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What did the 8086 (and 8088) do upon encountering an illegal instruction?

Use of undocumented opcodesHow did the Z80 instruction set differ from the 8080?What conventions and language extensions did people use to program the 8086 and 80286?Why did the PDP-11 include a JMP instruction?What was the IBM PC cost saving for using the 8088 vs 8086?Did the Intel 8086/8088 not guarantee the value of SS:SP immediately after RESET?Uses for the halt instruction?8086 pinout and address space limitHow to keep the instruction prefetcher filled upWhat instructions for the 8086 and subsequent x86 CPUs are not available in Long Mode?How did the 8086 interface with the 8087 FPU coprocessor?

7

I was reading Tanenbaum's "Operating Systems, Design and Implementation" 3rd edition (The MINIX Book), where I encountered the following quote that surprised me:

For instance, the 8086 and 8088 processors do not support detection of illegal instruction operation codes, but this capability is available on the 286 and above, which trap on an attempt to execute an illegal opcode.

What did the 8086 and 8088 do when it an encountered an illegal instruction?

instruction-set 8086 microprocessor 8088

share|improve this question

asked 8 hours ago

Joe DJoe D

1362

New contributor

Joe D is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Possible duplicate of Use of undocumented opcodes
  
  – Stephen Kitt
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  The suggested duplicate isn’t an exact duplicate of the question, but some of the answers to it also answer this question.
  
  – Stephen Kitt
  4 hours ago
  

  
  
  
  

add a comment | 

7

I was reading Tanenbaum's "Operating Systems, Design and Implementation" 3rd edition (The MINIX Book), where I encountered the following quote that surprised me:

For instance, the 8086 and 8088 processors do not support detection of illegal instruction operation codes, but this capability is available on the 286 and above, which trap on an attempt to execute an illegal opcode.

What did the 8086 and 8088 do when it an encountered an illegal instruction?

instruction-set 8086 microprocessor 8088

share|improve this question

asked 8 hours ago

Joe DJoe D

1362

New contributor

Joe D is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Possible duplicate of Use of undocumented opcodes
  
  – Stephen Kitt
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  The suggested duplicate isn’t an exact duplicate of the question, but some of the answers to it also answer this question.
  
  – Stephen Kitt
  4 hours ago
  

  
  
  
  

add a comment | 

I was reading Tanenbaum's "Operating Systems, Design and Implementation" 3rd edition (The MINIX Book), where I encountered the following quote that surprised me:

For instance, the 8086 and 8088 processors do not support detection of illegal instruction operation codes, but this capability is available on the 286 and above, which trap on an attempt to execute an illegal opcode.

What did the 8086 and 8088 do when it an encountered an illegal instruction?

instruction-set 8086 microprocessor 8088

share|improve this question

asked 8 hours ago

Joe DJoe D

1362

New contributor

Joe D is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

asked 8 hours ago

Joe DJoe D

1362

New contributor

Joe D is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

asked 8 hours ago

Joe DJoe D

1362

New contributor

Joe D is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 8 hours ago

Joe DJoe D

1362

New contributor

Joe D is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 8 hours ago

Joe DJoe D

1362

2 Answers
2

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8

Illegal opcodes were just instructions that hadn't been fully defined by the chip designers – a little like Undefined Behaviour in C, but much more predictable. Many people called these "undocumented instructions", because they functioned just like ordinary instructions, on the particular versions of the particular chips on which they are found. There was no special handling to prevent these instructions from executing.

Most of them were just NOPs (either because they weren't wired up to anything or because they did stuff like writing a register to itself) or duplicates of other instructions (because the instruction decoder ignored some bits when it didn't need to pay attention to them), but some of them were more interesting.

For instance, SALC, which does:

if (carry flag set)

  AL = 0xFF;

else

  AL = 0x00;

This can be used as a translation layer between C code and certain assembly returning conventions. It is equivalent to SBB AL, AL, except that it does not clobber flags.

For more information, see:

• 
  Stephen Kitt's answer to Use of undocumented opcodes.


• 
  supercat's answer to Use of undocumented opcodes, which has a good explanation of where they come from.


• A related question on Stack Overflow

share|improve this answer

edited 16 mins ago

Cody Gray

1,149521

answered 8 hours ago

wizzwizz4♦wizzwizz4

8,915642110

add a comment | 

6

It might be better to think about it this way:

On the 286 and above, a new thing was defined called an "illegal instruction", and this new thing came with a new behavior -- a trap that was generated when one was encountered. Before the 286, there was no such thing as an illegal instruction, just undocumented ones.

Your question then becomes, "what did the 8086 and 8088 do when encountering undocumented instructions" and the answer is simply that the behavior was undocumented. Some of them appeared to do nothing (i.e. they produced the same apparent result as a NOP instruction), while others did odd things, or simply the same thing as another opcode.

Here's a WayBack link that may shed more light on the situation:
http://web.archive.org/web/20190321200321/http://www.os2museum.com/wp/undocumented-8086-opcodes-part-i/

share|improve this answer

answered 8 hours ago

Ken GoberKen Gober

8,37112542

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  This gets confusing because the 186 was introduced just after the 286, but it also generates an “illegal instruction” exception, so it’s technically “186 and above”.
  
  – Stephen Kitt
  4 hours ago
  

  
  
  
  

add a comment | 

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8

Illegal opcodes were just instructions that hadn't been fully defined by the chip designers – a little like Undefined Behaviour in C, but much more predictable. Many people called these "undocumented instructions", because they functioned just like ordinary instructions, on the particular versions of the particular chips on which they are found. There was no special handling to prevent these instructions from executing.

Most of them were just NOPs (either because they weren't wired up to anything or because they did stuff like writing a register to itself) or duplicates of other instructions (because the instruction decoder ignored some bits when it didn't need to pay attention to them), but some of them were more interesting.

For instance, SALC, which does:

if (carry flag set)

  AL = 0xFF;

else

  AL = 0x00;

This can be used as a translation layer between C code and certain assembly returning conventions. It is equivalent to SBB AL, AL, except that it does not clobber flags.

For more information, see:

• 
  Stephen Kitt's answer to Use of undocumented opcodes.


• 
  supercat's answer to Use of undocumented opcodes, which has a good explanation of where they come from.


• A related question on Stack Overflow

share|improve this answer

edited 16 mins ago

Cody Gray

1,149521

answered 8 hours ago

wizzwizz4♦wizzwizz4

8,915642110

add a comment | 

8

Illegal opcodes were just instructions that hadn't been fully defined by the chip designers – a little like Undefined Behaviour in C, but much more predictable. Many people called these "undocumented instructions", because they functioned just like ordinary instructions, on the particular versions of the particular chips on which they are found. There was no special handling to prevent these instructions from executing.

Most of them were just NOPs (either because they weren't wired up to anything or because they did stuff like writing a register to itself) or duplicates of other instructions (because the instruction decoder ignored some bits when it didn't need to pay attention to them), but some of them were more interesting.

For instance, SALC, which does:

if (carry flag set)

  AL = 0xFF;

else

  AL = 0x00;

This can be used as a translation layer between C code and certain assembly returning conventions. It is equivalent to SBB AL, AL, except that it does not clobber flags.

For more information, see:

• 
  Stephen Kitt's answer to Use of undocumented opcodes.


• 
  supercat's answer to Use of undocumented opcodes, which has a good explanation of where they come from.


• A related question on Stack Overflow

share|improve this answer

edited 16 mins ago

Cody Gray

1,149521

answered 8 hours ago

wizzwizz4♦wizzwizz4

8,915642110

add a comment | 

Illegal opcodes were just instructions that hadn't been fully defined by the chip designers – a little like Undefined Behaviour in C, but much more predictable. Many people called these "undocumented instructions", because they functioned just like ordinary instructions, on the particular versions of the particular chips on which they are found. There was no special handling to prevent these instructions from executing.

Most of them were just NOPs (either because they weren't wired up to anything or because they did stuff like writing a register to itself) or duplicates of other instructions (because the instruction decoder ignored some bits when it didn't need to pay attention to them), but some of them were more interesting.

For instance, SALC, which does:

if (carry flag set)

  AL = 0xFF;

else

  AL = 0x00;

This can be used as a translation layer between C code and certain assembly returning conventions. It is equivalent to SBB AL, AL, except that it does not clobber flags.

For more information, see:

• 
  Stephen Kitt's answer to Use of undocumented opcodes.


• 
  supercat's answer to Use of undocumented opcodes, which has a good explanation of where they come from.


• A related question on Stack Overflow

share|improve this answer

edited 16 mins ago

Cody Gray

1,149521

answered 8 hours ago

wizzwizz4♦wizzwizz4

8,915642110

if (carry flag set)

  AL = 0xFF;

else

  AL = 0x00;

share|improve this answer

edited 16 mins ago

Cody Gray

1,149521

answered 8 hours ago

wizzwizz4♦wizzwizz4

8,915642110

edited 16 mins ago

Cody Gray

1,149521

edited 16 mins ago

Cody Gray

1,149521

answered 8 hours ago

wizzwizz4♦wizzwizz4

8,915642110

answered 8 hours ago

wizzwizz4♦wizzwizz4

8,915642110

6

It might be better to think about it this way:

On the 286 and above, a new thing was defined called an "illegal instruction", and this new thing came with a new behavior -- a trap that was generated when one was encountered. Before the 286, there was no such thing as an illegal instruction, just undocumented ones.

Your question then becomes, "what did the 8086 and 8088 do when encountering undocumented instructions" and the answer is simply that the behavior was undocumented. Some of them appeared to do nothing (i.e. they produced the same apparent result as a NOP instruction), while others did odd things, or simply the same thing as another opcode.

Here's a WayBack link that may shed more light on the situation:
http://web.archive.org/web/20190321200321/http://www.os2museum.com/wp/undocumented-8086-opcodes-part-i/

share|improve this answer

answered 8 hours ago

Ken GoberKen Gober

8,37112542

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  This gets confusing because the 186 was introduced just after the 286, but it also generates an “illegal instruction” exception, so it’s technically “186 and above”.
  
  – Stephen Kitt
  4 hours ago
  

  
  
  
  

add a comment | 

6

It might be better to think about it this way:

On the 286 and above, a new thing was defined called an "illegal instruction", and this new thing came with a new behavior -- a trap that was generated when one was encountered. Before the 286, there was no such thing as an illegal instruction, just undocumented ones.

Your question then becomes, "what did the 8086 and 8088 do when encountering undocumented instructions" and the answer is simply that the behavior was undocumented. Some of them appeared to do nothing (i.e. they produced the same apparent result as a NOP instruction), while others did odd things, or simply the same thing as another opcode.

Here's a WayBack link that may shed more light on the situation:
http://web.archive.org/web/20190321200321/http://www.os2museum.com/wp/undocumented-8086-opcodes-part-i/

share|improve this answer

answered 8 hours ago

Ken GoberKen Gober

8,37112542

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  This gets confusing because the 186 was introduced just after the 286, but it also generates an “illegal instruction” exception, so it’s technically “186 and above”.
  
  – Stephen Kitt
  4 hours ago
  

  
  
  
  

add a comment | 

It might be better to think about it this way:

On the 286 and above, a new thing was defined called an "illegal instruction", and this new thing came with a new behavior -- a trap that was generated when one was encountered. Before the 286, there was no such thing as an illegal instruction, just undocumented ones.

Your question then becomes, "what did the 8086 and 8088 do when encountering undocumented instructions" and the answer is simply that the behavior was undocumented. Some of them appeared to do nothing (i.e. they produced the same apparent result as a NOP instruction), while others did odd things, or simply the same thing as another opcode.

Here's a WayBack link that may shed more light on the situation:
http://web.archive.org/web/20190321200321/http://www.os2museum.com/wp/undocumented-8086-opcodes-part-i/

share|improve this answer

answered 8 hours ago

Ken GoberKen Gober

8,37112542

share|improve this answer

answered 8 hours ago

Ken GoberKen Gober

8,37112542

answered 8 hours ago

Ken GoberKen Gober

8,37112542

answered 8 hours ago

Ken GoberKen Gober

8,37112542