Technical Journal

Stuff I hack

Relocatable vs. Position-Independent Code (or, Virtual Memory isn't Just For Swap)

Myth: “Virtual memory” is the mechanism that a kernel uses to make more memory available than is actually physically installed, by setting aside a disk partition for the overflow and copying pages between memory and disk as needed.

I acquired this belief very early in my programming career, but it turns out that swapping pages to disk is merely one of the many things that “virtual memory” makes possible.

Fact: “Virtual memory” is a hardware (CPU) mechanism, which, every single time memory is accessed, references a kernel-specified data structure called a “page table” to arbitrarily frobnicate the high bits of the address, which is called “translating” from a “linear address” to a “physical address”. (The page table gets cached by a translation lookaside buffer, so the lookup is usually quite efficient!)

This fact became very real to me this week as I made a kernel from scratch: I was moderately surprised that I needed to set up a page table, when I had always thought of virtual memory as a somewhat advanced kernel feature. Today, I learned how “relocatable” and “PIC” – terms I’d encountered in the past and never really understood – suddenly make sense in this context.

Kernel from Scratch

One of my 3 major goals for Hacker School was to create a bootable, 64-bit kernel image from scratch, using only nasm and my text editor. Well, folks, one down, two to go.

Hello, kernel!

Octopress workflow

Today is my second day at Hacker School, and I decided to set up a little bit of tooling for blogging about what I do here. The first tool I set up (following the recommendations of many Hacker Schoolers and alums) was Octopress, a static site generator designed for GitHub Pages and implemented atop Jekyll. (The page you’re reading right now is Octopress-generated.) I followed the admirably thorough Octopress documentation for installation, initial configuration, deployment with Github Pages, and theme customization1. But I wanted even more convenience. So, I’m here to introduce you to the blog command (the same one I used to write this very post).

davidad@zayin ~/octopress> blog
Enter a title for your post:

blog is a bash script, pretty specific to my own setup (vim, chrome, OSX), but it could be adapted to other environments.

a davidad production