C Expert Programming Chapter 1 C: Through the Fog of Time and Space 1.1 The Prehistoric Phase of the C Language

The C language was born from a failed project---the Multics project.This project attempts to build a very large operating system that can be applied to very small hardware systems.Multics became a treasure trove of engineering lessons, but it also paved the way for C to embody "small is beautiful".

Ken Thompson Dennis Ritchie
Thompson PDP-7 New Operating System (Assembly Language)
Brian Kernighan 1970 named UNXI(1970.1.1)
Thompson created the B language, which he used for researchThe language BCPL has been simplified so that B's interpreter can reside in the PDP-7's only 8KB of memory.Because of the memory constraints of the hardware system, it only allowed to place an interpreter, not a compiler, and the resulting inefficiency prevented the use of the B language for system programming of UNIX itself.

1965-7 (BCPL)---"1969 (B)---"1971 (New B)---"1972-3 (early C)
Language
UNIX (with PDP-7 written in assembly language)---"UNIX (written in PDP-11 assembly language)---"UNIX (written in C)
        Operating system
  PDP-7 PDP-11

IBM 360

Honeywell 635

Exactly, UNIX appeared earlier than C (that's why UNIX's system time is measured in seconds from January 1, 1970, because that's when it was created.

Golden Rule for Compiler Designers: Efficiency is (Almost) Everything
Things to Care About: Meaningful Error Messages; Good Documentation; Product Support

Compiler efficiency includes: running efficiency (the speed at which the code runs) and compilation efficiency (the speed at which executable code is generated).Except for some development and learning environments, operational efficiency plays a decisive role.

There are a number of compiler optimizations that increase compile time, but reduce run time.There are also optimizations (such as weeding out dead code and ignoring run-time checks, etc.) that reduce compile time and run time while reducing memory usage.The downside of these optimizations is that invalid running results in the program may not be discovered.

Dennis Ritchie invented the efficiency-focused "New B" and succeeded.
B language promotes the idea that "referencing an array element is equivalent to a reference to a pointer plus an offset".The B language also maintains the typeless characteristics of the BCPL language, and its only operand is the word of the machine.Thomposon invented the ++ and -- operators and added them to the PDP-7's B compiler.They still exist in the C language.
// In C, the statement that copies one character of a string:
*p++ = *s++;
can be compiled into PDP-11 code extremely efficiently:
moveb (r0)+, (r1)+
Thompson PDP-11 (Assembly)

After PDP-11, untyped languages ​​quickly became anachronistic.This processor features hardware support for several data types of different lengths that the B language cannot express.
"New B"->C uses compilation mode to introduce the type system, and each variable must be declared before use.