MOVE opcode and C APIs in RPGLE

Question

Not considering code readability, performance and operating system version,
what's the difference between MOVE opcode and a C API such as ATOI?
How does MOVE internally works?

Example:

MOVE      ALPHAFLD  NUMRCFLD
EVAL      NUMRCFLD = ATOI(ALPHAFLD)

In old O.S. (< V3R7) when should I use MOVE and when should I use C APIs?

Answer 1

atoi() and atof() have "always" been available. But RPG programmers have been able to use %INT/%INTH or %DEC/%DECH to convert strings to numerics since V5R2, around the year 2002. %DECH and %INTH do rounding.

MOVE sometimes handles numerics in a bizarre way. For example, let's say you have a variable with 5 digits and 1 decimal position with a current value of 9876.5. Now you MOVE the value '123' to that variable. Try this code to see what bizarre result is in variable "num". Hint: It's not 123.

 D num             s              5p 1 inz(9876.5) 
 D msg             s             20                
 C                   move      '123'         num   
 C                   eval      msg = %char(num)    
 C     msg           dsply                         
 C                   return