# 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 theory, just a very real, very “pitfall” thing — the kernel and user programs fighting over territory, nearly ending in “mutual destruction.”
🚨 1MB is “Prime Real Estate,” But It’s Been Sold N Times Already
On old x86 computers, 1MB (that’s 0x100000) is a “sacred address.” Why? Because back in the day, the CPU could only see memory below 1MB at startup, so everyone agreed:
“Kernel, don’t wander off — just live at the 1MB spot, so we can find you easily.”
What happened? Guess what? — Now all programs want to go there by default!
You write a C program, compile it, and the linker sees: “Hey, no address specified? I’ll put it at 1MB by default!” You write a kernel, and the linker also says: “Hey, not specified? I’ll put it at 1MB too!”
💥 Two families moving into the same house at the same time, opening the door — “Hey, who are you? I’ve rented this place for three years!”
Kernel: I’m the system, I have to be here! User program: I’m hello world, I have a right to exist too!
Result? The kernel just boots up, hasn’t even had time to shout “I’m the kernel,” and gets overwritten by the user program’s code… The system blue-screens, reboots, starts again, gets overwritten again… You’re forever stuck in the “I just came alive, then got killed” loop.
🤯 So What Do We Do? The Kernel Can’t Be “Evicted” Every Day, Right?
The old-timers came up with a clever trick: trampoline
What’s a trampoline? Simply put:
First, put a small “guard code” at 1MB. It does nothing else — just one thing: move the entire kernel from 1MB to a safe place, then jump there and continue working.
It’s like moving house:
1. You first pile all your furniture at the door (1MB)
2. Then hire a temp worker (trampoline)
3. They help you move the sofa, TV, fridge — everything to the new house (3MB)
4. After moving, they shout: “OK, go to your new house now!”
5. You open the door and continue living in the new house.
Sounds cool? But — it’s really troublesome!
You have to write assembly, calculate addresses, make sure pointers don’t get corrupted, ensure every instruction can “jump without thinking”… By the time you’re done, half your hair is gone.
✅ But! We’re Developers, Not Saints — If We Can Be Lazy, We Will!
You ask me: “Can we not do the trampoline?”
Of course we can!
Let’s just “assign” the kernel to 3MB!
3MB = 0x300000. Have you ever seen any ordinary program dare to crowd into this spot? No! Nobody dares! This spot is the kernel’s “VIP Private Villa”!
So you just modify the linker script (linker.ld), just a few lines:
“`
SECTIONS {
. = 0x300000; /* Kernel base address set to 3MB */
…
}
“`
It’s that simple!
What’s the effect?
| Before | After |
|---|---|
| Kernel loaded at 1MB → overwritten by user program → crash | Kernel moves directly to 3MB → nobody bothers → rock solid |
| Need to write trampoline, lots of assembly, error-prone | Only one line changed, compile, run, success |
| Debugging like guessing riddles | Debugging like shopping at a supermarket, addresses crystal clear |
You don’t even need to care about words like “relocation,” “pointer correction,” or “address offset.” You’re just “choosing a house,” not “building a house.”
💡 Why Do I Recommend You Do This?
Because you’re a learner, not an OS engineer. You’re not trying to write Linux; you just want to:
Get your kernel running
Make your printf(“Hello Kernel!”); display something
Understand what “memory layout” means
There’s no need to torture yourself for the sake of a “classic solution.”
Trampoline is the “textbook answer,” but loading directly at 3MB is “programmer’s wisdom” — if it runs, doesn’t crash, and doesn’t give you headaches, it’s a good solution!
✅ Summary: Remember This in One Sentence
Don’t let the kernel and user programs fight over 1MB! Just have it live at 3MB — saves worry, saves effort, and saves your hair.
You don’t need a trampoline; you just need to change one line in the linker script, and then — your kernel can rest easy from then on.
📌 Bonus Tip: How to Know Exactly Where Your Kernel is Loaded?
A simple trick:
“`bash
# Check the kernel ELF file load address
readelf -l mykernel.elf | grep LOAD
# Or check the symbol table
nm mykernel.elf | grep _start
“`
You can also print the kernel’s own address at runtime:
“`c
// In kernel entry code
extern uint32_t _start;
printk("Kernel loaded at: 0x%xn", &_start);
“`
If the output shows 0x300000 — congratulations, your kernel is living comfortably!
🎯 Conclusion: The Purpose of This Article
This article uses a real problem (kernel load address conflict) to explain:
– Why 1MB is a special address
– What is a trampoline (the classic solution)
– Why you can avoid it (3MB solution)
– Most importantly: don’t be afraid to modify linker scripts
The linker script is the kernel’s “floor plan.” Master it, and you control the memory layout.
🌟 Sometimes, the best solution isn’t the most complex one, but the simplest one that works.
#OS #KernelDevelopment #MemoryLayout #LinkerScript #BootProcess #Trampoline #WritingFromScratch
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