Used to be my personal blog. Now, it covers programming, low-level development, operating systems, and more.

Kernel/Boot: Cope with a Relocation by a Bootloader in 32-bit x86 Assembly Code

A kernel should be relocatable by bootloaders as this is the only guarantee that it doesn’t interfere with addresses already in use by the firmware. This is especially true for the legacy boot flow on x86, i.e., without UEFI. Update 2024-01-29: I just published a new blog post focusing more on the background. It might be worth a read for you! TL;DR Introduction There are multiple ways to make a kernel relocatable. One Read more…

By Philipp Schuster, ago

How Does the “File Size is Smaller Than Mem Size” Optimization Work in GNU ld for the .bss Section?

This article explains how the GNU linker (GNU ld) can save disk space for symbols in the .bss section. TL;DR: If a section is of type SHT_NOBITS and if it is the last section in a LOAD segment, GNU ld uses this optimization. When you build software with a compiled language, such as C, certain symbols, which may originate in variable declarations, may be initialized to zero. An example are the following two Read more…

By Philipp Schuster, ago
Symbolic Picture: .bss is part of .data section

GNU ld: Linking .bss into .data to Ensure that Mem Size Equals File Size For Each LOAD Segment (.bss in a PROGBITS Section)

Update: I found another variant. I appended it to the end of the article. Original post: Some rudimentary ELF loaders require that p_memsz equals p_filesz for each LOAD segment in order to simplify the loading of the file. For example, the microkernels NOVA and Hedron have this restriction to bootstrap their roottasks (inital process). In this blog post, I’m going to provide a solution how this can be achieved when an ELF is Read more…

By Philipp Schuster, ago

CLI Utility to Calculate Indices for Page Tables

I just want to drop a short notice that I created a small CLI utility that I think is helpful. paging-calculator helps you to calculate the indices for a given virtual address that are used for the physical address lookup at different page table levels. Paging is a mechanism that operating systems use for memory management. Check it out on crates.io or GitHub. You can install it with cargo install paging-calculator. It works Read more…

By Philipp Schuster, ago
Screenshot of the `paging-calculator` CLI utility
acpid - Terminal Screenshot

Configure acpid From BusyBox in a Custom initrd

I have a setup with a Linux Kernel and a custom initrd, hence, no full Linux distribution. The initrd is meant to be as minimal as it can be. As a consequence, even basic things such as power-off on power-button pressed must be configured manually. There is no init system available – instead, I have to provide the init system. The initrd contains utilities from busybox. In this blog post, I show you Read more…

By Philipp Schuster, ago

The Probably Simplest x86 Driver Written in Assembly – Printing to QEMU’s debugcon-Device

This is an educational resource I wish I had when I started digging into low-level and operating systems during my early time of my studies at the university. However, dear past me, I got your back! Do you want to see a minimal way to communicate with (virtual) hardware and all of that written in pure assembly? Here you go. But at first, a few background information. How do we communicate with hardware? Read more…

By Philipp Schuster, ago
Code Snippet: Part of the QEMU Debugcon Driver written in Assembly