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Questions tagged [real-mode]

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6 votes
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Can you write to the GDT on Windows 95 from protected mode? Why, and how?

Can you write to the GDT (Global Descriptor Table) on Windows 95 from protected mode? Why, and how? In some old notes of mine, I have: "In Windows 95 the descriptor tables are accessible from ...
AlphaCentauri's user avatar
4 votes
4 answers

The move to protected mode on x86 [closed]

When I learned programming, the programmer of the CPU had direct memory access. That is, the Z80, the 8086/8088, 6502, and so on. I don't know of any real use of the 186, but at least the 286 ...
ABM K's user avatar
  • 472
24 votes
4 answers

How can a 32-bit x86 CPU start with reset vector 0xFFFFFFF0 even though it starts in 16-bit real mode?

As far as I know, x86 CPUs start up in 16-bit 'real' mode (maybe some don't). The reset vector is 0xFFFFFFF0 (in most CPUs and in this context) In this 16-bit real mode, we can only access 16 bit ...
Example person's user avatar
23 votes
1 answer

How much slower was the 286 in protected mode?

I am given to understand that if the 80286 is run in protected mode, it is slightly slower than real mode, due to memory protection checks taking extra clock cycles. Just how much slower is it? Either ...
rwallace's user avatar
  • 62.8k
1 vote
2 answers

8086 assembly relative addressing issue [closed]

This tiny program puts a few A's into the text mode buffer of an ms-dos pc. But only works if the number of repetitions (CX) is loaded with a direct value. If I try to read that from memory, i.e. ...
Dercsár's user avatar
  • 695
21 votes
2 answers

How can a protected-mode Watcom C program access memory that a real-mode interrupt service returned in a segment:offset register pair?

I'm using Open Watcom 1.9 to write some 286 real mode, and 386 protected mode software for DOS in C. In real mode, I can retrieve a pointer to the system ROM character bitmaps in real mode using int ...
knol's user avatar
  • 12.5k
12 votes
2 answers

Are .COM executable binaries real mode or protected mode?

Windows and DOS binary executable files with the .EXE extension have an MZ header in them and nowadays also a PE header. But before these there used to also be .COM binary executable files and they ...
hippietrail's user avatar
  • 6,684
9 votes
5 answers

How does the ‘real mode flat model’ work?

I am interested in old Intel processors. I'll try to be as concise as possible. I have been studying both Jeff Duntemann's Assembly Language Step-by-Step - Chapter 4 and Modes of Memory Addressing on ...
Ady's user avatar
  • 139
41 votes
3 answers

80286 can switch from real mode to protected mode - but why not back?

It's been a while, but I've read in a system programming book that you could switch your Intel 80286 CPU from the normal real mode to a more powerful protected mode. I clearly remember that they said ...
Byte Commander's user avatar