Intel 80287, 80286 manual Call Instruction

BASIC INSTRUCTION SET

Direct JMP outside of the current code segment. Direct JMP instructions that specify a target location outside the current code segment contain a full 32-bit pointer. This pointer consists of a selector for the new code segment and an offset within the new segment.

Example: JMP FAR_NEWCODE_FOO. Places the selector contained in the instruction into CS and the offset into IP. The program resumes execution at this location in the new code segment.

Indirect JMP outside of the current code segment. Indirect JMP instructions that specify a target location outside the current code segment use a double-word variable to specify the pointer.

Example: JMP NEWCODE. NEWCODE the first word of two consecutive words in memory which represent the new pointer. NEWCODE contains the new offset for IP and the word follow- ing NEWCODE contains the selector for CS. The program resumes execution at this location in the new code segment. (Protected mode programs treat this differently. See Chapters 6 and 7).

Direct JMP outside of the current code segment to a call gate. If the selector included with the instruc- tion refers to a call gate, then the processor ignores the offset in the instruction and takes the pointer of the routine being entered from the call gate.

JMP outside of current code segment may only go to the same level.

Example: JMP CALL_GATE_FOO. The selector in the instruction refers to the call gate CALL_GATE]OO, and the call gate actually provides the new contents of CS and IP to specify the address of the next instructions.

Indirect JMP outside the current code segment to a call gate. If the selector specified by the instruc- tion refers to a call gate, the processor ignores the offset in the double-word and takes the address of the routine being entered from the call gate. The JMP instruction uses the same format to indirectly specify a task gate or a task state segment.

Example: JMP CASE_TABLE [BX1. The instruction refers to the double-word in the array of point- ers called CASE_TABLE. The specific double-word chosen depends on the value in BX when the instruction executes. The selector portion of this double-word selects a call gate, and the processor takes the address of the routine being entered from the call gate.

3.6.1.2 CALL INSTRUCTION

CALL (Call Procedure) activates an out-of-iine proceciure, saving on the sla"k iht; addre•• uf the instruction following the CALL for later use by a RET (Return) instruction. An intrasegment CALL places the current value of IP on the stack. An intersegment CALL places both the value of IP and CS on the stack. The RET instruction in the called procedure uses this address to transfer control back to the calling program.

A long CALL instruction that invokes a task-switch stores the outgoing task's task state segment selec- tor in the incoming task state segment's link field and sets the nested task flag in the new task. In this case, the IRET instruction takes the place of the RET instruction to return control to the nested task.

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