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Cx51 User's Guide
Cx51 Introduction Compiling Programs Language Extensions Preprocessor Advanced Programming 
8051 Derivatives 
Analog Devices MicroConverter B2 Series Atmel 89x8252 and Variants Cast/Evatronix Dual DPTR Dallas 320, 420, 520, and 530 Dallas 390, 400, 5240, and Variants Arithmetic Accelerator Infineon C517, C509, 80C537, and Variants Data Pointers High-speed Arithmetic Library Routines NXP and Atmel Dual DPTR NXP 80C51MX Architecture NXP 8xC750, 8xC751, and 8xC752 Error Messages Library Reference AppendixAtmel 89x8252 and Variants
The Atmel 89x8252 and variants provide 2 data pointers which may be used for memory access. Using multiple data pointers may improve the speed of library functions like memcpy, memmove, memcmp, strcpy, and strcmp.
The MODA2 directive instructs the Cx51 Compiler to generate code that uses both data pointers in your program.
The Cx51 Compiler uses at least one data pointer in an interrupt function. If an interrupt function is compiled using the MODA2 directive, both data pointers are saved on the stack. This happens even if the interrupt function uses only one data pointer.
To conserve stack space, you may compile interrupt functions with the NOMODA2 directive. The Cx51 Compiler does not use the second data pointer when this directive is used.
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