Chapter 2 - Basic Protected Mode

Why use protected mode?

Many drawbacks of real mode, most notably the fact that real mode programs can only access 1 MB of the computer's physical memory, motivated Intel to create a totally differant environment known as protected mode. Protected mode was introduced with the 80286 processor around 1990 (before the advent of the 80286 processor, computers ran exclusively in real mode). Although the protected mode architecture underwent significant changes with the advent of the 80386 processor, it has changed very little since then. Even the computers of today are essentially the same as the 386 computers of ten years ago except that today's computers are much faster and contain more memory. Consequently, the basic protected mode environment is the same as it was on 386 computers. Most of what we will cover in this chapter has, in fact, remained the same since the 286 computers. Consequently, the best place to look for detailed documentation on the protected mode of today is to consult the documentation on the 80386 processor of a decade ago.
Intel's 80386 reference manual, which has been converted to HTML by a number of independent people and organizations, describes protected mode in gross detail.

Address mapping in Protected Mode

In protected mode, logical address are not converted to physical addresses by the shift-and-add method, as in real mode. Instead, segments are used as indecies into one of two special tables, called the Global Descriptor Table (GDT) and the Local Descriptor Table (LDT). The GDT and LDT are stored in memory with the following structure.

Each table can contain any arbitrary number of entries and each entry, or Descriptor is eight bytes long. Each descriptor describes several attributes about a segment in memory, such as the segment base, access rights, etc. The physical address and the size of the GDT and LDT are stored in special registers called the GDTR and the LDTR. The GDTR and the LDTR can only be accessed by special instructions (standard instructions like "mov" cannot access them).

When the processor converts a logical address into a physical address, it uses a step-by-step process which is outlined as follows:

• The value of the segment field of a logical address, called a selector, references a descriptor in either the GDT or the LDT. If bit 3 of the selector is set, the selector refers to a descriptor in the LDT. If this bit is cleared, the selector refers to an entry in the GDT. Bits 0-2 form the RPL. The full significance of the RPL is beyond the scope of this site, except to say that for our purposes, all bits in the RPL should be zero. Bits 4-15 store the index number of the descriptor to be accessed. This means that one can find the index merely by shifting the selector value to the right by four bits. In other words : Index = Selector >> 4. (Bits 0-3 are automatically thrown out by the shift.)
• The above table states that index 0 of either the GDT or the LDT references a NULL descriptor. If a NULL descriptor is referenced, memory access is not allowed and an error occurs (In windows, this causes the "This program has performed an illegal operation..." dialog box). For example, referencing logical addresses such as 0000:1234, 0004:5678 and 0000:ABCD are not allowed but logical addresses such as 0010: 0000 are allowed.
• Huge quantities of information are packed into each 8-byte descriptor (the precise format of a descriptor will be discussed later) . One attribute is the segment base. Once the processor finds the descriptor that the selector refers to, the segment base of that descriptor is consulted. This base is added to the offset of the logical address being referenced and the result is the relevant phyiscal address. In other words: Physical address = segment base of relevant descriptor + offset.

Examples

Suppose the GDT and the LDT contain the following data:

Here are the physical addresses that would result from some sample logical addresses.

There are many important points to make note of concerning the resolution of logical addresses under protected mode:

• Each logical address is no longer guarenteed to always refer to the same physical address. For example, if logical address 0008:0000 currently points to physical address 00000000h, the same logical address can be made to reference physical address 56780 simply by finding the first descriptor in the GDT and changing the segment base.
• In real mode, all 65536 possible segments are valid. In protected mode, this is not the case. We have already discussed that referencing index 0 of the GDT and LDT are not allowed. In addition, it should be noted that the GDT and LDT each have a finite size and referencing descriptors located beyond the end of the GDT or LDT is not allowed either. For example, if the GDT only contains 3 descriptors (plus the NULL descriptor), segments 20, 28, 30, and 38 cannot be used.
• The segment base in a descriptor table is a 32-bit number. This means that up to 4 GB of memory can be accessed in protected mode (limited of course by the amount of memory physically installed on your particular computer). The 1 MB limit of real mode does not apply.

So far, we have only scratched the surface of protected mode, but we have covered enough to try some problems. It should be noted that although we have covered how protected mode works, we have not discussed how to actually switch to protected mode. For now, we will assume that all DOS programs will run in real mode and all windows program will run in protected mode (the Windows operating system switches the computer from real mode to protected mode when it loads).

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