Keil C51 version 9.59 83.4 mb Keil Elektronik is pleased to announce the availability new version of C51 C Compiler (version 9.59) for the 8051 microcontroller is the most popular 8051 C compiler in the world. It provides more features than any other 8051 C compiler available today. What’s New in this release: The following sections list the changes instituted in each release of the C51 toolset.