tss Struct Reference

The Task-State Segment (TSS). More...

Data Fields
uint16_t	back_link
uint16_t void *	esp0:16
	Ring 0 stack virtual address. More...
uint16_t	ss0
uint16_t void *	esp1:16
	Ring 0 stack segment selector. More...
uint16_t	ss1
uint16_t void *	esp2:16
uint16_t	ss2
uint16_t uint32_t	cr3:16
void(*	eip )(void)
uint32_t	eflags
uint32_t	eax
uint32_t	ecx
uint32_t	edx
uint32_t	ebx
uint32_t	esp
uint32_t	ebp
uint32_t	esi
uint32_t	edi
uint16_t	es
uint16_t uint16_t	cs:16
uint16_t uint16_t uint16_t	ss:16
uint16_t uint16_t uint16_t uint16_t	ds:16
uint16_t uint16_t uint16_t uint16_t uint16_t	fs:16
uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t	gs:16
uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t	ldt:16
uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t	trace:16
uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t	bitmap

Detailed Description

The Task-State Segment (TSS).

Instances of the TSS, an x86-specific structure, are used to define "tasks", a form of support for multitasking built right into the processor. However, for various reasons including portability, speed, and flexibility, most x86 OSes almost completely ignore the TSS. We are no exception.

Unfortunately, there is one thing that can only be done using a TSS: stack switching for interrupts that occur in user mode. When an interrupt occurs in user mode (ring 3), the processor consults the ss0 and esp0 members of the current TSS to determine the stack to use for handling the interrupt. Thus, we must create a TSS and initialize at least these fields, and this is precisely what this file does.

When an interrupt is handled by an interrupt or trap gate (which applies to all interrupts we handle), an x86 processor works like this:

• If the code interrupted by the interrupt is in the same ring as the interrupt handler, then no stack switch takes place. This is the case for interrupts that happen when we're running in the kernel. The contents of the TSS are irrelevant for this case.
• If the interrupted code is in a different ring from the handler, then the processor switches to the stack specified in the TSS for the new ring. This is the case for interrupts that happen when we're in user space. It's important that we switch to a stack that's not already in use, to avoid corruption. Because we're running in user space, we know that the current process's kernel stack is not in use, so we can always use that. Thus, when the scheduler switches threads, it also changes the TSS's stack pointer to point to the new thread's kernel stack. (The call is in thread_schedule_tail() in thread.c.)

See [IA32-v3a] 6.2.1 "Task-State Segment (TSS)" for a description of the TSS. See [IA32-v3a] 5.12.1 "Exception- or Interrupt-Handler Procedures" for a description of when and how stack switching occurs during an interrupt.

Definition at line 51 of file tss.c.

Field Documentation

back_link

uint16_t tss::back_link

Definition at line 53 of file tss.c.

bitmap

uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t tss::bitmap

Definition at line 65 of file tss.c.

Referenced by tss_init().

cr3

uint16_t uint32_t tss::cr3

Definition at line 59 of file tss.c.

cs

uint16_t uint16_t tss::cs

Definition at line 65 of file tss.c.

ds

uint16_t uint16_t uint16_t uint16_t tss::ds

Definition at line 65 of file tss.c.

eax

uint32_t tss::eax

Definition at line 63 of file tss.c.

ebp

uint32_t tss::ebp

Definition at line 64 of file tss.c.

ebx

uint32_t tss::ebx

Definition at line 63 of file tss.c.

ecx

uint32_t tss::ecx

Definition at line 63 of file tss.c.

edi

uint32_t tss::edi

Definition at line 64 of file tss.c.

edx

uint32_t tss::edx

Definition at line 63 of file tss.c.

eflags

uint32_t tss::eflags

Definition at line 62 of file tss.c.

eip

void(* tss::eip) (void)

Definition at line 61 of file tss.c.

es

uint16_t tss::es

Definition at line 65 of file tss.c.

esi

uint32_t tss::esi

Definition at line 64 of file tss.c.

esp

uint32_t tss::esp

Definition at line 64 of file tss.c.

esp0

uint16_t void* tss::esp0

Ring 0 stack virtual address.

Definition at line 53 of file tss.c.

Referenced by tss_update().

esp1

uint16_t void* tss::esp1

Ring 0 stack segment selector.

Definition at line 55 of file tss.c.

esp2

uint16_t void* tss::esp2

Definition at line 57 of file tss.c.

fs

uint16_t uint16_t uint16_t uint16_t uint16_t tss::fs

Definition at line 65 of file tss.c.

gs

uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t tss::gs

Definition at line 65 of file tss.c.

ldt

uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t tss::ldt

Definition at line 65 of file tss.c.

ss

uint16_t uint16_t uint16_t tss::ss

Definition at line 65 of file tss.c.

ss0

uint16_t tss::ss0

Definition at line 55 of file tss.c.

Referenced by tss_init().

ss1

uint16_t tss::ss1

Definition at line 57 of file tss.c.

ss2

uint16_t tss::ss2

Definition at line 59 of file tss.c.

trace

uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t uint16_t tss::trace

Definition at line 65 of file tss.c.

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The documentation for this struct was generated from the following file:

• src/userprog/tss.c