Embedded Systems/Microprocessor Architectures

The chapters in this section will discuss some of the basics in microprocessor architecture. They will discuss how many features of a microprocessor are implemented, and will attempt to point out some of the pitfalls (speed decreases and bottlenecks, specifically) that each feature represents to the system.

Memory Bus

In a computer, a processor is connected to the RAM by a data bus. The data bus is a series of wires running in parallel to each other that can send data to the memory, and read data back from the memory. In addition, the processor must send the address of the memory to be accessed to the RAM module, so that the correct information can be manipulated.

Multiplexed Address/Data Bus

In old microprocessors, and in some low-end versions today, the memory bus is a single bus that will carry both the address of the data to be accessed, and then will carry the value of the data. Putting both signals on the same bus, at different times is a technique known as "time division multiplexing", or just multiplexing for short. The effect of a multiplexed memory bus is that reading or writing to memory actually takes twice as long: half the time to send the address to the RAM module, and half the time to access the data at that address. This means that on a multiplexed bus, moving data to and from the memory is a very expensive (in terms of time) process, and therefore memory read/write operations should be minimized. It also makes it important to ensure algorithms which work on large datasets are cache efficient.

Demultiplexed Bus

The opposite of a multiplexed bus is a demultiplexed bus. A demultiplexed bus has the address on one set of wires, and the data on another set. This scheme is twice as fast as a multiplexed system, and therefore memory read/write operations can occur much faster.

Bus Speed

In modern high speed microprocessors, the internal CPU clock may move much faster than the clock that synchronizes the rest of the microprocessor system. This means that operations that need to access resources outside the processor (the RAM for instance) are restricted to the speed of the bus, and cannot go as fast as possible. In these situations, microprocessors have 2 options: They can wait for the memory access to complete (slow), or they can perform other tasks while they are waiting for the memory access to complete (faster). Old microprocessors and low-end microprocessors will always take the first option (so again, limit the number of memory access operations), while newer, and high-end microprocessors will often take the second option.

I/O Bus

Any computer, be it a large PC or a small embedded computer, is useless if it has no means to interact with the outside world. I/O communications for an embedded computer frequently happen over a bus called the I/O Bus. Like the memory bus, the I/O bus frequently multiplexes the input and output signals over the same bus. Also, the I/O bus is moving at a slower speed than the processor is, so a large numbers of I/O operations can cause a severe performance bottleneck.

It is not uncommon for different IO methods to have separate buses. Unfortunately, it is also not uncommon for the electrical engineers designing the hardware to cheat and use a bus for more than 1 purpose. Doing so can save the need for extra transistors in the layout, and save cost. For example, a project may use the USB bus to talk to some LEDs that are physically close by. These different devices may have very different speeds of communication. When programming IO bus control, make sure to take this into account.

In some systems, memory mapped IO is used. In this scheme, the hardware reads its IO from predefined memory addresses instead of over a special bus. This means you'll have simpler software, but it also means main memory will get more access requests.

In a later section of this book, IO Programming, we discuss programming the IO bus.