Monthly Archives: August 2023

How the hardware/software interface works

It’s really all about memory. But to start at the beginning, the rough stack looks like this:

  • Userspace application
  • Kernel driver
  • Hardware device

I find it easier to think about this from the middle out. On Linux, the kernel exposes hardware devices as files backed by the /dev virtual filesystem. Userspace can do normal syscalls like open, read, write, and mmap on them, as well as the less typical ioctl (for more arbitrary, device-specific functionality).1.

The files are created by kernel drivers which are modules of kernel code whose sole purpose is to interface with and abstract hardware so it can be used by other parts of the operating system, or userspace. They are implemented implemented using internal driver “frameworks” in the kernel, e.g. the I2C or SPI frameworks. When you interface with a file in /dev, you are directly triggering callback handlers in a driver which execute in the process context.

That’s how userspace interfaces with the kernel. How do drivers interface with hardware? These days, mostly via memory mapped I/O (MMIO)2. This is when device hardware “appears” at certain physical addresses, and can be interfaced with via load and store instructions using an “API” that the device defines. For example, you can read data from a sensor by simply reading a physical address, or write data out to a device by writing to an address. The technical term for the hardware component these reads/writes interface with is “registers” (i.e. memory mapped registers).

(Aside: Other than MMIO, the other main interface the kernel has with hardware is interrupts, for interrupt driven I/O processing (as opposed to polling, which is what MMIO enables). I’m not very knowledgeable about this, so I won’t get into it other than to say drivers can register handlers for specific IRQ (interrupt requests) numbers, which will be invoked by the kernel’s generic interrupt handling infrastructure.)

Using MMIOs looks a lot like embedded bare metal programming you might do on a microcontroller like a PIC. At the lowest level, a kernel driver is really just embedded bare metal programming.

Here’s an example of a device driver for UART (serial port) hardware for ARM platforms: linux/drivers/tty/serial/amba-pl011.c. If you’re debugging an ARM Linux system via a serial connection, this is might be the driver being used to e.g. show the boot messages.

The lines like:

cr = readb(uap->port.membase + UART010_CR);

are where the real magic happens.

This is simply doing a read from a memory address derived from some base address for the device, plus some offset of the specific register in question. In this case it’s reading some control information from a Control Register.

#define UART010_CR		0x14	/* Control register. */


Device interfaces may range from having just a few to many registers.

To go one step deeper down the rabbit hole, how do devices “end up” at certain physical addresses? How is this physical memory map interface implemented?3

The device/physical address mapping is implemented in digital logic outside the CPU, either on the System on Chip (SOC) (for embedded systems), or on the motherboard (PCs)4. The CPU’s physical interface include the address, data, and control buses. Digital logic converts bits of the address bus into signals that mutually exclusively enable devices that are physically connected to the bus. The implementations of load/store instructions in the CPU set a read/write bit appropriately in the Control bus, which lets devices know whether a read or write is happening. The data bus is where data is either transferred out from or into the CPU.

In practice, documentation for real implementations of these systems can be hard to find, unless you’re a customer of the SoC manufacturer. But there are some out there for older chips, e.g.

Here’s a block diagram for the Tegra 2 SoC architecture, which shipped in products like the Motorola Atrix 4G, Motorola Droid X2, and Motorola Photon. Obviously it’s much more complex than my description above. Other than the two CPU cores in the top left, and the data bus towards the middle, I can’t make sense of it. (link)

While not strictly a “System on Chip”, a class PIC microcontroller has many shared characteristics of a SoC (CPU, memory, peripherals, all in one chip package), but is much more approachable.

We can see the single MIPS core connected to a variety of peripheral devices on the peripheral bus. There’s even layers of peripheral bussing, with a “Peripheral Bridge” connected to a second peripheral bus for things like I2C and SPI.

How to be happy

Note to self:

  1. Remember: You are already enough just as you are, right here, right now. You don’t need to achieve or do anything. 1
  2. Remember: The only competition in life โ€” if you must think of it that way โ€” is to know yourself as fully as possible, and act with maximum authenticity towards that truth.
  3. Remember: All things considered, you have it good โ€” many around the world would kill to switch places and inherit every single one of your problems.

It will never be easier than right now

This is a mindset I use to help with procrastination. It first came to me in my senior year on university when I needed to do lab reports. At that point, I had been doing lab reports for 8 years โ€” ever since the start of high school. And throughout that whole time, they were always excruciating.

But I realized that they were excruciating partly because I always waited until the days before the deadline to do them, which was about a week after the actual lab. By that point, the details of the lab were much fuzzier, making the lab report way harder.

It occurred to me that even though all I wanted to do after the lab is forget everything about it and push it off to the side, that exact moment โ€” right after the lab โ€” would be the easiest moment to ever do the report. As more time passes, it will strictly get harder as I begin to lose the context of the lab.

So I sucked it up and started to immediately go to the library right after the lab and simply do the report right then. It worked very well and I only wished I had started the habit years earlier.

I try to remember this lesson and apply it to my life now. If there are situations where I need to do something, and no additional information will arrive that will influence how the job gets done, I try to do it as quickly as possible to take advantage of the context fresh in my brain.

TIL: Debugging microcontrollers may require hardware breakpoints

I was debugging a microcontroller recently and was surprised to see that I was limited to 4 breakpoints. That surprised me because I’m usually only used to seeing that kind of limitation when using watchpoints, not breakpoints. (See my youtube video for more on this).

But it makes sense after all โ€” code running on this microcontroller is different than in a process in an OS’s userspace because the code will probably be flashed into some kind of ROM (Read Only Memory) (or EEPROM). Since the code is unwritable at a hardware level, the typical method of inserting software breakpoint instructions isn’t possible. So the alternative is to use the microprocessor’s support for hardware debugging, which occupies hardware resources and is thus finite.

WIP: Business models can have significant technical impact

Business models are an interesting topic that lives directly at the boundary between business and technology.

The business model describes an abstract plan for how the business is to function sustainably, and is a full time job to develop in and of itself. Then, it must be implemented in the product using technology.

For example, a typical SaaS business model involves subscription pricing at various intervals (monthly & yearly), with a discount given for the yearly plan in exchange for more money paid up front and longer commitment. There may or may not be a limited time free trial period, or alternatively a limited time guaranteed refund. Furthermore, there may be different pricing tiers that unlock more advanced features.

Ultimately, this is all going to end up as code that models the various pricing tiers, plans, timing deadlines, and enforces security (i.e. making sure the advanced features are only available to pro users). Stripe is a common service used to model arbitrary business models and execute payments processing. They provide client libraries offering data models for concepts like users and plans.

While subscription models have grown in popularity as software shifts to the web, it’s not the only model. The older model of buying discrete software packaged versions still exists, mostly used by vendors of desktop software. In this model, customers pay a larger sum for unlimited use of a specific major version of software. Then they can optionally pay again to upgrade to a newer major version when it’s released. A variant of this exists where there’s no upgrade fee (i.e. “lifetime free updates”). Another model exists called “Rent to own” which is like a subscription, except payments stop at a certain point after which there is free unlimited use.

Whether the software runs on the client or server is another dimension to consider which may influence the business model โ€” server side software is most commonly sold using subscriptions these days.

Client Server
SubscriptionAdobeSaaS Web Apps
One time payment per versionDash, Ableton Live
One time payment, lifetime free updatesFL Studio, Tailwind CSS (technically more assets than software)SaaS Web Apps Lifetime Plans (e.g Roam Research)
Rent to ownAudio plugins via Splice

The business model impacts technical strategy in a few ways.

  1. Branching and release management
  2. Compatibility of document artifacts

Subscriptions, “lifetime free updates”, and “rent to own” are simplest in terms of branching and release management. All users are expected to run the latest software (because it’s either free or automatic, in the case of server side), so development can largely happen on a single main branch which releases are cut from.

“One time payment per version” is more complex because potentially multiple major versions of the software need to be maintained in parallel. Ideally bugfixes in Version 2 would also be applied to V3 and V4 when appropriate, but no new feature development should make its way back to V2 and risk being included in binaries send to users that only paid for V2. For this situation, long term release branches make sense as they provide code isolation, though they require some mechanism to forward bugfixes, etc.

The second aspect is document artifact compatibility. Again in this scenario, subscriptions, “lifetime free updates”, and “rent to own” are simpler in that all users can be assumed to be running on the latest version โ€” or at least can be told to upgrade at no cost.

“One time payment per version” is again more complex because multiple versions of the software exist in the wild, all producing artifacts of slightly different versions. If artifacts may be sent between users of different versions, it creates a mess of trying dealing with all the compatibility issues that may arise.