updated 08-Task_management.md to be better structured

course/08-Task_management.md

@@ -144,7 +144,7 @@ Several possible ways to switch to a task:
 ## Switching to a task (2/2)
 
 - Here, we will describe how to switch to a nested task by calling the `call far` instruction
-- Very simple: one simply executes the `call far` instruction with the TSS selector of the task to switch to in argument:
+- Very simple: simply execute the `call far` instruction with the **TSS selector** of the task to switch to in argument:
 
   ```{.assembler .small}
   ; switch to the task referenced by
@@ -152,56 +152,7 @@ Several possible ways to switch to a task:
   call far <TSS selector>
   ```
 
-[//]: # ----------------------------------------------------------------
-## Task switching: steps (simplified)
-
-\small
-
-The CPU automatically peforms these operations during a task switch to a \textcolor{mygreen}{new task}:
-
-1) Reads the `tr` register to obtain the \textcolor{myorange}{current task}'s TSS
-1) Saves the current CPU context into the \textcolor{myorange}{current task}'s TSS
-1) Using the \textcolor{mygreen}{new task}'s TSS selector, loads the `tr` register to point to the \textcolor{mygreen}{new task}'s TSS
-1) Loads the \textcolor{mygreen}{new task}'s context (from its TSS) into the CPU
-   - includes `cr3` = \textcolor{mygreen}{new task}'s address space (= memory mapping)
-   - `cr3` points to the \textcolor{mygreen}{new task}'s page directory
-1) Resumes the execution of the \textcolor{mygreen}{new task} (from the restored `eip`)
-
-[//]: # ----------------------------------------------------------------
-## Switching back to the previous task
-
-\footnotesize
-
-Switching back to the \textcolor{mypurple}{previous task} is trivial:
-
-- Execute the `iret` assembly instruction
-
-What happens (simplified) when the CPU executes `iret`?
-
-1) Reads the `tr` register to obtain the \textcolor{mygreen}{current task}'s TSS
-1) Saves the current CPU context into the \textcolor{mygreen}{current task}'s TSS
-1) Loads the `tr` register with the value of the "Previous Task Link" field of the \textcolor{mygreen}{current task}'s TSS
-1) Loads the \textcolor{mypurple}{previous task}'s context (from its TSS) into the CPU
-   - includes `cr3` = \textcolor{mypurple}{previous task}'s address space
-1) Resumes the execution of the \textcolor{mypurple}{previous task}
-
-[//]: # ----------------------------------------------------------------
-## Stack switching
-
-- A stack switch occurs during a task switch from a **lesser privilege level to a higher** privilege level
-  - E.g. \textcolor{mygreen}{ring 3} $\rightarrow$ \textcolor{myred}{ring 0}: `ss` and `esp` are loaded from the current task's `TSS.ss0` and `TSS.esp0` values
-- Typically happens when:
-  - a task (user mode) performs a system call (\textcolor{mygreen}{ring 3} $\rightarrow$ \textcolor{myred}{ring 0})
-  - user code (\textcolor{mygreen}{ring 3}) is interrupted by a hardware interrupt or a processor exception
-
-[//]: # ----------------------------------------------------------------
-## Back to task switching
-
-- As previously seen, the CPU can switch to a task using a TSS **selector**:
-  ```{.assembler .small}
-  call far <TSS selector>
-  ```
-- What is a TSS **selector**?
+- What is a **\textcolor{myred}{TSS selector}**?
 
 [//]: # ----------------------------------------------------------------
 ## Global Descriptor Table (GDT)
@@ -257,7 +208,22 @@ Example of a GDT defining 3 tasks:
 
 
 [//]: # ----------------------------------------------------------------
-## Task switching revisited: in-depth steps
+## Task switching: steps (simplified)
+
+\small
+
+The CPU automatically peforms these operations during a task switch to a \textcolor{mygreen}{new task}:
+
+1) Reads the `tr` register to obtain the \textcolor{myorange}{current task}'s TSS
+1) Saves the current CPU context into the \textcolor{myorange}{current task}'s TSS
+1) Using the \textcolor{mygreen}{new task}'s TSS selector, loads the `tr` register to point to the \textcolor{mygreen}{new task}'s TSS
+1) Loads the \textcolor{mygreen}{new task}'s context (from its TSS) into the CPU
+   - includes `cr3` = \textcolor{mygreen}{new task}'s address space (= memory mapping)
+   - `cr3` points to the \textcolor{mygreen}{new task}'s page directory
+1) Resumes the execution of the \textcolor{mygreen}{new task} (from the restored `eip`)
+
+[//]: # ----------------------------------------------------------------
+## Task switching: in-depth steps
 
 \footnotesize
 
@@ -275,7 +241,7 @@ The CPU automatically peforms these operations during a task switch:
 1) Resumes the execution of the new task (from the restored `eip`)
 
 [//]: # ----------------------------------------------------------------
-## Task switching: important notes
+## Task register
 
 - The task register, `tr`, is loaded using the `ltr` instruction
   - requires a TSS selector as operand
@@ -285,10 +251,35 @@ The CPU automatically peforms these operations during a task switch:
   - `ltr` requires a TSS selector which implies a properly initialized and loaded GDT
 
 [//]: # ----------------------------------------------------------------
-## Task initialization, static data, done once
+## Switching back to the previous task
 
 \footnotesize
 
+Switching back to the \textcolor{mypurple}{previous task} is trivial:
+
+- Execute the `iret` assembly instruction
+
+What happens (simplified) when the CPU executes `iret`?
+
+1) Reads the `tr` register to obtain the \textcolor{mygreen}{current task}'s TSS
+1) Saves the current CPU context into the \textcolor{mygreen}{current task}'s TSS
+1) Loads the `tr` register with the value of the "Previous Task Link" field of the \textcolor{mygreen}{current task}'s TSS
+1) Loads the \textcolor{mypurple}{previous task}'s context (from its TSS) into the CPU
+   - includes `cr3` = \textcolor{mypurple}{previous task}'s address space
+1) Resumes the execution of the \textcolor{mypurple}{previous task}
+
+[//]: # ----------------------------------------------------------------
+## Stack switching
+
+- A stack switch occurs during a task switch from a **lesser privilege level to a higher** privilege level
+  - E.g. \textcolor{mygreen}{ring 3} $\rightarrow$ \textcolor{myred}{ring 0}: `ss` and `esp` are loaded from the current task's `TSS.ss0` and `TSS.esp0` values
+- Typically happens when:
+  - a task (user mode) performs a system call (\textcolor{mygreen}{ring 3} $\rightarrow$ \textcolor{myred}{ring 0})
+  - user code (\textcolor{mygreen}{ring 3}) is interrupted by a hardware interrupt or a processor exception
+
+[//]: # ----------------------------------------------------------------
+## Task initialization, static data, done once
+
 1. Reserve a GDT entry (TSS descriptors) for the initial TSS 
    - used to save the CPU context **before** switching to the first task
    - to be done **once**
@@ -297,13 +288,10 @@ The CPU automatically peforms these operations during a task switch:
 1. Loads the task register with the initial TSS selector
 1. Reserve a GDT entry (TSS descriptors) for task T
 1. Allocate a data structures for task T:
-   - \scriptsize page directory
-   - \scriptsize TSS structure and TSS selector
-   - \scriptsize kernel stack
-   - \scriptsize anything else that might be useful
-1. Initialize T's page directory with desired memory mappings, typically:
-   - \scriptsize framebuffer (identity, user privileged)
-   - \scriptsize RAM (identity, kernel privileged)
+   - page directory
+   - TSS structure and TSS selector
+   - kernel stack
+   - anything else that might be useful
 
 [//]: # ----------------------------------------------------------------
 ## Task load workflow (1/3)
@@ -314,21 +302,23 @@ The CPU automatically peforms these operations during a task switch:
 [//]: # ----------------------------------------------------------------
 ## Task load workflow (2/3)
 
-\small
-
 1. Initialize task T's data structures:
-   - \footnotesize initialize T's TSS descriptor (so it points to T's TSS)
-   - \footnotesize compute T's TSS selector
-   - \footnotesize initialize T's context (TSS structure)
-1. Initialize T's page directory with memory mapping where app (code + data) will be loaded:
-   - \footnotesize allocate frames to store app + stack
-   - \footnotesize contiguously map these frames at the desired virtual address
+   - initialize T's TSS descriptor (so it points to T's TSS)
+   - compute T's TSS selector
+   - initialize T's context (TSS structure)
+
+1. Initialize T's page directory with desired memory mappings, typically:
+   - framebuffer (identity, \textcolor{mygreen}{user privileged})
+   - RAM (identity, \textcolor{myred}{kernel privileged})
+   - memory area where app will be loaded (\textcolor{mygreen}{user privileged}):
+     - allocate frames to store app + **stack**
+     - contiguously map these frames at the desired virtual address
 1. \textcolor{myorange}{Copy the app (multiboot module) into T's allocated content}
 
 [//]: # ----------------------------------------------------------------
 ## Task load workflow (3/3)
 
-\textcolor{myorange}{Copy the app (multiboot module) into T's allocated content}
+**\textcolor{myorange}{Copy the app (multiboot module) into T's allocated content}**
 
 \footnotesize