask info after a RESET is DX (apparently also
sometimes the high word of EDX on some 486s).
This page lists some component and revision ID's found in the DX register
for the 386SX, 386DX, 486SX and 486DX models from Intel.
CPU: DX: Step:
386SX 2304h A0
2305h B
2306h C
2308h D1
386DX 0303h B0 - B10
0305h D0
0308h D1 & D2
486SX 0420h A0
486DX 0000h A1
0401h Bn
0302h C0
0404h D0
0410h cAn
0411h cBn
CLEAR1 Clears a specific bit to 0 (NEC V20/30 only)
??????????????????????????????????????????????????????????????????????????????
Mnemonic: CLEAR1 reg/mem,CL/immediate
Opcode : CLEAR1 r/m8,CL : 0F 12 [mod:000:r/m] (5/14 clocks)
CLEAR1 r/m8,imm3 : 0F 1A [mod:000:r/m] imm (6/15 clocks)
CLEAR1 r/m16,CL : 0F 13 [mod:000:r/m] (5/14 clocks)
CLEAR1 r/m16,imm4: 0F 1B [mod:000:r/m] imm (6/15 clocks)
CLEAR1 CY : F8 (NEC nomenclature for Intel's CLC)
CLEAR1 DIR : FC (NEC nomenclature for Intel's CLD)
Bug in : Rarely documented, except in NEC manuals
Function:
Clears the specified bit in the register/memory operand. The bit number (CL
or immediate) is ANDed with 07 (for 8-bit operands) or 0F (for 16-bit
operands) to get a valid bit number. No flags are affected by this
operation, except by CLEAR1 CY and CLEAR1 DIR.
The first (smaller) clock count of each pair is for register operands.
Note that 0F is treated as <POP CS> on the 88/86 and prefixes newer
instructions on 286+ CPUs.
(Supplied by Anthony Naggs)
See Also: NECINS, EXT, TEST1, NOT1, SET1
CMP4S Subtraction of packed BCD strings (NEC V20/30 only)
??????????????????????????????????????????????????????????????????????????????
Mnemonic: CMP4S
Opcode : 0F 26 (7+19n clocks, n is the number of bytes per operand)
Bug in : Rarely documented, except in NEC manuals
Function:
Subtracts the packed BCD string at DS:SI from the packed BCD string at
ES:DI, but does not store the result. The length of the string, in BCD
digits, is specified in CL. Unlike Intel string operations CL, DI & SI are
unchanged by the operation. The Zero Flag (ZF) is set if the result is zero.
The Carry Flag (CF) and Overflow Flag (OF) appear to be set by the
subtraction of the most significant digits.
Note that 0F is treated as <POP CS> on the 88/86 and prefixes newer
instructions on 286+ CPUs.
(Supplied by Anthony Naggs)
See Also: ADD4S, SUB4S, ROL4, ROR4
CMPS Compare String Bytes, Word or Dword
??????????????????????????????????????????????????????????????????????????????
Mnemonic: CMPS
Opcode : A6 (Bytes)
A7 (Words)
66 A6 (Bytes)
66 A7 (DWords)
Bug in : Early 286 in protected mode
Function:
Compares two strings in memory.
Repeated version (REP CMPS) in early 286 protected mode has a bug that
shows when, during execution, a segment limit exception or IO Privilege
Level Exception occurs.
In that case the exception handler sees the value of CX as it was at the
start of the REP instruction. SI and DI however reflect the correct index
of the elements currently scanned at the time of the exception.
Workaround: Do not scan beyond segment limits or into memory mapped I/O
areas.
CMPXCHG op1,op2 Compare and Exchange
??????????????????????????????????????????????????????????????????????????????
Mnemonic: CMPXCHG
Opcode : 0F B0 reg,mem/reg (Byte)
0F B1 reg,mem/reg (Word)
66 0F b0/b1 (Byte / DWord)
Bug in : pre-B step 486
Function:
Compares the accumulator (8,16 or 32 bit form) with op1 by internally
subtracting op1 from the accumulator and setting ZF according to the result.
If ZR, op2 is copied to op1, otherwise op1 is loaded into the accumulator.
On the A-step of the 486, this Mnemonic was coded using the opcodes for
the, discarded, A- to B0-step 386 instructions XBTS (a6) and IBTS (a7).
Because of software conflicts with software written for the early 386 DX the
opcodes for the 486 were changed to the ones above starting with the B step.
Note that some 386 software won't run on older 386es and some 486
software will not run on early 486es when using this instruction.
CPUID Identify CPU on 486 and higher CPUs
??????????????????????????????????????????????????????????????????????????????
Mnemonic: CPUID
Opcode : 0F A2
Bug in : Is undocumented for 486, seems not to work on tested AMD 486s
Officially introduced as a new instruction with the Pentium.
Function:
Identifies CPU and revision information for the installed CPU. Note that
Intel officially introduced CPUID only with the Pentium processor.
It seems the instruction was unofficially introduced in the later
486 CPUs as well. Discovered by Christian Ludloff (see acknowledgements).
Supported by the UMC U5S 486 clones as well.
Executing it on an early 486 yields an Invalid Opcode Exception.
To safely use this instruction, an exception handler must be installed.
A safer workaround though is to test whether the ID bit in EFLAGS is set.
If so, the CPU supports CPUID. See <EFLAGS> image.
The instruction expects input in the EAX register and outputs information
in the EAX, EBX, ECX and EDX registers.
Input: EAX = 0000 0000 : Check CPU 486+ installed
Output: after CPUID:
EAX = 0000 0001 : OK, instruction supported
EBX = 756e 6547 : 'uneG'
EDX = 4965 6e69 : 'Ieni'
ECX = 6c65 746e : 'letn'
effectively the CPU says 'GenuineIntel'
Officially this returns a 'vendor string', which may indicate other than
Intel strings for OEMs.
The UMC U5S-33 returns 'UMC UMC UMC ' or ' UMC UMC UMC' (untested).
Input: EAX = 0000 0001 : Obtain model specific information
Output: after CPUID:
EAX = RRRR RFMS : revision information
R = Reserved Zero, but reserved
F = Family (4=486, 5=Pentium)
M = Model (3 on tested 486DX-2/66, 1 on tested Pentium/60)
S = Stepping (5 on tested 486DX-2/66, 3 on tested Pentium/60)
EBX = RRRR RRRR
R = Reserved Zero, but reserved
ECX = RRRR RRRR
R = Reserved Zero, but reserved
EDX = xxxx xxxx : Bitmapped features, 1 means option available
Bit 0 = FPU built-in (supported on 486 and Pentium)
Bit 1 = V-86 mode extensions present
Bit 2 = I/O breakpoints possible
Bit 3 = 4 MB paging supported
Bit 4 = Time Stamp Counter present
Bit 5 = Has Pentium compatible Model Specific Registers
Bit 6 = Reserved (0)
Bit 7 = Machine Check Exception supported (P5 only)
Bit 8 = CMPXCHG8B supported (apparently Pentium only)
Bits 9-31 Reserved
Assume zero if bit is not mentioned.
Note that this instruction is not supported on all 486 CPUs. However,
Christian Ludloff has tested it on some 486 DX and 486 SX models, in
addition to the Pentium/60 and found them to be present on those machines.
Any step and model information you find this instruction to run on is
welcomed. Please forward it to Christian.
Apparently all new(er) Intel CPUs are equipped with (some) of these
extensions, not just the Pentium.
CR0-4 register layout (386+)
??????????????????????????????????????????????????????????????????????????????
= CR0: Some bits remain from the Machine Status Word of the 286.
Bit 31 16 0
?????????????????????????????????????????????????????????????????
?P?C?N?r?r?r?r?r?r?r?r?r?r?A?r?W?r?r?r?r?r?r?r?r?r?r?n?e?t?E?m?p?
?????????????????????????????????????????????????????????????????
??? ? ? ????????????????????? ? ? ? ? ? ?
????? ??????????????????????? ? ? ? ? ? ? ?
???NW Not Write through (1 if write through) ? ? ? ? ? ? ? ?
???CD Cache Disable (1 if disabled) ? ? ? ? ? ? ? ?
???PE Paging Enabled ? ? ? ? ? ? ? ?
AC Alignment mask (1=masked)?????????????????? ? ? ? ? ? ? ?
WP Write Protect (1 if read-only pages protected)? ? ? ? ? ?
NE Numeric Error (1 if errors should be ignored)?? ? ? ? ? ?
ET Extension Type (1=387 type FPU,0=287 type FPU)??? ? ? ? ?
TS Task Switch (1=task switch has occurred)??????????? ? ? ?
EP Emulate Processor Extension ????????????????????????? ? ?
(1=execute exception 7 on FPU codes) ? ?
MP Math Present (1=_FPU_ will handle FPU codes)??????????? ?
PE Protection Enabled (1=Protected mode activated)??????????
If EP=1 and MP=0, the FPU codes will be handled by software routines
via exception 7. Coprocessor emulators use this property.
= CR1: Is reserved
= CR2: Linear 32-bit address of Page Fault
= CR3: Page Directory Base Register (386+)
Bit 31 16 0
?????????????????????????????????????????????????????????????????
?x?x?x?x?x?x?x?x?x?x?x?x?x?x?x?x?x?x?x?x?r?r?r?r?r?r?r?p?P?r?r?r?
?????????????????????????????????????????????????????????????????
??????Page Directory Base Register????? ? ? PDBR
(used in the Paging process implemented on the 386+) ? ?
? ?
Page-level Cache Disable (486+)???????????????????????? ? PCD
Page-level Writes Transparent (486+)????????????????????? PWT
= CR4: Extended Machine Control (Pentium+)
Bit 31 16 0
?????????????????????????????????????????????????????????????????
?r?r?r?r?r?r?r?r?r?r?r?r?r?r?r?r?r?r?r?r?r?r?r?r?r?M?r?p?D?T?P?V?
?????????????????????????????????????????????????????????????????
Machine Check Enable (1=enabled)??????????????????? ? ? ? ? ? MCE
Page Size Extension (1=4 Mb paging instead of 4 Kb)???? ? ? ? ? PSE
Debugging Extension (1=breakpoints also valid for I/O)??? ? ? ? DE
Time Stamp instruction Disable (1=RDTSC only with CPL=0)??? ? ? TSD
Protected mode Virtual Interrupts (1=use VI flag in PM)?????? ? PVI
Virtual86 mode Virtual Interrupts (1=use VI flag in VM)???????? VME
The VME bit allows a V86 (or VM) task to use the 'virtual' interrupt
flag. Setting and clearing the interrupt flag (IF) in EFLAGS is no
longer intercepted by the V86 Monitor program (a very time consuming
procedure), instead, the Pentium+ sets and clears the VI flag in
EFLAGS, instead of the IF flag. This saves task switches to the
monitor to handle the CLI and STI instructions and thus a lot
of time in general purpose 8086 programs running in V86 mode.
The PVI bit allows the same for Protected Mode procedures who would
otherwise need supervision by a different task. That is:
Tasks with CPL<0 may now call tasks with CPL=0 without crashing
the system, but only under specific circumstances.
The TSD bit changes the CPL-sensitivity of the RDTSC (Read Time
Stamp Counter) instruction, a built-in CPU counter which is
incremented every internal clockpulse.
When TSD is 0, <RDTSC> is accessible for all CPL levels.
With TSD set to 1 however, RDTSC is available only to tasks with
CPL=0.
The DE bit allows the Pentium+ to set breakpoints in I/O space
using the breakpoint registers. The R/W coding 10b is used to
indicate that the breakpoint is in I/O space on the Pentium+.
The 10b encoding was marked as 'invalid' for pre-Pentium CPUs.
The PSE bit determines the size of the pages controlled by the
Paging Unit. With PSE = 0, the Paging mechanism uses 4 Kb pages.
With PSE set to 1 however, the Paging mechanism uses 4 Mb pages.
The MCE bit is used to allow generation of a Machine Check Exception.
This exception is the result of a Parity error _within_ the Pentium
or an active BUSCHK signal (low) on pin T3 (upper right hand corner,
fourth pin from right, third from top when pin A1 is upper left
corner, TOP view). The exception is vectored through interrupt 18d
(or 12h). Execution after this exception may void system integrity.
The Machine Check Address register holds the value of the address
bus at the moment the event took place.
The Machine Check Type register holds the type of bus access at the
time the event took place.
Both these registers are internal 64 bit registers which can only be
read through the instruction <RDMSR> (Read Model Specific Register).
See also <WRMSR> (Write Model Specific Register).
EFLAGS register layout (8088 to Pentium & NEC)
??????????????????????????????????????????????????????????????????????????????
Bit 31 16 0
?????????????????????????????????????????????????????????????????
?r?r?r?r?r?r?r?r?r?r?c?p?v?a?V?R?M?N?IOP?O?D?I?T?S?Z?r?A?r?P?r?C?
?????????????????????????????????????????????????????????????????
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Carry
CPUID available ?????? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Parity
Virtual Interrupt Pending? ? ? ? ? ? ? ? ? ? ? ? ? ?Aux carry
Virtual Interrupt flag ??? ? ? ? ? ? ? ? ? ? ? ? ????????? Zero
Alignment check ???????????? ? ? ? ? ? ? ? ? ? ??????????? Sign
Virtual-86 mode enabled ?????? ? ? ? ? ? ? ? ? Trap (step mode)
Resume flag ???????????????????? ? ? ? ? ? ??? Interrupt enable
Mode Flag ???????????????????????? ? ? ? ????? Direction (1=up)
Nested Task ???????????????????????? ? ??????????????? Overflow
??? I/O privilege level 0..3
Note: the Mode Flag is supported only on the NEC V20/30,
it is reserved on Intel CPUs.
The diagram below shows the names for each bit as referenced to in most
books, along with the CPU in which the bit was =officially= introduced.
Description: Name: CPU introduced:
CPUID available???????????????ID Pentium
Virtual Interrupt Pending?????VIP Pentium
Virtual Interrupt flag????????VI Pentium
Alignment Check Flag??????????AC 486
Virtual-86 Mode Flag??????????VM 386
Resume Flag???????????????????RF 386
Mode Flag (8O8O emulation on)?MD V20/V30 only
Nested Task???????????????????NT 286
I/O privilege level 0..3??????IOPL 286
Overflow Flag?????????????????OF 86
Direction Flag (1=up)?????????DF 86
Interrupt Flag (1=enabled)????IF 86
Trap Flag (single step mode)??TF 86
Sign Flag?????????????????????SF 86
Zero Flag?????????????????????ZF 86
Auxiliary carry Flag??????????AF 86
Parity Flag???????????????????PF 86
Carry Flag????????????????????CF 86
(8080 is written here as 8O8O to avoid visual confusion with the 8088).
(Mode Flag supplied by Anthony Naggs)
EXT Extract bit field (NEC V20/30 only)
??????????????????????????????????????????????????????????????????????????????
Mnemonic: EXT reg8,reg8 / EXT reg8,imm4
Opcode : 0F 33 [mod:reg:r/m] (26-55 clocks)
Bug in : Rarely documented, except in NEC manuals
Function:
Loads AX with bit field data. Bit field length is specified by the lowest
four bits of the second operand, more significant bits in AX are set to
zero. DS:SI specify the first memory location to read, and the low 4-bits
of the first operand specify the bit start position. The bit field can
cross a byte boundary. After each complete data transfer, SI and the first
operand are automatically updated to point to the next bit field.
Note that 0F is treated as <POP CS> on the 88/86 and prefixes newer
instructions on 286+ CPUs.
(Supplied by Anthony Naggs)
See Also: NECINS, TEST1, NOT1, CLEAR1, SET1
FPO2 Floating Point Operation 2 (NEC V20/30 only)
??????????????????????????????????????????????????????????????????????????????
Mnemonic: FPO2 fp-op / FPO2 fp-op,mem
Opcode : 0110011X [mod:XXX:r/m] (2/11 clocks)
Bug in : Rarely documented, except in NEC manuals
Function:
Intended to communicate with NEC maths co-processors. The NEC "FPO1" opcode
corresponds to Intel's "ESC" prefix for co-processor instructions. Although
data sheets exist for NEC maths co-processors, they have never been
manufactured.
Note that the 386+ CPUs implement the opcodes 66 and 67 as Operand Size and
Address Size prefixes respectively.
(Supplied by Anthony Naggs)
HLT Halt the processor
??????????????????????????????????????????????????????????????????????????????
Mnemonic: HLT
Opcode : F4
Bug in : No bug, handy use of instruction described below
Function:
Halts the processor, CPU restarts only when external event takes place such
as RESET activation, NMI request on NMI lines or maskable interrupt request
on INTR when interrupts are enabled.
Handy to use with following piece of code:
STI ; enable interrupts
lazy:
HLT ; suspend CPU internal bus clock
IN AL,60h ; Key pressed !
CMP AL,whatever_key
JNE lazy ; was not our key, just go back to sleep.
If the CPU is not going to be used for any processing tasks (hence is idle)
one may execute the code above to cool down the CPU because it stops the
internal CPU bus clock. It also saves (some) energy.
IBTS op1,op2 Insert Bit String
??????????????????????????????????????????????????????????????????????????????
Mnemonic: IBTS op1,op2
Opcode : 0F A7
Bug in : 386, 486 conflicting instruction opcode.
Function:
Obsolete instruction which was introduced on the A step of the 386 and
removed on the B1 step of the 386. The opcode a7 is used by the A step 486
to function as part of the CMPXCHG instruction. Because of software
conflicts (some compilers generated code for IBTS and its counterpart XBTS)
Intel decided to change the opcode for CMPXCHG on the B step of the 486.
Do NOT use IBTS in general purpose 386 or 486 applications.
IMUL Integer, signed, Multiply
??????????????????????????????????????????????????????????????????????????????
Mnemonic: IMUL op
IMUL op1,op2
IMUL op1,op2,op3
IMUL op1,op3
Opcode : F6w [mod:101:r/m] disp
Bug in : Apparently no bug, timing formula may be handy
Function:
It is mentioned here because of the timing formula.
The clocks used on 386 and 486 equal 9 or ceiling(log2(multiplier))+6.
Depending on which one is bigger.
Add an additional 3 clocks if multiplier is a memory operand.
See <MUL> for 32-bit MUL bugs.
INS Input String from IO port
??????????????????????????????????????????????????????????????????????????????
Mnemonic: INS, INSB, INSW, INSD
Opcode : AA, AB
Bug in : early 286, some 386, early 486, NEC conflicting mnemonic: INS
Function:
Reads values from a port address in DX into a string at ES:DI or ES:EDI
in memory. When used with the REPcondition prefix, CX or ECX contains the
number of values to read.
There is also a NEC specific instruction with the conflicting mnemonic INS,
see <NECINS> or select <NEC specific instructions> from the mnemonic list
page for more information regarding that instruction.
Bugs in the 286;
If, in protected mode, ES would contain a null selector or ES:DI would
point beyond the segment limit when executing the single INS, causing
exception 0dh, the 0d exception handler would point to the instruction
following INS and not to it.
If, in protected mode, during the repeated version of the instruction, a
segment limit or IOPL exception occurred, the exception handler would see
the CX value as it was before the start of the instruction, DI would reflect
the proper index at the time of the exception though. This type of bug
also occurs with the CMPS instruction.
Bugs in the 386:
The value of CX or ECX after the REPcondition version is not correct when
the instruction is followed by a PUSH, POP or memory reference. After
REP INS the value of CX, ECX is -1, not 0. Do not assume (E)CX to be 0.
When REP INS or INS is followed by an instruction that uses a different
address size or when they are followed by an instruction that references
the stack implicitly while the B bit of the SS descriptor is different than
the address size used by the instruction, INS will not properly update
the (E)DI and REP INS will not properly update the (E)CX register.
The actual address size used will be the one of the instruction following
the (REP) INS.
A workaround for this bug is to code a NOP with the same address size as the
INS right behind it by using the address size prefix byte 67h (when needed).
Bugs in the 486:
Early 486 may hang if the INS destination address spans across a doubleword
boundary, while not asserting BS16# or BS8#.
To avoid this, always align the string at a doubleword.
INS (NECINS) Insert bit field (NEC V20/30 only)
??????????????????????????????????????????????????????????????????????????????
Mnemonic: INS reg8,reg8 / INS reg8,imm4
Opcode : 0F 31 [mod:reg:r/m] (31-117 clocks)
Bug in : Rarely documented, except in NEC manuals
Function:
Stores bit field data from AX into memory. Bit field length is specified by
the lowest four bits of the second operand. ES:DI specify the first memory
location to write, and the low 4-bits of the first operand specify the bit
offset position. The bit field can cross a byte boundary. After each
complete data transfer, DI and the first operand are automatically updated
to point to the next bit field.
This mnemonic (INS) conflicts with the Intel mnemonic INS, which reads
a string from an I/O port. This Intel instruction has bugs which are listed
with the entry for <INS>. For clarity, this NEC version is referred to as
"NECINS" where possible in this list.
Note that 0F is treated as <POP CS> on the 88/86 and prefixes newer
instructions on 286+ CPUs.
(Supplied by Anthony Naggs)
See Also: EXT, TEST1, NOT1, CLEAR1, SET1
INVD Invalidate internal and external caches
??????????????????????????????????????????????????????????????????????????????
Mnemonic: INVD
Opcode : 0F 08
Bug in : some 486
Function:
INVD tells the processor that all data in both the internal as well as the
external caches is invalid. Data held in external write-back caches is
discarded.
If on some 486's a cache line fill is in progress while the INVD instruction
is being executed, that line is NOT invalidated and the buffer contents
is moved into the cache. Valid cache lines are ALWAYS used to satisfy
read requests on all 486's, regardless whether the cache is enabled or not.
Workaround is to disable the cache prior to flushing it like this:
MOV EAX,CR0
OR EAX,60000000h ; cache disable bits
PUSHFD
CLI
MOV BL,CS:here
OUT dummyport,dummydata
MOV CR0,EAX
here:
INVD
AND EAX,9fffffff ; cache enable, write-through
MOV CR0,EAX
POPFD
JMP Jump unconditionally.
??????????????????????????????????????????????????????????????????????????????
Mnemonic: JMP dest
Opcode : EB disp8
Bug in : A to C0 step of 486
Function:
JMP transfers execution to a location within -127 to +128 bytes from the
jump instruction. The bug occurs when the jump causes a General Protection
Violation while an NMI or INTR occur at exactly the same clockpulse.
Although very unlikely to occur, it is listed for completeness.
LAR Load Access Rights (Protected Mode)
??????????????????????????????????????????????????????????????????????????????
Mnemonic: LAR reg1,reg/mem
Opcode : 0F 02
Bug in : some 386
Function:
LAR Loads the Access rights of a descriptor in the Global Descriptor Table,
whose selector is reg/mem into reg1. When successful, ZF=1, otherwise ZF=0.
Some 386es allow access to selector 0 in the GDT leaving ZF=1.
Normally this should not be possible and produce the condition ZF=0.
Workaround would be to create an entry 0 in the GDT that consists of only
zeroes. This will cause access with a selector of 0 to fail and
produce ZF=0.
A data breakpoint set to the mem16 operand of LAR can be missed on some
386es if the segment with the selector at mem16 is not accessible.
(see also <debugging>)
286-LOADALL / 386-LOADALL
??????????????????????????????????????????????????????????????????????????????
Mnemonic: LOADALL
Opcode : 286 : 0F 05 (195 clocks)
386+: 0F 07 ( ? clocks)
Bug in : Is an undocumented opcode on 286,some 386,some early 486 ?
Support for this instruction has been dropped with the 486.
Function:
Loads virtually all processor registers with defined values from memory.
Initialises processor to specified state. Apparently aliased on the 286 by
opcode 0f 04.
The 286 LOADALL instruction reads a block of 102 bytes into the chip,
starting at address 000800 hex.
Memory description for LOADALL read area on 286:
(addresses are in hexadecimal, lengths in decimal)
0800: 6 N/A
0806: 2 MSW (Machine Status Word)
0808: 14 N/A
0816: 2 TR (Task Register)
0818: 2 FLAGS (286 Flags)
081a: 2 IP (Instruction Pointer)
081c: 2 LDT (Local Descriptortable)
081e: 2 DS (Data Segment)
0820: 2 SS (Stack Segment)
0822: 2 CS (Code Segment)
0824: 2 ES (Extra Segment)
0826: 2 DI (Destination Index)
0828: 2 SI (Source Index)
082a: 2 BP (Base Pointer)
082c: 2 SP (Stack Pointer)
082e: 2 BX (BX register)
0830: 2 DX (DX register)
0832: 2 CX (CX register)
0834: 2 AX (AX register)
0836: 6 ES cache (ES descriptor _cache_)
083c: 6 CS cache (CS descriptor _cache_)
0842: 6 SS cache (SS descriptor _cache_)
0848: 6 DS cache (DS descriptor _cache_)
084e: 6 GDTR (Global Descriptor Table)
0854: 6 LDT cache (Local Descriptor_cache_)
085a: 6 IDTR (Interrupt Descriptor table)
0860: 6 TSS cache (Task State Segment _cache_)
Descriptor caches layout:
3 bytes 24 bit physical address of segment
1 byte access rights byte, same format as access right byte
in a regular descriptor. The 'present' bit now
represents a 'valid' bit. If this bit is cleared
(zero) the segment is invalid and accessing it will
trigger exception 0dh.
The DPL (Descriptor Privilege Level) fields of the CS
and SS descriptor caches determine the CPL
(Current Privilege Level).
2 bytes 16 bit segment length limit.
This layout is the same for the GDTR and IDTR registers,
except that the access rights byte must be zero.
The register caches are internal CPU registers containing a copy of the last
'composed' address and access information loaded for a particular register
in protected mode (e.g. ES). An outline of the basics of 286 protected
mode register caching and register layout is beyond the scope of this file
The 386 LOADALL loads 204 (dec) bytes from the address at ES:EDI and resumes
execution in the specified state.
Memory description for LOADALL read area on 386+:
(addresses are in hexadecimal, lengths in decimal)
relative offset: Bytes: Registers:
0000: 4 CR0
0004: 4 EFLAGS
0008: 4 EIP
000c: 4 EDI
0010: 4 ESI
0014: 4 EBP
0018: 4 ESP
001c: 4 EBX
0020: 4 EDX
0024: 4 ECX
0028: 4 EAX
002c: 4 DR6
0030: 4 DR7
0034: 4 TR
0038: 4 LDT
003c: 4 GS (zero extended)
0040: 4 FS (zero extended)
0044: 4 DS (zero extended)
0048: 4 SS (zero extended)
004c: 4 CS (zero extended)
0050: 4 ES (zero extended)
0054: 12 TSS descriptor cache
0060: 12 IDT descriptor cache
006c: 12 GDT descriptor cache
0078: 12 LDT descriptor cache
0084: 12 GS descriptor cache
0090: 12 FS descriptor cache
009c: 12 DS descriptor cache
00a8: 12 SS descriptor cache
00b4: 12 CS descriptor cache
00c0: 12 ES descriptor cache
Descriptor caches layout:
1 byte zero
1 byte access rights byte, same as 286
2 bytes zero
4 bytes 32 bit physical base address of segment
4 bytes 32 bit segment length limit
LSL Load Segment Limit
??????????????????????????????????????????????????????????????????????????????
Mnemonic: LSL reg1,reg/mem
Opcode : 0F 03
Bug : some 386
Function:
Loads the limits of a segment in protected mode by reading GDT entry reg/mem
into reg1. Proper completion generates ZF=1, otherwise ZF=0.
Some 386es allow access to selector 0 in the GDT leaving ZF=1.
Normally this should not be possible and produce the condition ZF=0.
Workaround would be to create an entry 0 in the GDT that consists of only
zeroes. This will cause access with a selector of 0 to fail and
produce ZF=0.
Some 386es leave SP/ESP corrupted after successful completion of LSL, when
LSL is followed by an explicit stack reference, using instructions like
CALL, ENTER, LEAVE, IRET, RET, PUSH, POP, PUSHA, POPA, PUSHF and POPF.
System-induced exceptions or interrupts however do not corrupt SP/ESP in
that case. A workaround is to code a NOP after LSL.
A data breakpoint set to the mem16 operand of LSL can be missed on some
386es if the segment with the selector at mem16 is not accessible.
(see also <debugging>)
MOV Move data to and from registers and or memory
??????????????????????????????????????????????????????????????????????????????
Mnemonic: MOV involving CRx, DRx or TRx, MOV to SS, CS
Opcode : 0F 2n [mod:rrr:r/m], 8E [mod:sreg:r/m]
Bug in : some 88,some 86,some 386,all 386,A to C0 step of 486
Function:
MOV Moves data in and out of (special) registers and memory.
Some _very early_ 88 and 86 processors do not disable interrupts following
a MOV sreg,reg. This causes them to crash when an interrupt uses the stack
between MOV SS,reg and MOV SP,op. These versions carry a copyright message
for 1978 on the package. Later, corrected revisions, carry both 1978 and
1981 as the copyright year.
Normally interrupts would be disabled between the move to SS and execution
of the instruction following it on 88 and 86es. A workaround is to manually
disable the interrupts when reloading SS. The 286 and higher processors only
disable interrupts after a MOV SS, in contrast to earlier CPUs, including
the NECs, who do this with all MOV sreg,op instructions.
An unsolvable problem occurs when an unmaskable interrupt or exception
takes place while executing the instruction pair on an old 88 or 86.
There are conflicting messages though about this type of interrupts having
no effect on the bug.
On the 86 and 88, but not on the C-MOS versions 80C86 and 80C88, the
instruction MOV CS,op is valid and causes an unconditional jump.
The C-MOS versions, as well as the NEC V20 and V30 ignore this coding.
This may also be the case on the 186 but has not been tested.
The 286+ CPUs consider CS an invalid operand for this instruction and
generate exception 6 (Invalid opcode).
The opcode for the MOV CS,op is: 8e [mod:001:r/m] See also <POP CS>.
On some 386es, random breakpoint breaks occur from the debug registers
D0-D3 when a MOV from CR3, TR6 or TR7 is executed. This will continue until
after a jump instruction is executed. The actual contexts of D0-D3 is not
affected. Workaround is to disable breakpoints before the MOV from CR3,TR6
or TR7, execute a jmp right after the move and enabling breakpoints again.
See also <debugging>
On some 386es a MOV to SS may cause a code or data breakpoint set to the
instruction following the MOV to be missed if the instruction takes more
than two clocks. (see <debugging>)
On all 386es a MOV to or from CRx, TRx or DRx executes correctly regardless
of the mod field (the first two bits in the third byte of the opcode).
The mod should be 11b. Intel documentation for the 386 stated it was
undefined.
Some 386 assemblers and compilers may generate values other than 11b for
mod and fail on early 486es, causing an Invalid Opcode Exception, since they
do require the mod field to be correct. More recent 486es recognize the
aliased instructions as valid and execute them accordingly.
On all 386es, moves to or from DR4 and DR5 are aliased to DR6 and DR7.
On the early 486es these encodings are not recognized and generate an
Invalid Opcode Exception. More recent 486es do recognize these aliases and
execute them correctly.
On the A to C0 steps of the 486, loading TR5 with a reg32 operand may hang
the CPU if bits 0 and 1 (control bits) activate cache read, cache write or
flush. A workaround is:
JMP fetcher
ALIGN 16
fetcher:
NOP
IN AL,port ; Note that this corrupts EAX...
MOV TR5,EBX ; EBX contained the new TR5 value.
NOP
NOP
On the A to C0 step of the 486 loading a value into CR0 which disables the
cache may corrupt the cache. Forcing a prefetch will avoid this.
PUSHFD
CLI
MOV BL,CS:label
MOV CR0,EAX
label:
POPFD
NOP
Using EBX:
Note that using EBX under the Microsoft Windows 3.0 DOS box in standard mode
or after Microsoft Windows 3.0' termination after running standard mode, for
32-bit addressing in real or virtual 86 mode, is likely to crash the system
due to the fact that apparently the Windows 3.0 DOS box trashes EBX while
servicing interrupts, turning bit 18 of EBX to 1 and thus causing unwanted
segment violation errors. Use of EBX in calculations is likely to cause
spurious errors and may yield unpredictable behaviour of your code under
the aforementioned circumstances.
(MOV CS,op for NEC and 88/86, C88/C86, & 1978 copyright message
supplied by Anthony Naggs).
MOVS Move string of bytes, words or doublewords in memory
??????????????????????????????????????????????????????????????????????????????
Mnemonic: MOVSB / MOVSW / MOVSD
Opcode : A4 / A5 / 66 A5
Bug in : early 286 in PM, some 386
Function:
MOVS moves strings in memory. Possible units to move are byte, word and
doubleword. Typically the source is DS:(E)SI, the target ES:(E)DI
If the single instruction MOVS (not prefixed by REPx) is executed with a
NULL selector in ES or when ES:DI points beyond the segment limit while
executing the the single instruction, causing exception 0dh, the CS:IP
saved by the 0dh exception handler will point after the MOVS instruction,
instead of to it on some 286s.
If a segment limit exception or IOPL violation exception occurs during the
REPx prefixed form of MOVS in Protected Mode, some early 286 will reset CX
to its initial setting (before the REPx started) instead of showing CX as
it was at the time of the exception. SI and DI are not affected and show the
values they had at the time of the exception.
During debugging with breakpoints set, REP MOVS can cause data breakpoints
to be missed on some 386, see <debugging>.
If, on some 386es, MOVS is followed by an instruction which uses a different
address size, or by an instruction which implicitly references the stack
(like POP, PUSH, IRET, RET, CALL, ENTER, LEAVE, PUSHA, POPA, PUSHF and POPF)
while the D-bit for the stack is different from the current address size
used by the MOVS instruction, the destination register updated will depend
on the address size of the instruction that follows, rather than that of
the MOVS. This can result in the updating of only DI when EDI was meant
or EDI when only DI was meant.
The repeated form REPx MOVS has the same bug, but in addition to (E)DI,
also (E)SI is affected.
A workaround is to always code a NOP with the same address size after MOVS
and REPx MOVS.
Example:
(16-bit code segment)
MOVSW ; 16-bit addressing MOVS
NOP ; 16-bit addressing NOP
db 67h
MOVSW ; 32-bit addressing MOVS
db 67h
NOP ; 32-bit addressing NOP
(32-bit code segment)
MOVSD ; 32-bit addressing MOVS
NOP ; 32-bit addressing NOP
db 67h
MOVSD ; 16-bit addressing MOVS
db 67h
NOP ; 16-bit addressing NOP
MUL Unsigned Multiply 16 & 32-bit versions
??????????????????????????????????????????????????????????????????????????????
Mnemonic: MUL reg
Opcode : (66) F7 Ex
Bug in : 386
Function:
MUL multiplies ax with a 16-bit operand to form a 32-bit result in dx:ax.
The 32-bit version multiplies eax with a 32-bit operand to form a 64-bit
result in edx:eax.
Some 386es have a problem which redirects output from the 32-bit MUL
to the wrong parts of the wrong registers.
Typically the following happens:
Properly operating 32-bit version: Properly operating 16-bit version:
EAX: 'A':'B' EAX: 'A':'B'
EBX: 'C':'D' EBX: 'C':'D'
EDX: 'E':'F' EDX: 'E':'F'
CD x AB gives a result in EF:AB D x B gives a result in F:B
While executing the 32-bit MUL, the faulty CPU takes CD times AB and puts
the value it should have added to 'A' into 'F' while at the same time
adding the value it should have put into EF to AB.
No workaround other than to use 16-bit multiply.
Some 386's have a bug which generates incorrect values in 16-bit mode.
The iAPX program from IGEL (Chris Lueders) tests for this bug.
Intel apparently organized a replacement project to get the faulty chips
returned to factory for screening. After testing at Intel the faulty CPUs
were sold again to bulk buyers who installed them in 16-bit only machines.
These tested and failed chips carry the text "16-bit S/W only" or a single
sigma. The tested and passed chips carry a double sigma (??) on the package.
(supplied by Chris Lueders)
NEC V20/V30 introduction
??????????????????????????????????????????????????????????????????????????????
The NEC V series microprocessors are functionally similar to the 8086 design
which NEC licensed from Intel. The internal microcode and most NEC mnemonics
are different from Intel's, to avoid Intel copyright claims. Only the
NEC V20 & V30, pin compatible with 8088 & 8086 respectively, are usually
found in IBM compatible PCs.
The V20 and V30 are often supplied as an "upgrade kit" for PCs originally
equipped with an 88/86, as they execute most instructions in fewer clocks
and can be used at a higher clock rate than the Intel parts.
Occasionally single board PCs use the V40 & V50, which are based on the same
CPU core and have integrated peripheral functions. Other V series family
members diverge further from the Intel x86 series and are used in
controllers and instrumentation rather than PCs.
The V20 and V30 have four classes of extra instructions beyond those
present on the 86/88: