CALL : Jump to a sub-routine and save the return address

This instruction is one of the few that write to two different registers simultaneously:

• The source value (register or immediate) is written to PC
• The address of the next instruction (PC+2 or PC+4) is written in the destination register.

This allows the processor to jump to a subroutine (pointed to by the immediate or register value) and save the return address in a register (which gets written again in PC to return), as is demonstrated in this example:

.profile auto

; .... main program:
call my_function R5
; ....
HALT

;...

. my_function
; function starts with the return address in R5
  ; do something useful here (or not)
mov R5 PC ; return

Note that PC as a source or destination makes no sense :

• CALL PC R1 writes the instruction's address to PC, which creates an endless loop !
• CALL R1 PC jump to R1 but saves PC+2 to PC, which makes a conflict : should R1 or PC+2 go to PC ?