Task State Segment

The TSS holds ring-0 stack pointer and I/O map base for privilege switches on older multitasking models. Even on modern kernels, a minimal TSS is required so IST or ring transitions know which stack to use on interrupts from ring 3. For example, the x86 Linux per-CPU TSS mainly stores the ring-0 RSP for double-fault stacks.

Minimal fields

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TR register: Points to TSS selector in GDT (busy bit managed by CPU)
IST entries (64-bit TSS): Alternate stacks for critical exceptions
I/O bitmap offset: Set to TSS limit to disable port traps if unused
Hardware task switching is obsolete; software context switch replaces it

Busy bit rules

The CPU sets the TSS busy bit when loaded via LTR; do not manually edit it unless you know the double-fault implications. For minimal kernels, only ESP0 and SS0 fields matter; zero the rest. Place the TSS in memory that stays mapped after paging. The TR register holds the selector; verify it points to a system segment type 0x9 (available TSS) in the GDT.

Why for this exercise

You will define a TSS descriptor in the GDT and load it with LTR. This exercise requires a valid ESP0 and SS0 for future ring-3 entry or interrupt stack switching.

Implement TSS setup in C.

Requirements:

Define TSS structure with all relevant fields
Fill ESP0 and SS0 for kernel stack
Implement create_tss_descriptor() for GDT
Implement load_tr() that simulates LTR instruction
Show TSS fields printed
Show GDT TSS descriptor
Implement IO bitmap concept
Show that TSS is required for ring 3 -> ring 0 transitions
Print selector value for TSS
Show TR register loaded

Test:

Show TSS created
Show ESP0 and SS0 set
Show TSS descriptor in GDT
Show TR loaded
Show IO bitmap concept

Task State Segment

Minimal fields

nasm
Loading…

TR register: Points to TSS selector in GDT (busy bit managed by CPU)
IST entries (64-bit TSS): Alternate stacks for critical exceptions
I/O bitmap offset: Set to TSS limit to disable port traps if unused
Hardware task switching is obsolete; software context switch replaces it

Busy bit rules

Why for this exercise

You will define a TSS descriptor in the GDT and load it with LTR. This exercise requires a valid ESP0 and SS0 for future ring-3 entry or interrupt stack switching.

Implement TSS setup in C.

Requirements:

Define TSS structure with all relevant fields
Fill ESP0 and SS0 for kernel stack
Implement create_tss_descriptor() for GDT
Implement load_tr() that simulates LTR instruction
Show TSS fields printed
Show GDT TSS descriptor
Implement IO bitmap concept
Show that TSS is required for ring 3 -> ring 0 transitions
Print selector value for TSS
Show TR register loaded

Test:

Show TSS created
Show ESP0 and SS0 set
Show TSS descriptor in GDT
Show TR loaded
Show IO bitmap concept

Set Up the Task State Segment (TSS)

Topic Overview

Task State Segment

Minimal fields

Busy bit rules

Why for this exercise

Implementation Task

Progressive Hints

Resources

Learn by building

Set Up the Task State Segment (TSS)

Topic Overview

Task State Segment

Minimal fields

Busy bit rules

Why for this exercise

Implementation Task

Progressive Hints

Resources

Learn by building