Passing Values to Functions

The compiler uses registers A,B, R1, R2, R3, R4, Y0, and Y1 to pass parameter values to functions. Upon a function call, the compiler scans the parameter list from left to right, using registers for these values:

• The first two 8/16-bit integer values — Y0 and Y1.
• The first two 32-bit integer or float values — A and B.
• The first four pointer parameter values — R2, R3, R4, and R1 (in that order).
• The third and fourth 8/16-bit integer values — A and B (provided that the compiler does not use these registers for 32-bit parameter values).
• The third 8/16-bit integer value — B (provided that the compiler does not use this register for a 32-bit parameter value).
  Note: 64-bit parameter values are always passed on the stack.

The compiler passes the remaining parameter values on the stack. The system increments the stack by the total amount of space required for memory parameters. This incrementing must be an even number of words, as the stack pointer (SP) must be continuously long-aligned. The system moves parameter values to the stack from left to right, beginning with the stack location closest to the SP. Because a long parameter must begin at an even address, the compiler introduces one-word gaps before long parameter values, as appropriate.