Program control instruction procedure

PROCEDURES A procedure is a group of instructions (subroutine or function) that usually perform a specific task. Advantages: Disadvantages: (a)It is reusable section of the software that It takes the compiler a small amount is stored in memory once, but used as often of time to link the procedure and as necessary. return from it. (b)It saves memory space. (c)Makes easier to develop software. PROCEDURES How procedure links with main program The CALL instruction links to the procedure and the RET (return) instruction return from the procedure. The CALL instruction pushes the address (return address) of the instruction following the CALL on the stack. The RET instruction removes an address from the stack so the program return to the instruction following the CALL. A procedure begins with the PROC directive and ends with the ENDP directive. The PROC directive is followed by the type of procedure: NEAR (intrasegment) or FAR (intersegment). Format of Procedure XXX PROC NEAR/FAR ............................................ ............................................ ............................................ RET XXX ENDP Example: SUMS PROC NEAR ADD AX,BX ADD AX,CX ADD AX,DX RET SUMS ENDP N.B XXX is the name of level (both level name should be same) To call a procedure in main program write: CALL XXX PROCEDURES (continued) CALL instruction The CALL instruction transfer the flow of the program to the procedure. The CALL instruction differ from a jump instruction because a CALL saves a return address on the stack. Whenever a CALL instruction executes it: Pushes the IP or, CS:IP on the stack. Changes the value of IP or, CS:IP. Jumps to the procedure by new IP or, CS:IP address. Difference between JMP and CALL instruction JMP CALL Doesn’t use stack Uses stack Doesn’t return to the next instruction of Must return to the next instruction of JMP CALL PROCEDURES (continued) Types of CALL (a)Near CALL (b) Far CALL Difference between Near CALL and Far Call Near CALL Far CALL (1) Procedure located within the same code (1) Procedure located in the entire segment (±32KB) memory (1 MB) (2) 3-byte instruction (2) 5-byte instruction (3) Only IP content is replaced by (IP± (3) Both CS and IP contentsare displacement) replaced by new CS and IP address (4) Stack stores only return IP address (2 byte) (4) Stack stores the return CS and IP address. (4 byte) RET instruction The Return (RET) Instruction removes a 16-Bit number (near return) from the stack and places it into IP or removes a 32- Bit number (far return) and places it into IP And CS. The Near and far return instructions are both defined in the procedure’s PROC directive, which automatically selects the proper return instruction.