UART - Raspberry Pi Hardware Reference - page 106

The serial port developer, wishing to communicate with a serial device or AVR
class microcontroller, will be very interested in raw mode. Using raw mode, the data you
transmit is sent unmodified to its destination. Likewise, the data received is received as it
was originally transmitted. Cooked mode, which is the norm, is a very different beast.
The original purpose of serial lines for Unix was the handling of user interaction
using terminal I/O (this is still true for the serial port console). Many terminal processing
functions were considered common enough among applications to centralize them in
the kernel. This saved the application from having to deal with these physical aspects and
lead to consistency in their handling. This terminal handling is affectionately known as
cooked mode .
The main areas of cooked mode processing are as follows:

Input processing : The type of kernel processing performed on
serial input data (like backspace processing)

Output processing : The type of kernel processing performed
on serial output data (like converting a sent line feed into a
carriage return and line-feed pair)

Local processing : Involving input and output, processing
features such as echo

Control processing : Other serial controls

We can get a sense of how raw mode differs from cooked mode by looking at what
cfmakeraw(3) changes. Looking at

term.c_iflag &= ~ (IGNBRK | BRKINT | PARMRK | ISTRIP
| INLCR | IGNCR | ICRNL | IXON);

we see that the following input processing features are disabled:

Flag Description Setting

IGNBRK Ignore break Disabled

BRKINT Break reads as 0x00 Disabled

PARMRK Don't prefix with \377 (byte reads 0) Disabled

ISTRIP Strip off eighth bit Disabled

INLCR Translate NL to CR Disabled

IGNCR Ignore CR Disabled

ICRNL Translate CR to NL Disabled

IXON Enable output XON/XOFF Disabled