Hardware September 19, 2017

Computer hardware There are three major components of a computer: CPU Temporary storage (e.g., RAM) Persistent storage (e.g., hard drive)

RAM RAM (Random Access Memory) is a “scratchpad” where the computer stores data that is currently being used. Think of it as a storage area for data under the control of the CPU. RAM is called “temporary storage” because it only keeps its state while it is supplied with power. If you’re working on a Word document and the computer suddenly loses power, you lose everything since your last save. Examples: Several tabs open in Chrome—each tab’s data is in RAM. Editing an image in Photoshop—the image data is in RAM.

RAM Most computers currently come with 8-16 GB of RAM. Cohen’s MacBook Pro shipped with 16 GB. For the sake of comparison, my first computer (Mac LC II, circa 1992) featured 4 MB. BCPS devices max out at 12 GB. More RAM = better computer performance.

Persistent storage Two primary types: hard disk drive (HDD) and solid-state drive (SSD). HDD: Data stored as a magnetic pattern on a spinning disk. SSD: No moving parts; all data stored electronically. Also known as “flash memory.” Faster and more reliable than HDD, but more expensive (though less so than it used to be). Still much slower than RAM, so it is not a suitable substitute/replacement for RAM. Flash memory does not last forever. We know there is a limit on how long it can store data, but we don’t know what that limit is.

Persistent storage

CPU Recall: the CPU is the “brains” of the computer. CPU is responsible for executing the instructions that achieve the desired results. The CPU’s main job is to handle the execution of processes. A process is a single instance of a program being executed. A CPU is only capable of understanding very simple instructions. “Add these two numbers together.” “Determine if these two numbers are equal.”

CPU pipeline The CPU runs each instruction through a pipeline, which is similar to a factory assembly line. There are typically four different processes in the pipeline: Fetch: Determines the instruction that needs to be executed next. Decode: Determines what the CPU needs to do to execute this instruction. Execute: Runs the requested computation (e.g., performing an arithmetic operation) Writeback: Stores the result of the computation by writing it to memory (often in a special location in the CPU).

CPU pipeline

Parallelism Wouldn’t it be more efficient if we could multi-task and take advantage of the fact that different parts of the CPU are responsible for different processes? This is known as parallelism.

parallelism

Parallelism A multi-core processor combines multiple CPUs into one. Dual-core: two cores Quad-core: four cores Multiple cores allow the CPU to more effectively handle multiple programs running at once (e.g., writing an email while streaming Spotify). N.B. The relationship is not linear—a quad-core processor is not necessarily four times faster than one with a single core.

Clock speed A CPU’s clock speed measures how many cycles it can complete in one second. E.g., a 2 GHz processor can complete 2 billion cycles in one second. Moore’s law states that transistors get twice as small approximately every two years. Upshot: Every two years, chip capacity doubles OR chips get smaller and cheaper (if capacity is unchanged). This is sometimes stated as every 18 months based on the prediction of another Intel executive. This is an observation, not an actual scientific law.

Moore’s law in action One of the first computers capable of executing stored instructions was EDVAC (Electronic Discrete Variable Automatic Computer). Developed in the mid-1940s for the U.S. Army by UPenn. Weighed over 17,000 pounds and covered nearly 500 square feet of floor area. Could add two numbers together in roughly 864 microseconds (1 microsecond = 1 millionth of a second), for about 1,160 operations per second. One multiplication took 2,900 microseconds (so about 340 operations per second). Required 30 people per eight-hour shift to operate.

Moore’s law in action

Moore’s law in action IBM ASCC (Automatic Sequence Controlled Calculator), also called the Harvard Mark I, was developed for the U.S. Navy in the mid-1940s. Weighed roughly 10,000 pounds. Volume of 816 cubic feet: 51 ft. long, 8 ft. high, 2 ft. deep. Mark I could perform three additions or subtractions per second. A multiplication took 6 seconds. A division took 15.3 seconds. A logarithm or trigonometric function took over a minute.

Moore’s law in action

Moore’s law in action