Intel 210200-002 manual Call Call Procedure Call, Call procedure-name

CALL CALL PROCEDURE CALL

Operation:

if Inter-Segment then (SP) -- (SP) - 2 ((SP) + 1 :(SP)) -- (CS) (CS) -- SEG

(SP) -- (SP) - 2

((SP) +1 :(SP)) -- (IP) (IP) -- DEST

Flags Affected:

None

Description:

CALL procedure-name

CALL activates an out-of-line procedure, sav- ing information on the stack to permit a RET (return) instruction in the procedure to transfer control back to the instruction follow- ing the CALL. The assembler generates a dif- ferent type of CALL instruction depending on whether the programmer has defined the pro- cedure name as NEAR or FAR. For control to return properly, the type of CALL instruction must match the type of RET instruction that exits from the procedure. (The potential for a mismatch exists if the procedure and the CALL are contained in separately assembled programs.) Different forms of the CALL instruction allow the address of the target pro- cedure to be obtained from the instruction itself (direct CALL) or from a memory loca- tion or register referenced by the instruction (indirect CALL), In the following descrip- tions, bear in mind that the processor auto- matically adjusts IP to point to the next instruction to be executed before saving it on the stack.

For an intrasegment direct CALL, SP (the stack pointer) is decremented by two and IP is pushed onto the stack. The target procedure's relative displacement (up to ±32k) from the CALL instruction is then added to the instruction pointer. This CALL instruction

form is "self-relative" and appropriate for position-independent (dynamically relocat- able) routines in which the CALL and its target are moved together in the same segment.

An intrasegment indirect CALL may be made through memory or a register. SP is decre- mented by two; IP is pushed onto the stack. The target procedure offset is obtained from the memory word or 16-bit general register referenced in the instruction and replaces IP.

For an intersegment direct CALL, SP is decre- mented by two, and CS is pushed onto the stack. CS is replaced by the segment word con- tained in the instruction. SP again is decremented by two. IP is pushed onto the stack and replaced by the offset word in the instruction.

For an intersegment indirect CALL (which only may be made through memory), SP is decremented by two, and CS is pushed onto the stack. CS is then replaced by the content of the second word of the doubleword memory pointer referenced by the instruction. SP again is decremented by two, and IP is pushed onto the stack and replaced by the content of the first word of the doubleword pointer refer- enced by the instruction.

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