Chapter 03-2：CPU Multitasking
===

:::info

雖然現代作業系統早已捨棄 x86 CPU 原生支援的 multitasking，但是我們仍然可以透過這個章節的範例，一窺當初 80386 設計的 CPU level multitasking。
  1. LDT
  2. Task Gate and TSS

:::
>[time=Sat, Sep 6, 2025 11:47 PM]
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https://youtu.be/tACT3xxqcE4
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>[!Note] Overview of Segment Descriptors

# LDT

>Reference: [使用Local Descriptor Table (LDT)](https://jasonblog.github.io/note/os/1415.html)

>[!Note] LDT segment descriptor
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>[!Note] LLDT: Load LDT
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>[!Note] Memory Layout
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>[!Note] Code Flow
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## Source Code

https://github.com/srhuang/a-os/commit/248e558b13d57bfdfdb98e9b017d9115723c2144

## Compile

nasm boot/loader.s -I boot/ -o out/loader.bin 

## Put on hard disk

dd if=../code/out/loader.bin of=./60mb.img bs=512 count=4 seek=1 conv=notrunc

## Checkpoint



# Task Gate

>[!Note] Task State Segment (TSS)
>Reference: [Task State Segment](https://pdos.csail.mit.edu/6.828/2004/readings/i386/s07_01.htm)
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>[!Note] TSS Descriptor
>Reference: [Task Gate Descriptor](https://pdos.csail.mit.edu/6.828/2004/readings/i386/s07_04.htm)
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>[!Note] Task Gate Descriptor
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>[!Note] call task gate steps
>Reference: [Task Switching](https://pdos.csail.mit.edu/6.828/2004/readings/i386/s07_05.htm)
>1. Get new TSS Selector from task gate.
>2. Get new TSS Descriptor from GDT.
>3. Check new TSS Descriptor: P should be `1`, B should be `0`.
>4. Get old TSS position from TR Register and backup all registers.
>5. Load new TSS position to TR Register and restore all registers.
>6. Set new TSS Descriptor: B = `1`.
>7. Set Register elfags: NT = `1`.
>8. Set new TSS: `BACK LINK TO PREVIOUS TSS`.
>9. Jump to new TSS.

>[!Note] iret steps
>1. Set Register eflags: NT = `0`.
>2. Set current TSS Descriptor: B = `0`.
>3. Get current TSS position from TR Register and backup all registers.
>4. Get `BACK LINK TO PREVIOUS TSS`, and load it to TR register, and restore all registers.
>5. Jump to previous TSS.

>[!Note] Memory Layout
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>[!Note] Code Flow
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## Source Code

https://github.com/srhuang/a-os/commit/e8b9c9e13367e306fd4a011d4416145d82cbb571

## Compile

nasm boot/loader.s -I boot/ -o out/loader.bin 

## Put on hard disk

dd if=../code/out/loader.bin of=./60mb.img bs=512 count=4 seek=1 conv=notrunc

## Checkpoint