Process scheduling and multitasking in KnightOS September 2, 2014 on Drew DeVault's blog

I’m going to do some blogging about technical decisions made with KnightOS. It’s an open-source project I’ve been working on for the past four years to build an open-source Unix-like kernel for TI calculators (in assembly). It’s been a cool platform on top of which I can research low level systems concepts and I thought I’d share some of my findings with the world.

So, first of all, what is scheduling? For those who are completely out of the loop, I’ll explain what exactly it is and why it’s neccessary. Computers run on a CPU, which executes a series of instructions in order. Each core is not capable of running several instructions concurrently. However, you can run hundreds of processes at once on your computer (and you probably are doing so as you read this article). There are a number of ways of accomplishing, but the one that suits the most situations is preemtive multitasking. This is what KnightOS uses. You see, a CPU can only execute one instruction after another, but you can “raise an interrupt”. This will halt execution and move to some other bit of code for a moment. This can be used to handle various events (for example, the GameBoy raises an interrupt when a button is pressed). One of these events is often a timer, which raises an interrupt at a fixed interval. This is the mechanism by which preemptive multitasking is accomplished.

Let’s say for a moment that you have two programs loaded into memory and running, at addresses 0x1000 and 0x2000. Your kernel has an interrupt handler at 0x100. So if program A is running and an interrupt fires, the following happens:

  1. 0x1000 is pushed to the stack as the return address
  2. The program counter is set to 0x100 and the interrupt runs
  3. The interrupt concludes and returns, which pops 0x1000 from the stack and into the program counter.

Once the interrput handler runs, however, the kernel has a chance to be sneaky:

  1. 0x1000 is pushed to the stack as the return address
  2. The program counter is set to 0x100 and the interrupt runs
  3. The interrupt removes 0x1000 from the stack and puts 0x2000 there instead
  4. The interrupt concludes and returns, which pops 0x2000 from the stack and into the program counter.

Now the interrupt has switched the CPU from program A to program B. And the next time an interrupt occurs, the kernel can switch from program B to program A. This event is called a “context switch”. This is the basis of preemptive multitasking. On top of this, however, there are lots of ideas around which processes should get CPU time and when. Some systems have more complex schedulers, but KnightOS runs on limited hardware and I wanted the context switch to be short and sweet so that the running processes get as much of the CPU as possible. I’ll explain the simple KnightOS scheduling algorithm here. First, its goals:

What KnightOS uses is a simple round robin with the ability to suspend threads. That is, we have a list of processes and then some flags, among which is whether or not the processes is currently suspended. So say we have this list of processes in memory:

As process 1 is running and an interrupt fires, the kernel looks at this table and picks the next non-suspended process to run - process 2. On the next interrupt, it does it again, skipping process 3 and giving time to process 4.

To actually implement this, we have to think about the stack. KnightOS runs on z80 processors, which have a single stack and a shared memory space. The CPU uses the PC register to keep track of which address the current instruction is at. That is, say you compile this code:

ld a, 10
inc a
ld (hl), a

This compiles to the machine code 3E 0A 3C 77. Say we load this program at 0x8000 - then 0x8000 will point to ld a, 10. When the CPU finishes executing this instruction, it advances PC to 0x8002 (since ld a, 10 is a two-byte instruction). The next instruction it executes will be inc a, and then PC advances to 0x8003.

The stack is used for a lot of things. It can be used to save values, and it is used to call subroutines. It is also used for interrupts. It’s like the same stacks you use in higher level applications, but it’s at a very low level. When an interrupt fires, the current value of PC is pushed to the stack. Then PC is set to the interrupt routine, and then when that’s done the top of the stack is removed and placed into PC (effectively returning control to the original location). However, since the stack is used for much more than that, we have additional things to consider.

In KnightOS, when a new process starts, it’s allocated a stack in memory and the CPU’s stack pointer (SP) is set to its address. When an interrupt happens, we need to change the stack to point at some other process so it has time to run (since that’s where its PC is). However, we need to make sure that the first processes stack is left intact. Since we allocate a new stack for the next process, we can simply change SP to that processes stack. This will leave behind the value of PC that was pushed during the interrupt for the previous process, and lo and behlod a similar value of PC is waiting on top of the other processes stack.

So that’s it! We do a simple round robin, skipping suspended processes and following the procedure outlined above to switch between them. This is how KnightOS shares one CPU with several “concurrent” processes. Operating systems like Linux use more complicated schedulers with more interesting theory if you’d like some additional reading. And of course, since KnightOS is open source, you may enjoy reading all of our code for handling this stuff (in assembly):

Context switching

Stack allocation during process creation

We’re hanging out on #knightos on Freenode if you want to chat about cool low-level stuff like scheduling and memory management.

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