Chapter 09：User Process
===

:::info

作業系統的終極任務就是向 User Process 提供服務，所以本章的目標就是成功跳轉 User Process。本章的背景知識之前在 ==[Chapter 03-1：Privilege Level Switching](https://hackmd.io/@srhuang/BJO73NE5le)== 有詳細的範例實作，如果對於如何切換權限 (Ring0/Ring3) 還不熟悉的人可以先補充這部分的知識。
  1. Memory mgmt. for user process
  2. Load User Process
  3. Jump to User Process
:::
>[time=Sat, Nov 29, 2025 2:46 AM]
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https://youtu.be/er73Fo5T-dQ
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# Memory mgmt. for user process

>[!Note] Support user virtual address bitmap
>

>[!Note] Overview of memory mgmt.
>

>[!Note] Overview of memory bitmap
>

## Source Code
https://github.com/srhuang/a-os/commit/c7be2124ddbfc485325e08ac81158bf0b131fb48

## Compile

make all

## Put on hard disk

sh gen.sh

## Checkpoint

# Load User Process


>[!Note] hard disk layout
>

>[!Note] ELF
>

>[!Note] Section & Segment
>Segment = loader 使用的單位（程式執行需要的 memory 區塊）
>Section = linker 使用的單位（編譯/連結時的邏輯區塊）
>Segment 是由多個 section 合併形成的。
>Section 不一定會出現在任何 segment 裡（很多純工具用的 section）。
>

## Source Code
https://github.com/srhuang/a-os/commit/dadee4c58be66de81054e4b215cb04ff0dbe0558

## Compile

make all

## Put on hard disk
==gen.sh==

dd if=../code/out/mbr.bin of=./60mb.img bs=512 count=1 conv=notrunc
dd if=../code/out/loader.bin of=./60mb.img bs=512 count=4 seek=1 conv=notrunc
dd if=../code/out/kernel.bin of=./60mb.img bs=512 count=200 seek=5 conv=notrunc
dd if=../code/out/usr.bin of=./60mb.img bs=512 count=100 seek=205 conv=notrunc

sh gen.sh

## Checkpoint

ll out/usr.bin


readelf -h -l out/usr.bin


# Jump to User Process

>[!Note] TSS data structure
>

>[!Note] TSS Descriptor
>

>[!Note] User Process Stack
>

>[!Note] User Process Kernel Stack (Ring 0 --> Ring 3)
>

>[!Note] Interrupt Triggered Kernel Stack (Ring 3 --> Ring 0)
>

>[!Note] System Call Kernel Stack (Ring 3 --> Ring 0)
>

>[!Warning] mmap_min_addr
>The default value for mmap_min_addr varies by Linux distribution and kernel version, but it's commonly set to 65536 (64 KiB) for security to prevent NULL-pointer dereference exploits
>`cat /proc/sys/vm/mmap_min_addr`
>我們設定最低的 user space virtual address is `0x10000`

## Source Code
https://github.com/srhuang/a-os/commit/3f5c978caa14576bef935050a6221ed889b6e870

## Compile

make all

## Put on hard disk

sh gen.sh

## Checkpoint

vb 0x2b:0x80480a0
c
info gdt
info tab