Brief Introduction to Computer System Architecture

App Inventor is a “high level” programming language. That means we create programs without having to know about the underlying operating system or hardware components of our device. The software engineers that create operating system software are “low level” programmers who must be familiar with the details of the hardware.

To understand the SPECTRE and MELTDOWN exploits, we need to have a basic understanding of computer systems – particularly the CPU or processor – and how it operates.

This video is a high level, simplified introduction to the basic elements of a computer system. I emphasize “simplified”! I have an Intel processor manual from a couple of years ago that has over 3,400 pages!

A future video will look at how high level programs are converted into “machine instructions” that are processed by the CPU. After that, we will look at how SPECTRE works to read memory that should be protected.

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In the above video, I did not define “RAM” memory and what it means. RAM means “Random Access Memory”. A typical modern PC has 8 to 16 gigabytes of RAM memory. Many smart phones have 4 to 6 gigabytes of RAM memory. While both RAM and FLASH are types of memory, they are not the same thing.

This short video introduces types of memory used in computers, smart phones and electronic devices including ROM, PROM, EPROM, EEPROM, FLASH, RAM (both DRAM and SRAM), and a brief history of the now very old “magnetic core” memory.

The major difference between ROM, PROM, EPROM, EEPROM/FLASH is that these memories retain their stored values even if power is turned off.

RAM memory, however, loses its content if power is turned off. Some times a battery backup unit is attached to RAM to keep the memory “alive” even if the overall system power is turned off. RAM memory is otherwise fast to use and has become remarkably inexpensive.

“DRAM” means “dynamic RAM” and “SRAM” means “static RAM”.

Most of our devices use DRAM because it is cheaper and each bit takes up less circuit space (than SRAM) so more memory can be packed into a smaller space. The reason it is called “dynamic” RAM is because the memory must be continuously refreshed. Each bit is stored as a tiny capacitive charge. Because the charge slowly bleeds off, the charge must be periodically refreshed – if not, the values stored in memory will gradually fade away (so to speak).

A special circuit continuously reads and then rewrites each bit so that the charge stored at each bit location is refreshed and does not fade away.

 

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