Keil Logo

Dallas 390, 400, 5240, and Variants

The Dallas Semiconductor 8xC390, 8xC400, 5240, and variants provide additional MCU modes that are fully supported by the Keil compiler.

Contiguous mode allows you to create large programs that exceed the classic 8051's 64K limit. The ROM(D512K) and ROM(D16M) directives instruct the Cx51 Compiler to generate code for the contiguous mode. The far memory type is used to access variables and constants using 24-bit DPTR addressing mode (in contiguous mode).


  • The contiguous mode requires the extended LX51 Linker/Locater and the extended AX51 Macro Assembler that are available only in the PK51 Professional Developers Kit.

In addition to the extended address space, the DS8xC390, DS8xC400, and DS5240 provide auto toggle and decrement features for the dual data pointers. The\KEIL\C51\LIB\C51DS2T.LIB library contains accelerated versions of the memcpy, memmove, memcmp, strcpy, and strcmp functions that use these features. For non-contiguous mode (classic 8051 mode) applications, you must add this library to your project to use the dual DPTR of these devices. The contiguous mode C library already contains the library routines for the auto toggle and decrement features.

The DS8xC390, DS8xC400, and DS5240 offer an extended stack space that is configured in the START390.A51 startup file.

  Arm logo
Important information

This site uses cookies to store information on your computer. By continuing to use our site, you consent to our cookies.

Change Settings

Privacy Policy Update

Arm’s Privacy Policy has been updated. By continuing to use our site, you consent to Arm’s Privacy Policy. Please review our Privacy Policy to learn more about our collection, use and transfers
of your data.