LX51 User's Guide

Common Area

The common code area may be accessed by all banks. It typically includes routines and constant data that must always be accessible. For example:

• Reset vector,
• Interrupt vectors,
• Interrupt service routines,
• String constants,
• Bank switching routines,
• Inter-bank jump table,
• and so on.

The following code sections must always be located in the common area:

• Reset and Interrupt Vectors
  Reset and interrupt jump entries must remain in the common area, since the code bank selected by the hardware at is not known at the time of the CPU reset or interrupt. The linker locates absolute code segments in the common area in each case.
• Code Constants
  Constant values (strings, tables, etc.) which are defined in the code area must be stored in the common area unless you can guarantee that the code bank containing the constant data is selected at the time they are accessed by program code. You can relocate these segments in code banks by means of control statements.
• Interrupt Functions
  Interrupt code generated by the Cx51 Compiler must be located in the common area. However, interrupt functions can call functions in other code banks. The linker produces a warning when an attempt is made to locate a Cx51 interrupt function in a code bank.
• Bank Switch Code
  The code required for switching code banks as well as the associated inter-bank jump table are located in the common area since they are required by all banks. By default, the linker automatically locates these segments in the common area. Do not attempt to locate them into bank areas.
• Library Functions
  Intrinsic run-time library functions used by the Cx51 Compiler or the PL/M-51 Compiler must be located in the common area. It is likely that the bank switch code uses the same registers that are used to transfer values to these library routines. Therefore, the linker always locates runtime library program sections in the common area. Do not locate these program sections in other bank areas.

It is difficult to estimate the size of the common area since it depends on the particular software application and hardware constraints. If the ROM area that is dedicated as common area is not large enough to contain the entire common code, the linker duplicates the remaining part of the common code area into the beginning of each code bank. This is also the case if your hardware does not provide a common code area section in the ROM space.