Task State Segment

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The Task State Segment (TSS) is a special data structure for x86 processors which holds information about a task. The TSS is primarily suited for hardware multitasking, where each individual process has its own TSS. In Software multitasking, one or two TSS's are also generally used, as they allow for entering ring0 code after an interrupt.


x86 Structure

offset 31-16 15-0
0x00 reserved LINK
0x04 ESP0
0x08 reserved SS0
0x0C ESP1
0x10 reserved SS1
0x14 ESP2
0x18 reserved SS2
0x1C CR3
0x20 EIP
0x28 EAX
0x2C ECX
0x30 EDX
0x34 EBX
0x38 ESP
0x3C EBP
0x40 ESI
0x44 EDI
0x48 reserved ES
0x4C reserved CS
0x50 reserved SS
0x54 reserved DS
0x58 reserved FS
0x5C reserved GS
0x60 reserved LDTR
0x64 IOPB offset reserved

TSS in software multitasking

For each CPU which executes processes possibly wanting to do system calls via interrupts, one TSS is required. The only interesting fields are SS0 and ESP0. Whenever a system call occurs, the CPU gets the SS0 and ESP0-value in its TSS and assigns the stack-pointer to it. So one or more kernel-stacks need to be set up for processes doing system calls. Be aware that a thread's/process' time-slice may end during a system call, passing control to another thread/process which may as well perform a system call, ending up in the same stack. Solutions are to create a private kernel-stack for each thread/process and re-assign esp0 at any task-switch or to disable scheduling during a system-call.

Setting up a TSS is straight-forward. An entry in the Global Descriptor Table is needed (see also the GDT Tutorial), specifying the TSS' address as "base", TSS' size as "limit", 0x89 (Present|Executable|Accessed) as "access byte" and 0x40 (Size-bit) as "flags". In the TSS itself, the members "SS0", "ESP0" and "IOPB offset" are to be set:

  • SS0 gets the kernel datasegment descriptor (e.g. 0x10 if the third entry in your GDT describes your kernel's data)
  • ESP0 gets the value the stack-pointer shall get at a system call
  • IOPB may get the value sizeof(TSS) (which is 104) if you don't plan to use this io-bitmap further (according to mystran in http://forum.osdev.org/viewtopic.php?t=13678)

The actual loading of the TSS must take place in protected mode and after the GDT has been loaded. The loading is simple as:

mov ax, 0x??  ;The descriptor of the TSS in the GDT (e.g. 0x28 if the sixths entry in your GDT describes your TSS)
ltr ax        ;The actual load

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