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I am interested in old Intel processors. I'll try to be as concise as possible.

1) In real mode flat model, does the CPU and operating system see all 1MB but only programs see 64KB or the operating system and the CPU is also limited to 64KB (16-bit)?

2) Do 16-bit processors run in both real mode flat model and real mode segmented model but 32-bit processors run in protected mode flat model?

3) What are the examples (Windows) of real mode flat model, real mode segmented model and protected mode flat model (like MS-DOS, Windows 3.1 to Windows XP - or higher)? (Can't find it in the Internet)

Thanks in advance for reply,
Adi

I’m assuming you’re asking about x86 processors, not the older 8-bit CPUs.

1. Real mode is always segmented, and everything (CPU, operating system, programs, even peripherals on the CPU bus) has access to all the system’s address space up to just over 1 MiB (1 MiB strictly before the 286). You can write programs without paying attention to segments, and you’ll be limited to 64 KiB, but nothing enforces that; this is the memory model used by default for COM programs under DOS. There’s no practical distinction between “real mode flat model” and “real mode segmented model”.




2. The 8086, 8088, and 80186 CPUs only run in real mode. The 80286 introduced 16-bit protected mode with a 24-bit address space. The 80386 introduced 32-bit protected mode, which still relies on segments (but segments can provide direct access to all 4 GiB of address space). Newer CPUs support all the modes available in older CPUs.




3. Up to Windows 3.0 included, Windows could run in real mode, using segments. Windows/386, Windows 3.0 and later could also run in protected mode, on 286s (Windows 3.0) and 386s. Windows NT, Windows 95 and all subsequent versions of Windows run in 32-bit protected mode. MS-DOS always ran in real mode; there were several DOS extenders which allowed programs to run in protected mode on top of DOS, such as DOS/4GW.

There is an “unreal flat mode” on 386s and later, which allows programs to run in real mode while accessing all 4 GiB, but it’s difficult to use and never gained much popularity. Sometimes this is referred to as “flat real mode”, when running 16-bit code with 32-bit registers and 4 GiB segment limits.

Most other 386-based operating systems run in 32-bit protected mode, generally with a flat model where segments cover all of the address space.

1) All software sees the full 20bit address space, since there is no distinction between kernel and user land.

2) Flat mode is simply all segment descriptors set to maximum length and base offset 0. This is not feasible in 16 bit protected mode (80286), but most 80386 operating systems will use this mode in protected mode. There are other alternatives supported by the hardware, however

3) MS-DOS in real mode was always segmented. There are DOS extenders that allow flat modes beyond 16 bit addressing. 16 bit windows is designed in such way it is mostly transparent whether the cpu is running in real or protected mode.