Please list some software/s and links which can help me build firmware for Atmel 89C2051 micro-controller. Thanks.
The Atmel AT89C2051 is a 20 pin version of the popular 8051 microcontroller. The program memory (Flash) is limited to 2K and all the instructions related to jumping or branching should be restricted such that the destination address falls within the physical program memory space of the device. The Atmel AT89C2051 also has all of the normal 128 bytes of RAM as well as the built-in UART.
The datasheet (PDF link) is often a good place to start when developing firmware for a microcontroller.
Since the Atmel AT89C2051 is fully compatible with the MCS-51 architecture, and can be programmed using the MCS-51 instruction set, http://www.8052.com should be a good resource.
http://www.google.com/search?q=89C2051+site:www.8052.com
The open source Small Device C Compiler (SDCC) toolchain can be used to produce the necessary firmware files for programming the Atmel chip.
The Atmel C51ASM assembler is a two-pass macro assembler for the Atmel AT89 Family of 8051 microcontrollers and, according to the documentation, supports the AT89C2051.
The Stack Overflow questions tagged 8051 may also be of use.