C51: ROM(COMPACT)

Information in this article applies to:

• C51 Version 5.50 and later

SYMPTOMS

I'm using the Keil 8051 C Compiler V5.0 with bank switching. We are looking for code space savings, and I've found that the linker reports the following gaps in our application:

BANK1   EFB9H     0031H     INBLOCK      ?PR?_KEYTOCONTROLCODE?PROGRAM
                EFEAH     0016H                  *** GAP ***
BANK1   F000H     026DH     INBLOCK      ?PR?DIAGNOSTICS?DIAG

BANK1   F637H     0031H     INBLOCK      ?PR?_CONTROLCODETOKEY?PROGRAM
                F668H     0198H                  *** GAP ***
BANK1   F800H     03F1H     INBLOCK      ?PR?PROGRAM?PROGRAM

Is there something that I can do to minimize these gaps in the linker's placement of modules?

CAUSE

These gaps are caused by using the ROM(COMPACT) directive to reduce program function size. When you compile with the ROM(COMPACT) directive, each segment must fit in an INBLOCK (2K page). With ROM(COMPACT), JMPs coded inside a function are implemented using the AJMP instruction, therefore, each function MUST fit within a 2K block.

RESOLUTION

If you change the large functions (or even the whole program) to be ROM(LARGE), the compiler uses LJMPS within the function and the function may cross over 2K boundaries.

MORE INFORMATION

• Refer to ROM in the Cx51 User's Guide.

Last Reviewed: Thursday, February 25, 2021