This preview shows page 1. Sign up to view the full content.
Unformatted text preview: JMP (jump) instruction unconditionally transfers program control to a destination instruction. The transfer is one-way; that is, a return address is not saved. A destination operand specifies the address (the instruction pointer) of the destination instruction. The address can be a relative address or an absolute address. A relative address is a displacement (offset) with respect to the address in the EIP register. The destination address (a near pointer) is formed by adding the displacement to the address in the EIP register. The displacement is specified with a signed integer, allowing jumps either forward or backward in the instruction stream. An absolute address is a offset from address 0 of a segment. It can be specified in either of the following ways: An address in a general-purpose register -- This address is treated as a near pointer, which is copied into the EIP register. Program execution then continues at the new address within the current code segment. 7-20 Vol. 1 PROGRAMMING WITH GENERAL-PURPOSE INSTRUCTIONS An address specified using the standard addressing modes of the processor -- Here, the address can be a near pointer or a far pointer. If the address is for a near pointer, the address is translated into an offset and copied into the EIP register. If the address is for a far pointer, the address is translated into a segment selector (which is copied into the CS register) and an offset (which is copied into the EIP register). In protected mode, the JMP instruction also allows jumps to a call gate, a task gate, and a task-state segment. Call and return instructions -- The CALL (call procedure) and RET (return from procedure) instructions allow a jump from one procedure (or subroutine) to another and a subsequent jump back (return) to the calling procedure. The CALL instruction transfers program control from the current (or calling procedure) to another procedure (the called procedure). To allow a subsequent return to the calling procedure, the CALL instruction saves the current contents of the EIP register on the stack before jumping to the called procedure. The EIP register (prior to transferring program control) contains the address of the instruction following the CALL instruction. When this address is pushed on the stack, it is referred to as the return instruction pointer or return address. The address of the called procedure (the address of the first instruction in the procedure being jumped to) is specified in a CALL instruction the same way as it is in a JMP instruction (see "Jump instruction" on page 7-20). The address can be specified as a relative address or an absolute address. If an absolute address is specified, it can be either a near or a far pointer. The RET instruction transfers program control from the procedure currently being executed (the called procedure) back to the procedure that called it (the calling procedure). Transfer of control is accomplished by copying the return instruction pointer from the s...
View Full Document
This note was uploaded on 10/01/2013 for the course CPE 103 taught by Professor Watlins during the Winter '11 term at Mississippi State.
- Winter '11