Implement Your Own Operating System (Part 04)

In this article, we will discuss Segmentation. Segmentation means accessing the memory through segments. Segments are portions of the address space, possibly overlapping, specified by a base address and a limit. To address a byte in segmented memory, you use a 48-bit logical address: 16 bits that specify the segment and 32-bits that specify what offset you want within that segment. The offset is added to the base address of the segment, and the resulting linear address is checked against the segment’s limit — see the figure below. If everything works out fine (including access-rights checks ignored for now) the result is a linear address. When paging is disabled, then the linear address space is mapped 1:1 onto the physical address space, and the physical memory can be accessed.

Translation of logical addresses to linear addresses.

To enable segmentation you need to set up a table that describes each segment — a segment descriptor table. In x86, there are two types of descriptor tables: the Global Descriptor Table (GDT) and Local Descriptor Tables (LDT). An LDT is set up and managed by user-space processes, and all processes have their own LDT. LDTs can be used if a more complex segmentation model is desired — we won’t use it. The GDT is shared by everyone — it’s global.

As we discuss in the virtual memory and paging sections, segmentation is rarely used more than in a minimal setup, similar to what we do below in the coding part.

Ok, Let’s move to the code!

1.Accessing Memory

Most of the time when accessing memory there is no need to explicitly specify the segment to use. The processor has six 16-bit segment registers: cs, ss, ds, es, gs and fs. The register cs is the code segment register and specifies the segment to use when fetching instructions. The register ss is used whenever accessing the stack (through the stack pointer esp), and ds is used for other data accesses. The OS is free to use the registers es, gs and fs however, it wants.

Below is an example showing implicit use of the segment registers:

func:
        mov eax, [esp+4]
        mov ebx, [eax]
        add ebx, 8
        mov [eax], ebx
        ret

The above example can be compared with the following one that makes explicit use of the segment registers:

func:
        mov eax, [ss:esp+4]
        mov ebx, [ds:eax]
        add ebx, 8
        mov [ds:eax], ebx
        ret

You don’t need to use ss it for storing the stack segment selector, or ds for the data segment selector. You could store the stack segment selector in ds and vice versa. However, in order to use the implicit style shown above, you must store the segment selectors in their indented registers.

So we should make a file called gdt.s like this:

Don’t forget to add gdt.o file to Makefile .

2.Loading the Global Descriptor Table (GDT):

The Global Descriptor Table (GDT)

A GDT/LDT is an array of 8-byte segment descriptors. The first descriptor in the GDT is always a null descriptor and can never be used to access memory. At least two segment descriptors (plus the null descriptor) are needed for the GDT because the descriptor contains more information than just the base and limit fields. The two most relevant fields for us are the Type field and the Descriptor Privilege Level (DPL) field.

Table 3–1 in chapter 3 of the Intel manual [33] specifies the values for the Type field. The table shows that the Type field can’t be both writable and executable at the same time. Therefore, two segments are needed: one segment for executing code to put in cs (Type is Execute-only or Execute-Read) and one segment for reading and writing data (Type is Read/Write) to put in the other segment registers.

The DPL specifies the privilege levels required to use the segment. x86 allows for four privilege levels (PL), 0 to 3, where PL0 is the most privileged. In most operating systems (eg. Linux and Windows), only PL0 and PL3 are used. However, some operating systems, such as MINIX, make use of all levels. The kernel should be able to do anything, therefore it uses segments with DPL set to 0 (also called kernel mode). The current privilege level (CPL) is determined by the segment selector in cs.

The segments needed are described in the table below.

Loading the GDT into the processor is done with the lgdt assembly code instruction, which takes the address of a struct that specifies the start and size of the GDT. It is easiest to encode this information using a “packed struct” as shown in the following example:

struct gdt {
        unsigned int address;
        unsigned short size;
    } __attribute__((packed));

If the content of the eax the register is the address to such a struct, then the GDT can be loaded with the assembly code shown below:

lgdt [eax]

It might be easier if you make this instruction available from C, the same way as was done with the assembly code instructions in and out.

After the GDT has been loaded the segment registers need to be loaded with their corresponding segment selectors. The content of a segment selector is described in the figure and table below:

Bit:     | 15                                3 | 2  | 1 0 |
Content: | offset (index)                      | ti | rpl |

Finally, we should create the following two files to Loading the GDT:

Then we can call sgments_install_gdt function from our kmain.c :

After All, are working fine you can see the following output in os terminal:

Well done! you just successfully integrated segmentation. The completed OS from my Github is here. We will meet the next article Implement Your Own Operating System (Part 05).

Reference: The Little OS Book

Thank you!