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C251 C Compiler

The Keil C251 compiler unlocks the features and performance of the MCS® 251 Architecture. It is easy to use and minimizes the learning curve for starting your 251 projects. Extensions incorporated into the C251 compiler give you full access to all CPU resources and derivatives. The C251 compiler is the most efficient and flexible development tool set available. With support for all derivatives and full compatibility with all major emulator vendors, C251 is clearly the best choice for your projects.

It supports symbolic access to all features of the 251 architecture including the Atmel Wireless & Micro devices and the Intel USB devices.

The C251 compiler translates a C source file into a relocatable object module. The object file contains full symbolic information for debugging with the µVision Debugger or an in-circuit emulator. In addition to the object file, the C251 compiler generates a list file which may optionally include symbol table and cross reference information.


  • Ten basic data types, including 32-bit and 64-bit IEEE floating-point
  • Four memory models including small, medium, compact, and large
  • Program size up to 16 Mbytes
  • Direct access to all 251 hardware components
  • Source code compatible with the Keil C51 compiler
  • C interrupt functions with optional register bank switching
  • Aggressive code optimization techniques
  • Register parameters and register variables with global optimization
  • Direct interface to Keil A51, A251, and C51 and to Intel ASM51 and PL/M-51
  • Flexible variable allocation with near, far, huge, bit, ebit, code, data, bdata, ebdata, idata, xdata, and pdata memory types
  • Complete symbol and type information for source-level debugging
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