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Can an x86 CPU running in real mode be considered to be basically an 8086 CPU?


The start of x86: Intel 8080 vs Intel 8086?How do you put a 286 in Protected Mode?How do accelerators and CPU cards work on the Apple II?How to use the “darker” CGA palette using x86 Assembly?Examples of operating systems using hardware task switching of x86 CPUsDid the 286 go out of its way to follow the 8088 bus protocol?How did people program for Consoles with multiple CPUs?













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When an x86 CPU is running in real mode, can it be considered to be basically an 8086 CPU (or maybe 8088)? Or are there differences between the two?










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    1















    When an x86 CPU is running in real mode, can it be considered to be basically an 8086 CPU (or maybe 8088)? Or are there differences between the two?










    share|improve this question









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      1












      1








      1








      When an x86 CPU is running in real mode, can it be considered to be basically an 8086 CPU (or maybe 8088)? Or are there differences between the two?










      share|improve this question









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      user12245 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.












      When an x86 CPU is running in real mode, can it be considered to be basically an 8086 CPU (or maybe 8088)? Or are there differences between the two?







      cpu x86






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      share|improve this question









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      share|improve this question




      share|improve this question








      edited 1 hour ago









      Stephen Kitt

      39.3k8160172




      39.3k8160172






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      asked 2 hours ago









      user12245user12245

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          2 Answers
          2






          active

          oldest

          votes


















          7














          An x86 CPU running in real mode is intended to be backwards-compatible with an 8086 or 8088, but there do end up being a number of differences, for example:



          • newer CPUs run faster (in general);

          • newer CPUs add new instructions (and, with the 386, new registers, since the 32-bit registers can be used in real mode);

          • instruction timing — the speed of individual CPU instructions — varies from one family to another; some instructions run more slowly on newer CPUs;

          • implementation details vary, and in some cases, can affect run-time behaviour — for example, varying prefetch queue lengths mean that self-modifying code may not work on CPUs other than the model it was written for;

          • some instructions behave differently — for example, PUSH SP on an 8086 increments SP after pushing it, whereas on a 286 it increments SP before pushing it, so the value on the stack is different;

          • bus interactions (LOCK prefixes) behave differently on the 8086/8088 compared to all later CPUs;

          • illegal opcodes which run without error on the 8086 produce exceptions on later CPUs;

          • the 8086 has no instruction length limit, whereas instructions which are too long will produce exceptions on later CPUs;

          • segment wraparounds inside an instruction or word access work on the 8086 but trap on later CPUs;

          • stack wraparounds work on the 8086 but will shut down a 286 or later;

          • divide errors behave differently on the 8086/8088 compared to all later CPUs.

          The 8086 also has a few bugs which were fixed in later CPUs, but that generally doesn’t matter — all it means is that the workarounds which were needed on 8086/8088 are no longer necessary on later CPUs. (One example is the handling of interrupted instructions with multiple prefixes.)



          Software which is actually affected by differences other than speed is very rare indeed, and you can count on the vast majority of software still technically working on a modern x86 CPU in real mode. Speed is another matter; famously, programs written using Turbo Pascal fail with an “Error 200” on CPUs faster than a 200MHz Pentium, and many games don’t cope well with faster CPUs (but some CPUs can be slowed down in creative ways).






          share|improve this answer

























          • Great write up, except speed issues are not really due a changed/extended ISA - they would occure as well back then when speed up occured - after all, having a 10 MHz 8086 was already in the early 1980s a way to screw programs made for a 4.77 MHz 8088

            – Raffzahn
            2 hours ago











          • The fact that it applies to 8MHz v. 4.77 MHz 8086s doesn’t mean it stops applying when comparing any other CPU to an 8086/8088 ;-).

            – Stephen Kitt
            2 hours ago











          • True, and I don't want to put this down, as it (may) be the most obvious (and usually intended) effect. Just, as far as I understand the intention of the question is about any difference originated in a changed/extended ISA, not a higher clock frequency - after all, we always can clock down faster CUs (at least I hope so :))

            – Raffzahn
            2 hours ago











          • Another difference between the 8086 and chips with protected mode: the 286+ didn't wrap around addresses at the 1MB mark in real mode, so IBM had to add an A20 line mask to work around that issue: although not generally an issue on PCs, it could theoretically happen if the A20 line has been enabled to access the UMB region and a program tried using wraparound to access low memory. It's pretty unlikely however: I've never seen a program crash in that manner!

            – ErikF
            18 mins ago


















          0















          When an x86 CPU is running in real mode, can it be considered to be basically an 8086 CPU (or maybe 8088)?




          As so often it depends on your value of 'basically' (and there is no user visible difference between 8086 and 8088 beside speed).




          Or are there differences between the two?




          Well, it's so far the same, as every (modern) x86 operating in real mode will execute pure 8086 programs (*1) adhering to what were legal (*2) instructions (*3) on the 8086.



          At the same time they are able to execute later extensions as well while in real mode. So it is possible to write 32-bit real mode programs, or use additional registers and instructions in real mode.



          So a x86 isn't the same but for most parts (and depending on the CPU used) a compatible superset of an 8086.




          *1 - Lets ignore 'external' hardware differences for this.



          *2 - There are a few instructions that changed over time, including basic 8086 ones. They may cause incompatibilities in rare circumstances.



          *3 - There are some non-instruction combinations (i.e. prefixes) that were ignored on 8086 but will throw interrupts on later CPUs or result in addressing errors. This is a classic case of later restrictions on less well defined behaviour (like double segment prefix and the like).






          share|improve this answer

























          • As usual, receiving a down vote is cool - but without any reasoning its rather senseless if not cowardly, isn't it? So, what part made you hitting the button?

            – Raffzahn
            1 hour ago







          • 1





            Have an upvote to balance things out ;-).

            – Stephen Kitt
            1 hour ago











          • @StephenKitt Thanks. Cool. (BTW, your answer is the better, more detailed anyway (sans the speed issue that is :))) Still I'd like to know what people consider worth a downvote. I assume it's well know that I don't doge any confrontation. To me there's nothing more helpful in finding a good answer than people applying critical thinking on what has been said. So I'm genuinely interested in what seams to be downvoteworthy.

            – Raffzahn
            8 mins ago












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          2 Answers
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          2 Answers
          2






          active

          oldest

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          active

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          active

          oldest

          votes









          7














          An x86 CPU running in real mode is intended to be backwards-compatible with an 8086 or 8088, but there do end up being a number of differences, for example:



          • newer CPUs run faster (in general);

          • newer CPUs add new instructions (and, with the 386, new registers, since the 32-bit registers can be used in real mode);

          • instruction timing — the speed of individual CPU instructions — varies from one family to another; some instructions run more slowly on newer CPUs;

          • implementation details vary, and in some cases, can affect run-time behaviour — for example, varying prefetch queue lengths mean that self-modifying code may not work on CPUs other than the model it was written for;

          • some instructions behave differently — for example, PUSH SP on an 8086 increments SP after pushing it, whereas on a 286 it increments SP before pushing it, so the value on the stack is different;

          • bus interactions (LOCK prefixes) behave differently on the 8086/8088 compared to all later CPUs;

          • illegal opcodes which run without error on the 8086 produce exceptions on later CPUs;

          • the 8086 has no instruction length limit, whereas instructions which are too long will produce exceptions on later CPUs;

          • segment wraparounds inside an instruction or word access work on the 8086 but trap on later CPUs;

          • stack wraparounds work on the 8086 but will shut down a 286 or later;

          • divide errors behave differently on the 8086/8088 compared to all later CPUs.

          The 8086 also has a few bugs which were fixed in later CPUs, but that generally doesn’t matter — all it means is that the workarounds which were needed on 8086/8088 are no longer necessary on later CPUs. (One example is the handling of interrupted instructions with multiple prefixes.)



          Software which is actually affected by differences other than speed is very rare indeed, and you can count on the vast majority of software still technically working on a modern x86 CPU in real mode. Speed is another matter; famously, programs written using Turbo Pascal fail with an “Error 200” on CPUs faster than a 200MHz Pentium, and many games don’t cope well with faster CPUs (but some CPUs can be slowed down in creative ways).






          share|improve this answer

























          • Great write up, except speed issues are not really due a changed/extended ISA - they would occure as well back then when speed up occured - after all, having a 10 MHz 8086 was already in the early 1980s a way to screw programs made for a 4.77 MHz 8088

            – Raffzahn
            2 hours ago











          • The fact that it applies to 8MHz v. 4.77 MHz 8086s doesn’t mean it stops applying when comparing any other CPU to an 8086/8088 ;-).

            – Stephen Kitt
            2 hours ago











          • True, and I don't want to put this down, as it (may) be the most obvious (and usually intended) effect. Just, as far as I understand the intention of the question is about any difference originated in a changed/extended ISA, not a higher clock frequency - after all, we always can clock down faster CUs (at least I hope so :))

            – Raffzahn
            2 hours ago











          • Another difference between the 8086 and chips with protected mode: the 286+ didn't wrap around addresses at the 1MB mark in real mode, so IBM had to add an A20 line mask to work around that issue: although not generally an issue on PCs, it could theoretically happen if the A20 line has been enabled to access the UMB region and a program tried using wraparound to access low memory. It's pretty unlikely however: I've never seen a program crash in that manner!

            – ErikF
            18 mins ago















          7














          An x86 CPU running in real mode is intended to be backwards-compatible with an 8086 or 8088, but there do end up being a number of differences, for example:



          • newer CPUs run faster (in general);

          • newer CPUs add new instructions (and, with the 386, new registers, since the 32-bit registers can be used in real mode);

          • instruction timing — the speed of individual CPU instructions — varies from one family to another; some instructions run more slowly on newer CPUs;

          • implementation details vary, and in some cases, can affect run-time behaviour — for example, varying prefetch queue lengths mean that self-modifying code may not work on CPUs other than the model it was written for;

          • some instructions behave differently — for example, PUSH SP on an 8086 increments SP after pushing it, whereas on a 286 it increments SP before pushing it, so the value on the stack is different;

          • bus interactions (LOCK prefixes) behave differently on the 8086/8088 compared to all later CPUs;

          • illegal opcodes which run without error on the 8086 produce exceptions on later CPUs;

          • the 8086 has no instruction length limit, whereas instructions which are too long will produce exceptions on later CPUs;

          • segment wraparounds inside an instruction or word access work on the 8086 but trap on later CPUs;

          • stack wraparounds work on the 8086 but will shut down a 286 or later;

          • divide errors behave differently on the 8086/8088 compared to all later CPUs.

          The 8086 also has a few bugs which were fixed in later CPUs, but that generally doesn’t matter — all it means is that the workarounds which were needed on 8086/8088 are no longer necessary on later CPUs. (One example is the handling of interrupted instructions with multiple prefixes.)



          Software which is actually affected by differences other than speed is very rare indeed, and you can count on the vast majority of software still technically working on a modern x86 CPU in real mode. Speed is another matter; famously, programs written using Turbo Pascal fail with an “Error 200” on CPUs faster than a 200MHz Pentium, and many games don’t cope well with faster CPUs (but some CPUs can be slowed down in creative ways).






          share|improve this answer

























          • Great write up, except speed issues are not really due a changed/extended ISA - they would occure as well back then when speed up occured - after all, having a 10 MHz 8086 was already in the early 1980s a way to screw programs made for a 4.77 MHz 8088

            – Raffzahn
            2 hours ago











          • The fact that it applies to 8MHz v. 4.77 MHz 8086s doesn’t mean it stops applying when comparing any other CPU to an 8086/8088 ;-).

            – Stephen Kitt
            2 hours ago











          • True, and I don't want to put this down, as it (may) be the most obvious (and usually intended) effect. Just, as far as I understand the intention of the question is about any difference originated in a changed/extended ISA, not a higher clock frequency - after all, we always can clock down faster CUs (at least I hope so :))

            – Raffzahn
            2 hours ago











          • Another difference between the 8086 and chips with protected mode: the 286+ didn't wrap around addresses at the 1MB mark in real mode, so IBM had to add an A20 line mask to work around that issue: although not generally an issue on PCs, it could theoretically happen if the A20 line has been enabled to access the UMB region and a program tried using wraparound to access low memory. It's pretty unlikely however: I've never seen a program crash in that manner!

            – ErikF
            18 mins ago













          7












          7








          7







          An x86 CPU running in real mode is intended to be backwards-compatible with an 8086 or 8088, but there do end up being a number of differences, for example:



          • newer CPUs run faster (in general);

          • newer CPUs add new instructions (and, with the 386, new registers, since the 32-bit registers can be used in real mode);

          • instruction timing — the speed of individual CPU instructions — varies from one family to another; some instructions run more slowly on newer CPUs;

          • implementation details vary, and in some cases, can affect run-time behaviour — for example, varying prefetch queue lengths mean that self-modifying code may not work on CPUs other than the model it was written for;

          • some instructions behave differently — for example, PUSH SP on an 8086 increments SP after pushing it, whereas on a 286 it increments SP before pushing it, so the value on the stack is different;

          • bus interactions (LOCK prefixes) behave differently on the 8086/8088 compared to all later CPUs;

          • illegal opcodes which run without error on the 8086 produce exceptions on later CPUs;

          • the 8086 has no instruction length limit, whereas instructions which are too long will produce exceptions on later CPUs;

          • segment wraparounds inside an instruction or word access work on the 8086 but trap on later CPUs;

          • stack wraparounds work on the 8086 but will shut down a 286 or later;

          • divide errors behave differently on the 8086/8088 compared to all later CPUs.

          The 8086 also has a few bugs which were fixed in later CPUs, but that generally doesn’t matter — all it means is that the workarounds which were needed on 8086/8088 are no longer necessary on later CPUs. (One example is the handling of interrupted instructions with multiple prefixes.)



          Software which is actually affected by differences other than speed is very rare indeed, and you can count on the vast majority of software still technically working on a modern x86 CPU in real mode. Speed is another matter; famously, programs written using Turbo Pascal fail with an “Error 200” on CPUs faster than a 200MHz Pentium, and many games don’t cope well with faster CPUs (but some CPUs can be slowed down in creative ways).






          share|improve this answer















          An x86 CPU running in real mode is intended to be backwards-compatible with an 8086 or 8088, but there do end up being a number of differences, for example:



          • newer CPUs run faster (in general);

          • newer CPUs add new instructions (and, with the 386, new registers, since the 32-bit registers can be used in real mode);

          • instruction timing — the speed of individual CPU instructions — varies from one family to another; some instructions run more slowly on newer CPUs;

          • implementation details vary, and in some cases, can affect run-time behaviour — for example, varying prefetch queue lengths mean that self-modifying code may not work on CPUs other than the model it was written for;

          • some instructions behave differently — for example, PUSH SP on an 8086 increments SP after pushing it, whereas on a 286 it increments SP before pushing it, so the value on the stack is different;

          • bus interactions (LOCK prefixes) behave differently on the 8086/8088 compared to all later CPUs;

          • illegal opcodes which run without error on the 8086 produce exceptions on later CPUs;

          • the 8086 has no instruction length limit, whereas instructions which are too long will produce exceptions on later CPUs;

          • segment wraparounds inside an instruction or word access work on the 8086 but trap on later CPUs;

          • stack wraparounds work on the 8086 but will shut down a 286 or later;

          • divide errors behave differently on the 8086/8088 compared to all later CPUs.

          The 8086 also has a few bugs which were fixed in later CPUs, but that generally doesn’t matter — all it means is that the workarounds which were needed on 8086/8088 are no longer necessary on later CPUs. (One example is the handling of interrupted instructions with multiple prefixes.)



          Software which is actually affected by differences other than speed is very rare indeed, and you can count on the vast majority of software still technically working on a modern x86 CPU in real mode. Speed is another matter; famously, programs written using Turbo Pascal fail with an “Error 200” on CPUs faster than a 200MHz Pentium, and many games don’t cope well with faster CPUs (but some CPUs can be slowed down in creative ways).







          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited 2 hours ago

























          answered 2 hours ago









          Stephen KittStephen Kitt

          39.3k8160172




          39.3k8160172












          • Great write up, except speed issues are not really due a changed/extended ISA - they would occure as well back then when speed up occured - after all, having a 10 MHz 8086 was already in the early 1980s a way to screw programs made for a 4.77 MHz 8088

            – Raffzahn
            2 hours ago











          • The fact that it applies to 8MHz v. 4.77 MHz 8086s doesn’t mean it stops applying when comparing any other CPU to an 8086/8088 ;-).

            – Stephen Kitt
            2 hours ago











          • True, and I don't want to put this down, as it (may) be the most obvious (and usually intended) effect. Just, as far as I understand the intention of the question is about any difference originated in a changed/extended ISA, not a higher clock frequency - after all, we always can clock down faster CUs (at least I hope so :))

            – Raffzahn
            2 hours ago











          • Another difference between the 8086 and chips with protected mode: the 286+ didn't wrap around addresses at the 1MB mark in real mode, so IBM had to add an A20 line mask to work around that issue: although not generally an issue on PCs, it could theoretically happen if the A20 line has been enabled to access the UMB region and a program tried using wraparound to access low memory. It's pretty unlikely however: I've never seen a program crash in that manner!

            – ErikF
            18 mins ago

















          • Great write up, except speed issues are not really due a changed/extended ISA - they would occure as well back then when speed up occured - after all, having a 10 MHz 8086 was already in the early 1980s a way to screw programs made for a 4.77 MHz 8088

            – Raffzahn
            2 hours ago











          • The fact that it applies to 8MHz v. 4.77 MHz 8086s doesn’t mean it stops applying when comparing any other CPU to an 8086/8088 ;-).

            – Stephen Kitt
            2 hours ago











          • True, and I don't want to put this down, as it (may) be the most obvious (and usually intended) effect. Just, as far as I understand the intention of the question is about any difference originated in a changed/extended ISA, not a higher clock frequency - after all, we always can clock down faster CUs (at least I hope so :))

            – Raffzahn
            2 hours ago











          • Another difference between the 8086 and chips with protected mode: the 286+ didn't wrap around addresses at the 1MB mark in real mode, so IBM had to add an A20 line mask to work around that issue: although not generally an issue on PCs, it could theoretically happen if the A20 line has been enabled to access the UMB region and a program tried using wraparound to access low memory. It's pretty unlikely however: I've never seen a program crash in that manner!

            – ErikF
            18 mins ago
















          Great write up, except speed issues are not really due a changed/extended ISA - they would occure as well back then when speed up occured - after all, having a 10 MHz 8086 was already in the early 1980s a way to screw programs made for a 4.77 MHz 8088

          – Raffzahn
          2 hours ago





          Great write up, except speed issues are not really due a changed/extended ISA - they would occure as well back then when speed up occured - after all, having a 10 MHz 8086 was already in the early 1980s a way to screw programs made for a 4.77 MHz 8088

          – Raffzahn
          2 hours ago













          The fact that it applies to 8MHz v. 4.77 MHz 8086s doesn’t mean it stops applying when comparing any other CPU to an 8086/8088 ;-).

          – Stephen Kitt
          2 hours ago





          The fact that it applies to 8MHz v. 4.77 MHz 8086s doesn’t mean it stops applying when comparing any other CPU to an 8086/8088 ;-).

          – Stephen Kitt
          2 hours ago













          True, and I don't want to put this down, as it (may) be the most obvious (and usually intended) effect. Just, as far as I understand the intention of the question is about any difference originated in a changed/extended ISA, not a higher clock frequency - after all, we always can clock down faster CUs (at least I hope so :))

          – Raffzahn
          2 hours ago





          True, and I don't want to put this down, as it (may) be the most obvious (and usually intended) effect. Just, as far as I understand the intention of the question is about any difference originated in a changed/extended ISA, not a higher clock frequency - after all, we always can clock down faster CUs (at least I hope so :))

          – Raffzahn
          2 hours ago













          Another difference between the 8086 and chips with protected mode: the 286+ didn't wrap around addresses at the 1MB mark in real mode, so IBM had to add an A20 line mask to work around that issue: although not generally an issue on PCs, it could theoretically happen if the A20 line has been enabled to access the UMB region and a program tried using wraparound to access low memory. It's pretty unlikely however: I've never seen a program crash in that manner!

          – ErikF
          18 mins ago





          Another difference between the 8086 and chips with protected mode: the 286+ didn't wrap around addresses at the 1MB mark in real mode, so IBM had to add an A20 line mask to work around that issue: although not generally an issue on PCs, it could theoretically happen if the A20 line has been enabled to access the UMB region and a program tried using wraparound to access low memory. It's pretty unlikely however: I've never seen a program crash in that manner!

          – ErikF
          18 mins ago











          0















          When an x86 CPU is running in real mode, can it be considered to be basically an 8086 CPU (or maybe 8088)?




          As so often it depends on your value of 'basically' (and there is no user visible difference between 8086 and 8088 beside speed).




          Or are there differences between the two?




          Well, it's so far the same, as every (modern) x86 operating in real mode will execute pure 8086 programs (*1) adhering to what were legal (*2) instructions (*3) on the 8086.



          At the same time they are able to execute later extensions as well while in real mode. So it is possible to write 32-bit real mode programs, or use additional registers and instructions in real mode.



          So a x86 isn't the same but for most parts (and depending on the CPU used) a compatible superset of an 8086.




          *1 - Lets ignore 'external' hardware differences for this.



          *2 - There are a few instructions that changed over time, including basic 8086 ones. They may cause incompatibilities in rare circumstances.



          *3 - There are some non-instruction combinations (i.e. prefixes) that were ignored on 8086 but will throw interrupts on later CPUs or result in addressing errors. This is a classic case of later restrictions on less well defined behaviour (like double segment prefix and the like).






          share|improve this answer

























          • As usual, receiving a down vote is cool - but without any reasoning its rather senseless if not cowardly, isn't it? So, what part made you hitting the button?

            – Raffzahn
            1 hour ago







          • 1





            Have an upvote to balance things out ;-).

            – Stephen Kitt
            1 hour ago











          • @StephenKitt Thanks. Cool. (BTW, your answer is the better, more detailed anyway (sans the speed issue that is :))) Still I'd like to know what people consider worth a downvote. I assume it's well know that I don't doge any confrontation. To me there's nothing more helpful in finding a good answer than people applying critical thinking on what has been said. So I'm genuinely interested in what seams to be downvoteworthy.

            – Raffzahn
            8 mins ago
















          0















          When an x86 CPU is running in real mode, can it be considered to be basically an 8086 CPU (or maybe 8088)?




          As so often it depends on your value of 'basically' (and there is no user visible difference between 8086 and 8088 beside speed).




          Or are there differences between the two?




          Well, it's so far the same, as every (modern) x86 operating in real mode will execute pure 8086 programs (*1) adhering to what were legal (*2) instructions (*3) on the 8086.



          At the same time they are able to execute later extensions as well while in real mode. So it is possible to write 32-bit real mode programs, or use additional registers and instructions in real mode.



          So a x86 isn't the same but for most parts (and depending on the CPU used) a compatible superset of an 8086.




          *1 - Lets ignore 'external' hardware differences for this.



          *2 - There are a few instructions that changed over time, including basic 8086 ones. They may cause incompatibilities in rare circumstances.



          *3 - There are some non-instruction combinations (i.e. prefixes) that were ignored on 8086 but will throw interrupts on later CPUs or result in addressing errors. This is a classic case of later restrictions on less well defined behaviour (like double segment prefix and the like).






          share|improve this answer

























          • As usual, receiving a down vote is cool - but without any reasoning its rather senseless if not cowardly, isn't it? So, what part made you hitting the button?

            – Raffzahn
            1 hour ago







          • 1





            Have an upvote to balance things out ;-).

            – Stephen Kitt
            1 hour ago











          • @StephenKitt Thanks. Cool. (BTW, your answer is the better, more detailed anyway (sans the speed issue that is :))) Still I'd like to know what people consider worth a downvote. I assume it's well know that I don't doge any confrontation. To me there's nothing more helpful in finding a good answer than people applying critical thinking on what has been said. So I'm genuinely interested in what seams to be downvoteworthy.

            – Raffzahn
            8 mins ago














          0












          0








          0








          When an x86 CPU is running in real mode, can it be considered to be basically an 8086 CPU (or maybe 8088)?




          As so often it depends on your value of 'basically' (and there is no user visible difference between 8086 and 8088 beside speed).




          Or are there differences between the two?




          Well, it's so far the same, as every (modern) x86 operating in real mode will execute pure 8086 programs (*1) adhering to what were legal (*2) instructions (*3) on the 8086.



          At the same time they are able to execute later extensions as well while in real mode. So it is possible to write 32-bit real mode programs, or use additional registers and instructions in real mode.



          So a x86 isn't the same but for most parts (and depending on the CPU used) a compatible superset of an 8086.




          *1 - Lets ignore 'external' hardware differences for this.



          *2 - There are a few instructions that changed over time, including basic 8086 ones. They may cause incompatibilities in rare circumstances.



          *3 - There are some non-instruction combinations (i.e. prefixes) that were ignored on 8086 but will throw interrupts on later CPUs or result in addressing errors. This is a classic case of later restrictions on less well defined behaviour (like double segment prefix and the like).






          share|improve this answer
















          When an x86 CPU is running in real mode, can it be considered to be basically an 8086 CPU (or maybe 8088)?




          As so often it depends on your value of 'basically' (and there is no user visible difference between 8086 and 8088 beside speed).




          Or are there differences between the two?




          Well, it's so far the same, as every (modern) x86 operating in real mode will execute pure 8086 programs (*1) adhering to what were legal (*2) instructions (*3) on the 8086.



          At the same time they are able to execute later extensions as well while in real mode. So it is possible to write 32-bit real mode programs, or use additional registers and instructions in real mode.



          So a x86 isn't the same but for most parts (and depending on the CPU used) a compatible superset of an 8086.




          *1 - Lets ignore 'external' hardware differences for this.



          *2 - There are a few instructions that changed over time, including basic 8086 ones. They may cause incompatibilities in rare circumstances.



          *3 - There are some non-instruction combinations (i.e. prefixes) that were ignored on 8086 but will throw interrupts on later CPUs or result in addressing errors. This is a classic case of later restrictions on less well defined behaviour (like double segment prefix and the like).







          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited 7 mins ago

























          answered 2 hours ago









          RaffzahnRaffzahn

          55.3k6136224




          55.3k6136224












          • As usual, receiving a down vote is cool - but without any reasoning its rather senseless if not cowardly, isn't it? So, what part made you hitting the button?

            – Raffzahn
            1 hour ago







          • 1





            Have an upvote to balance things out ;-).

            – Stephen Kitt
            1 hour ago











          • @StephenKitt Thanks. Cool. (BTW, your answer is the better, more detailed anyway (sans the speed issue that is :))) Still I'd like to know what people consider worth a downvote. I assume it's well know that I don't doge any confrontation. To me there's nothing more helpful in finding a good answer than people applying critical thinking on what has been said. So I'm genuinely interested in what seams to be downvoteworthy.

            – Raffzahn
            8 mins ago


















          • As usual, receiving a down vote is cool - but without any reasoning its rather senseless if not cowardly, isn't it? So, what part made you hitting the button?

            – Raffzahn
            1 hour ago







          • 1





            Have an upvote to balance things out ;-).

            – Stephen Kitt
            1 hour ago











          • @StephenKitt Thanks. Cool. (BTW, your answer is the better, more detailed anyway (sans the speed issue that is :))) Still I'd like to know what people consider worth a downvote. I assume it's well know that I don't doge any confrontation. To me there's nothing more helpful in finding a good answer than people applying critical thinking on what has been said. So I'm genuinely interested in what seams to be downvoteworthy.

            – Raffzahn
            8 mins ago

















          As usual, receiving a down vote is cool - but without any reasoning its rather senseless if not cowardly, isn't it? So, what part made you hitting the button?

          – Raffzahn
          1 hour ago






          As usual, receiving a down vote is cool - but without any reasoning its rather senseless if not cowardly, isn't it? So, what part made you hitting the button?

          – Raffzahn
          1 hour ago





          1




          1





          Have an upvote to balance things out ;-).

          – Stephen Kitt
          1 hour ago





          Have an upvote to balance things out ;-).

          – Stephen Kitt
          1 hour ago













          @StephenKitt Thanks. Cool. (BTW, your answer is the better, more detailed anyway (sans the speed issue that is :))) Still I'd like to know what people consider worth a downvote. I assume it's well know that I don't doge any confrontation. To me there's nothing more helpful in finding a good answer than people applying critical thinking on what has been said. So I'm genuinely interested in what seams to be downvoteworthy.

          – Raffzahn
          8 mins ago






          @StephenKitt Thanks. Cool. (BTW, your answer is the better, more detailed anyway (sans the speed issue that is :))) Still I'd like to know what people consider worth a downvote. I assume it's well know that I don't doge any confrontation. To me there's nothing more helpful in finding a good answer than people applying critical thinking on what has been said. So I'm genuinely interested in what seams to be downvoteworthy.

          – Raffzahn
          8 mins ago











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