C51: Function of ?C?ICALL and ?C?ICALL2

Information in this article applies to:

• C51 any Version

SYMPTOMS

When I use function pointers, the compiler outputs code that calls a library routine named ICALL as shown here.

              (*g) ();   // Call func indirectly
0004 AA06          MOV     R2,AR6
0006 A907          MOV     R1,AR7
0008 120000  E     LCALL   ?C?ICALL

What does this routine do and why is it necessary?

CAUSE

The ?C?ICALL library routine performs indirect function calls by:

• Loading DPL and DPH with the function address,
• Clearing the Accumulator,
• Executing JMP @DPTR+A.

Indirect function calls are handled with this function because there are 4 or 5 instructions necessary to execute the call. It is more memory efficient to have this routine if you have several indirect calls.

In addition there is a slightly different entry named ?C?ICALL2. This function expects the function address in DPL/DPH and therefore does not copy the function address.

SEE ALSO

• C51: ?C? Library Routines

Last Reviewed: Thursday, February 25, 2021