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C51: Reserve Memory at Absolute Bank Addresses

Information in this article applies to:

  • C51, all Versions
  • BL51, all Versions
  • LX51, all Versions


I am using the C51 Compiler and µVision with a code banking project. I have a 88KB CODE memory in the chip. The lower 32KB is assigned to the common area, and the rest is for the code banked area. I have 2 banks: one 32 kb and the other is 24 kb.

Is there a way to tell compiler/linker that not all the banks are of the same size?

I need the compiler linker to enforce the size issue in order to know any violations in size.


Yes, you can block the memory usage in the linker as described below:

When using the LX51 Linker/Locater, just enter under Options - LX51 Locate - Reserve: B1:0xE000-B1:0xFFFF. This blocks the memory area 0xE000 - 0xFFFF from being used by other variables.

When using the BL51 Linker/Locater, the solution is a bit more complicated. You can reserve the memory area 0xE000 - 0xFFFF in bank 1 with the following method:

  • Add a source module called B1_RESERVE.C with the following content:

    code unsigned char dummy[0x2000];
  • Enter under Options - BL51 - Misc - Misc: BANK1 (?CO?b1_reserve (0xE000)). This locates dummy to address 0xE000 in bank 1.
  • Enter under Options - BL51 - Misc - Overlay: * ! ?CO?b1_reserve. This avoids the linker Warning L16: UNCALLED SEGMENT, IGNORED FOR OVERLAY PROCESS.


  • Refer to RESERVE in the LX51 User's Guide.


Last Reviewed: Thursday, February 25, 2021

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