Intel 64 and IA-32 Architectures Software Developers Manual Volume 1, Basic Architecture

Vol. 1 7-31

PROGRAMMING WITH GENERAL-PURPOSE INSTRUCTIONS

segment registers (DS, ES, FS, GS, and SS). The transfers are always made to or

from a segment register and a general-purpose register or memory. Transfers

between segment registers are not supported.

The POP and MOV instructions cannot place a value in the CS register. Only the far

control-transfer versions of the JMP, CALL, and RET instructions (see Section

7.3.16.2, “Far Control Transfer Instructions”) affect the CS register directly.

7.3.16.2 Far Control Transfer Instructions

The JMP and CALL instructions (see Section 7.3.8, “Control Transfer Instructions”)

both accept a far pointer as a source operand to transfer program control to a

segment other than the segment currently being pointed to by the CS register. When

a far call is made with the CALL instruction, the current values of the EIP and CS

registers are both pushed on the stack.

The RET instruction (see “Call and return instructions” on page 7-21) can be used to

execute a far return. Here, program control is transferred from a code segment that

contains a called procedure back to the code segment that contained the calling

procedure. The RET instruction restores the values of the CS and EIP registers for the

calling procedure from the stack.

7.3.16.3 Software Interrupt Instructions

The software interrupt instructions INT, INTO, BOUND, and IRET (see Section

7.3.8.4, “Software Interrupt Instructions”) can also call and return from interrupt

and exception handler procedures that are located in a code segment other than the

current code segment. With these instructions, however, the switching of code

segments is handled transparently from the application program.

7.3.16.4 Load Far Pointer Instructions

The load far pointer instructions LDS (load far pointer using DS), LES (load far

pointer using ES), LFS (load far pointer using FS), LGS (load far pointer using GS),

and LSS (load far pointer using SS) load a far pointer from memory into a segment

pointer is loaded into the selected segment register and the offset is loaded into the

selected general-purpose register.

7.3.17 Miscellaneous Instructions

The following instructions perform operations that are of interest to applications

programmers. For the purpose of this discussion, these instructions are further

divided into subordinate subgroups of instructions that provide for:

• Address computations

• Table lookup