Simple Operating Systems （简称SOS）是一个可以运行在X86平台上（包括QEMU，BOCHS，VirtualBOX等模拟器上）的面向教学的微型嵌入式操作系统系列软件。从SOS-1到SOS9，逐步完成了一个从无（只有显示）到有（虚存、文件系统、线程、进程）的操作系统创建过程。通过分析、运行和修改此操作系统，会对操作系统的实际实现有更深入的的了解。这是第四部分SOS-4。
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