June, 2024

# Writing an OS Kernel from Scratch – Part 4: Entering Protected Mode (Detailed Segmented Memory Management) “Protected mode ≠ Paging! Its first line of defense is the segmentation mechanism that modern OSes ‘quietly bypass.'” In the previous few articles, we used GRUB to easily boot the kernel, output “Hello World,” and understood the Multiboot... » read more

# Quick Tip: Kernel Loaded at 1MB? Stop Joking, That Spot’s Already Taken! Have you ever wondered: Why does the OS kernel get loaded to the 1MB position? Can anyone go there? If I write a “hello world” program, what if it wants to live there too? Don’t worry, today we’re not talking about deep... » read more

H08 — DDR5 On-Die ECC and Sideband ECC: On-Chip Error Correction vs System ECC Key Insight: ECC is not just a technology. It is a commitment to data reliability. ECC Types Introduced in DDR5 DDR5 has three types of ECC, working at different levels: DDR5 ECC 的三层架构： Layer 1：On-Die ECC（片内 ECC） → DRAM 颗粒内部，每颗颗粒自己纠错 →... » read more

# Writing an OS Kernel from Scratch – Part 5: Enabling Paging — Building the Foundation of Virtual Memory “Segmentation is just a transition; paging is the true moat of modern operating systems. Today, we manually enable x86 paging, giving the kernel virtual memory!” In the previous article, we deeply understood protected mode and segmented... » read more

354 | Interrupt Controller: How Hardware Interrupts the CPU Key Insight: You press a key, and a character appears on the screen almost simultaneously — behind this, the CPU has never been “waiting” for the keyboard. It has been running its own code the whole time, until the hardware forcibly “interrupts” it. 1. Problem: How... » read more

K04 — When Memory Is Exhausted: How the OOM Killer Chooses Its Victims Key Insight: When memory is exhausted, the kernel needs a “referee” to decide who should leave. That is the OOM Killer. OOM Killer Trigger Conditions When system memory is critically low, Linux invokes the OOM Killer. The trigger is not “memory is... » read more

Writing an OS Kernel from Scratch | USB Protocol — From HID to High-Speed Devices Have you ever wondered: When you plug in a mouse, how does the system know it’s a mouse and not a keyboard? Why can a USB drive be used immediately, but an old printer requires a driver to be installed?... » read more

Writing an OS Kernel from Scratch | Context Switch — How the CPU Changes the Scene The scheduler decides to take process A off the CPU and put process B on the CPU — but how does the CPU know which line of code A paused at? Where does B resume from? How are register... » read more

S08 — How Does an Address Travel from the CPU to a DRAM Cell Key Insight: Every bit of data has its complete journey map. The Complete Path from CPU to DRAM How does an address issued by the CPU ultimately select a DRAM cell? It requires a complete address resolution process. Complete path (simplified):... » read more

K02 — Process Address Space: Virtual Address Management and VMA Key Insight: Each process’s address space is an independent map. Every region on the map is described by a VMA. What is a VMA (Virtual Memory Area) VMA (Virtual Memory Area) is a Linux kernel data structure describing a contiguous region in a process’s virtual... » read more