Mark Williams Let's C 3.x

The Mark Williams Let's C compiler was powerful C compiler for MS-DOS. Let's C was a subset of the Mark Williams C Programming System, and the first low cost entry-level professional C compiler for the IBM PC. It was also the first compiler to have a C source-level debugger.

It boasts the ability to create faster, smaller, and better optimized executables than any other compiler on the market at the time. It supports both large and small x86 models, and supports the 8087 math co-processor. It competed against compilers from Digital Research, Lattice, Computer Innovations, and Microsoft.

It includes a complete implementation of Kernighan and Ritchie C language, plus extensions to C implemented under UNIX. Includes an assembler, loader, archiver, advanced symbolic debugger. C shell, utilities and Micro-EMACS, a full-screen editor.

There were versions of the Mark Williams C Programming System for Unix and the Atari ST. Let's C is fully compatible, so C programs can easily be ported between the ST and IBM PC.

The Mark Williams Company was also notable as having produced Coherent, the first commercially viable Unix clone.