ASCII

What is it

American Standard Code for Information Interchange

ASCII is a standard data encoding format for electronic communication between computers.

  • ASCII is about converting binary number to a numeric value to a character or instruction

These are arbitrary mappings of numeric value to a human-readable character

  • Read further section on IBM vs Mac extended characters

This is in contrast to hexadecimal which is a sequential (less arbitrary) mapping of bits/bytes to a more human-legible format (0-9, then A-F)

Original ASCII

Original ASCII was a 7-bit code

  • 2^7 = 128 possible characters

1 bit was reserved bit for parity (way to look for errors)

First 0 to 31 were reserved as control characters

First character 32 represents blank space ( )

Last character 126 represents tilde (~)

Control characters are invisible (not printed on screen) characters that serve as instructions to peripherals such as printers or screens

Extended Characters

Modern ASCII uses extended character sets which uses all 8-bit allocation to double the number of characters:

  • 2^8 = 256 possible characters

IBM and Mac created their own extended character sets (because they were competitors), leading to diverging definition of extended character sets

  • 130 was used for accented e on Windows/Dos while Macs used 142

What is ASCII's relationship to binary number and decimal number

Source

Let's take typing on a keyboard as an example.

A keypress on a keyboard sends a very low voltage signal which is picked up by the I/O system on motherboard.

The I/O handler then sends the signal to the interrupt handler, which sends it as a keyboard interrupt to the operating system's keyboard driver.

The keyboard driver maps this high-priority interrupt signal to a binary value according to hardware's specific rules.

This binary representation of a keypress is what the operating system uses or sends to another program

When you press a g and h key, two voltages get translated to binary values of 0110 0111 and 0100 1000.

  • These values translate to decimal representation of 103 and 72

The ASCII code is a mapping between the binary representation to a decimal numeric value and a corresponding symbol which the computer paints on the screen.

Since hexadecimal can also represent binary number, you can also map ASCII to hexadecimal representation