Is real mode limited to 16-bit instructions on the 8086 processor?

In summary, the conversation discusses whether software running in real mode must use only 16-bit instructions. The answer is no, as on a 80386 or any later 32-bit CPU, the address-size override prefix and/or operand-size override prefix can be used to access 32-bit memory addresses and operands. However, the address is still limited to 20 bits. The source for this information is the Intel 64 And IA-32 Architectures Software Developer's Manuals, which are recommended for those programming in assembly for x86 processors.
  • #1
Ktx20
1
0
Hello
Does anyone know if its true that all software running in real mode must use only 16-bit instructions? is there an authorised sourcet that can aprove od deny it?
 
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  • #2
A true 8086 doesn't have 32 bit mode, but you asked about software running in real mode. On a 80386 or any later 32 bit cpu, I'm wondering what happens when a operand size prefix (66 hex) and/or effective address size prefix (67 hex) is used on an instruction in real mode?
 
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  • #3
No. While in real-address mode on the 80386 and up, software may use the address-size override prefix (0x67) and/or the operand-size override prefix (0x66) to access 32-bit memory addresses and/or operate on 32-bit operands, respectively.

However, please note that, when using the address-size override prefix in real-address mode, the address is still limited to 20 bits (i.e., you can only access up to memory address 0x000FFFFF). That is, unless you switch to unreal mode...

My source? Volume 1: Basic Architecture of the Intel 64 And IA-32 Architectures Software Developer's Manuals. How is that for an authorized source? Straight from the horse's mouth, so to speak, eh?

And, Ktx20, if you are going to be programming in assembly for x86 processors, you should REALLY get familiar with http://www.intel.com/products/processor/manuals/" [Broken].

There might be "educational" books out there, but Intel's PDFs were what I used to learn about the x86 architecture and how to program for it.
 
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1. What is the difference between the 8086 processor and real mode?

The 8086 processor is a microprocessor developed by Intel in the late 1970s. Real mode is a mode of operation in which the processor can access a maximum of 1 MB of memory. The 8086 processor operates in real mode by default, but it can also switch to other modes such as protected mode and virtual 8086 mode.

2. How does real mode impact system performance?

Real mode can limit system performance because it only allows the processor to access a maximum of 1 MB of memory. This can be a bottleneck for modern computing tasks that require more memory. However, real mode can also be useful for running legacy software that is designed to work within the 1 MB memory limit.

3. How do I switch from real mode to protected mode?

To switch from real mode to protected mode, the processor must be reset. This can be done through a software interrupt or by changing the value of a control register. Once in protected mode, the processor can access more memory and have additional features such as virtual memory and multitasking.

4. What is the purpose of real mode in modern computers?

Real mode is no longer commonly used in modern computers. However, it can still be useful for running legacy software or for certain boot processes. It is also important for understanding the history and development of computer systems and processors.

5. Can real mode and protected mode coexist?

Yes, real mode and protected mode can coexist in the same system. The processor can switch between these modes as needed depending on the task at hand. This allows for backward compatibility with older software while still taking advantage of the advanced features of protected mode.

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